CHANGES IN MIGRATION OF BIRDS OF PREY OVER THE PAST DECADE

CHANGES IN MIGRATION OF BIRDS OF PREY IN (a geographical region) OVER THE PAST DECADE

INTRODUCTION

Birds are one of the best groups of animals for monitoring the effects of climate change. They are day-time active, conspicuous, easy to identify, and are popular with many groups of people, including amateur birdwatchers and professional scientists. In many parts of Europe, their distributions and numbers, as well as the timing of their migrations and breeding seasons, have been well monitored for decades.

Migratory birds are likely to be more vulnerable than non-migrants because they can influenced

by conditions in three different geographic locations : their breeding grounds, their winter areas, and their migration routes. Individual birds also experience “carry-over effects,” such as when conditions experienced in wintering areas influence subsequent breeding success, or when conditions experienced on the breeding grounds influence subsequent over-winter survival.

Furthermore, field evidence indicates that large numbers of migrants can be are killed by storms

encountered when they are migrating. Climatologists predict that storms and other extreme events are likely to increase in frequency in the years ahead. Therefore we can expect that migrants will suffer greater storm-induced losses, which could cause noticeable reductions in populations regardless of other climate changes.

                   

REVIEW OF LITERATURE:

CLIMATE CHANGE

Earth’s climate is currently changing. Although c limate change has occurred throughout earth’s

history, the current rate of change, the fact that we are present to be impacted by it and the growing body of evidence indic ating that we are responsible for it, all suggest that we should attempt to reduce its impacts wherever possible. Over the course of the last century, global average surface temperature has increased by around 0.6ºC, and precipitation has increased, particularly over mid- and high latitudes.

These changes, in turn, have affected the extent of both global ic e cover (decreasing) and

sea- levels (increasing).

The ongoing increase in temperature resulted from increased concentrations of carbon dioxide

and other greenhouse gases in the atmosphere, which reduces radioactive heat loss from earth. As a

result of burning fossil fuels and other human activities, carbon dioxide concentrations have risen by 32% from about 280 ppm in pre- industrial times to about 370 ppm at present. If this trend continues, carbon dioxide levels are expected to exceed 400 ppm by the year 2100, causing a mean global temperature rise of 1-4 ºC in the coming century. Increased surface warming is likely to increase the frequency and intensity of climatic extremes, including tropical cyclones, flooding, and droughts.

Sea-levels are forecast to rise at rates of 30-50 cm per century, flooding many fertile delta regions and low-lying is lands. Over much of the world, glac ial areas will be restr icted to higher latitudes and altitudes, and animals and plants that depend upon them will be further restricted as well. Pollen records suggest that in most instances trees can shift their ranges by about 200-400 m annually Under the current rate of climate change, July isotherms are expected to advance northward at 4-5-Km annually. If trees were to track this rate of change they would need to migrate 10-25 times faster than the mean rate exhibited by the pollen record. In this situation it is difficult to predict the impacts of climate change on plants and animals.

Another problem in predicting future biota distribution is that large parts of the landscape have

now been converted to human use and, as such, are closed to most of the wild animals and plants. In fact, many areas provide no broad-front dispersal route, but, at best, a series of narrow interrupted corridors and stepping stones, thereby impeding or preventing the movements of many organisms. The ongoing situation is likely to favour plants with short response times, namely herbs, shrubs and fastgrowing trees, whereas slow-growing trees with long generation times and poor dispersal characteristics could be particularly disadvantaged. Although animals are more mobile than plants, they also are restricted to areas of suitable vegetation.

RESPONSES OF MIGRATORY BIRDS TO CLIMATIC CHANGES

A growing body of field and laboratory evidence indicates that far from being a static and

conservative trait; migration is a dynamic and flexible behaviour in birds that is greatly influenced by external factors. Thus we can expect that in addition to population effects in migrants, migratory behaviour, itself, is likely to change in association with climate change. And, indeed, many changes in migration already have been reported. Many migrants are migrating earlier in spring than formerly, and some are migrating later in autumn as well. As a result, individuals of some species stay for longer on their European breeding areas. Most examples of shifts of increased migratory behaviour involve species that have extended their breeding ranges into higher latitude areas where overwintering was not possible or was costly in the past. On the other hand, some species that once were entirely migratory are now partially migratory, with increasing numbers of individuals staying on their breeding grounds year-round. In yet other species, individuals are now migrating shorter distances than formerly, and are over-wintering farther north. One example of the latter is the increased proportion of White Storks that now over-winters in southern Spain, rather than migrating to Africa. In some species these changes may be beneficial or neutral to populations. In others they may be harmful. Almost all of these changes are associated with changes in food-availability, or with climatic conditions that are likely to affect food-supplies, such as milder winters.

Some of the observed changes in migratory behaviour appear to represent immediate behavioural

or “facultative” responses to prevailing conditions, whereas others may reflect genetic changes

brought about by natural selection. Despite difficulties of detecting the latter, there is evidence from a few species that indicates a genetic basis for changes in migration timing, and, at least for one species, a genetic basis for changes in migration intensity and the direction of migratory travel. Most changes in migratory behaviour are likely to start as facultative responses and then become genetically based as natural selection acts over time.

One situation that has come to light in studies of biological responses to climate change is that

different plants and animals often do not respond at the same speed and magnitude to climate change.

As a result migratory birds that once arrived on their breeding areas when their food-supplies were reaching their peaks now arrive either too early or too late to take immediate advantage of this situation. Furthermore, those arriving too late are likely to breed less successfully, resulting in population declines.

The breeding ranges of some European birds are already shifting north, as individuals withdraw

from southern portions of their ranges, while others spread north at the northern limits of their ranges.

A particular concern involving range shifts is the loss of mountain-top breeders, which may disappear from much of their range, as global warming reduces the extent of specific high-mountain habitats.

Some measures taken to combat the causes of climate change, such as the development of wind

farms, could themselves severely impact migratory birds. This is particularly so if wind farms are sited improperly along major migration routes, where large numbers of migrants could then be killed by colliding with rotor blades.

There is no doubt that bird migration originated in the tropics, or at least in tropical-subtropical

conditions. This is supported by the observation that most long-distance migrants of the northern

hemisphere have closely related, non-migratory or partially migratory forms in the tropics Migration under tropical conditions initially covered only short distances and from the beginning included partial migration. However partial migration may have evolved in has proved to be an extremely successful and adaptable life force, and has become increasingly widespread.

Once partial migration was genetically anchored in a species, the movement ecology of

populations within it could range from entirely sedentary to completely migratory depending upon ecological circumstances, and within the latter, intercontinental, long-distance migration could evolve as conditions mer ited. Selection experiments involving captive song birds suggest that the transformation from migratory to sedentary population (or vice versa) in the wild could occur within about 25 generations or 40 years.

Recent ice ages certainly played a major role in the development of bird migration in and out of

Europe. At the height of ice coverage in the Northern Hemisphere the avifauna of Europe was greatly reduced ,only to increase during intervening warm periods. The current bird migration system in Europe, which emerged at the end of the last ice age, 15 000 years ago, is still developing.

Migrant birds also are important vectors for different forms of life, including plants, fungi, algae,

and many microorganisms. As a result, migratory birds can be a major factor in determining the

distribution of other life forms. Populations of migrating birds can also serve as reservoirs for diseases and can spread disease-causing agents to humans, their livestock, and plant resources. In an overview of the subject, Gerlach (1979) lists viruses, Rickettsia, Chlamydia, bacteria, and fungi as disease vectors that can be spread by birds either through direct infection or through the ectoparasites carried by birds. Cases of toxoplasmosis and Haemosporidia ( i.e. protozoans) have been reported .This widespread transport has been closely studied. An examination of over 5,000 birds in Austria has revealed that many arboviruses are regularly transported by migratory birds. Transported infections include Q fever, typhus fever, pseudo-tuberculosis, Newcastle disease, salmonella, and, most recently, avian influenza H5 N1. As migration behaviour changes in European birds as a result of climate change, so will their role as transport agents of organisms that are important to humans.

4. IMPACT OF CLIMATE CHANGE IN MIGRATORY BIRDS

Change in the migratory behaviour of wild birds has attracted attention recently, as interest has grown in assessing the effects of human-induced climate change. If weather has become warmer, as it has over much of the world, one might expect birds to have responded accordingly, with migratory species overwintering at higher latitudes, or arriving earlier and departing later from their breeding grounds. Ringrecoveries, long-term observations of visible migration at migration watchsites, and regional records of first arrival and last departure in spring and autumn, respectively, all have played important roles in assessing the way that bird migration has changed and continues to change over time. The following sections provide examples of these changes.

CHANGES IN MIGRATION TIMING

Studies of long-term trends in arrival times of birds are mostly based on dates of first sightings, as it is these dates that are most frequently recorded, in some European localities for periods exceeding 300 years .The problem with first arrival dates is that many refer only to single individuals, which may not be representative of entire populations. Although median or mean arrival dates of populations of individuals in their breeding areas are more representative, they have been recorded less frequently, and chiefly in recent decades. Another source of migration timing are Bird Observatories where observations of visible migrants or trapping dates of other migrants are maintained throughout the migration seasons each year, enabling median or mean passage dates (and standard deviations) to be calculated. One approach in using these data has been to combine records from different Bird Observatories in the same region and calculate regional values.

Whereas arrival (or departure) dates refer to birds from a single population breeding in a

particular area, passage dates usually refer to birds from more than one breeding area, occupying a wide span of latitude, counted at a point on their migration. Some studies have compared first and median or mean passage dates from the same site over a period of years, and found the various dates to be correlated.

In years that were early, the total arrival period was prolonged. Despite methodological differences, long-term studies of migration timing tend to support each other’s findings.

SPRING DATES

Presumably as a result of long-term climate warming, many birds now arrive in their breeding

areas earlier in spring and depart later in autumn than in the past, spending from a few days to a few weeks longer each year in their summer quarters. Such changes have become apparent in a wide range of species at many localities in both Eurasia and North America

Nevertheless, not all species exhibit such changes. Exceptions may be the result of missing data or population declines that make it more difficult to detect the earliest arrivals and latest departures, as well as inflexibility in migration scheduling, or constancy in limiting factors in spite of climate change.

Of 983 Eurasian bird populations in which first arrival dates on the breeding grounds were

monitored over time, 59% showing no significant change, 39% arrived significantly earlier, and only 2% arrived significantly later. Both short-distance and long-distance migrants showed the same trends. From 222 populations for which mean passage dates could be calculated of time, 69% showed no change, 26% were significantly earlier, and only 5% were significantly later.

The average change of first arrival date over all species and sites was -0.373 days per year, while the equivalent figure for mean passage dates was -0.100 days per year. Both figures were statistically significant. It is not obvious why the two figures differed, but in general the mean migration dates were based on larger, more standardized data-sets.

Within the long term trends, arrival and migration dates fluctuated annually in line with local

temperature. For example, at the Rybachy Bird Observatory on the Courish Spit in the southeastern Baltic, warming during the 1930s and 1940s, and then in the 1960s and 1980s, was associated with significantly earlier spring migration in many species of song birds, whereas colder periods during the 1950s and 1970s were associated with later passage.

Most researchers have used annual temperatures from localities on the migration route or

breeding area, whereas others have used the winter-spring index of the North Atlantic Oscillation (NAO), a large-scale climate phenomenon influencing weather in this region that is calculated as the difference in normalized monthly values of atmospheric pressure in the Azores and Iceland. Positive values indicate warmer and wetter winter-spring weather (and by earlier spring migration) in northwest Europe and the opposite weather conditions and later arrival dates than usual in southern Europe. Typically, most birds arrived about 2.5-3.3 days earlier for every 1°C increase in spr ing temperature .A smaller number of studies available from North America revealed similar, although in eastern North America, long-term temperature change has been less marked than in Western Europe. In general, earlier arrival of migrants in spring leads to earlier breeding, as described as a recent trend in a range of species .

Earlier breeding, in turn, often gives rise to higher reproductive success

Despite strong correlations between arrival dates and temperature on the breeding grounds, much of the variance in arrival dates remains unaccounted for. Migration timing may also be influenced by weather along the migration route or in wintering areas, as well as by changes in weather including wind and barometric pressure, and by different factors such as food-supply.

Moreover, poor weather at one part of a migration route can stall migratory movements there, even though conditions may be favourable further along the route. Inter-spec ies differences, which have been demonstrated in every relevant study, could be diet-related, and further investigation is needed.

In comparing the changes that have occurred in the spring migration dates of different species,

several general patterns emerge:

     Greater changes have occurred in the migration dates of early-migrating species than of latermigrating species. This is associated with weather (including temperature) being more var iable earlier than later in the spring 1998.

     Greater changes have occurred in the arrival dates of short-distance than long-distance migrants – presumably because short-distance migrants generally arr ive earlier in spring (same point as above), and have more flexibility in their migration timings.

     Greater changes have occurred in the arrival dates of smaller bird than larger birds. This is possibly because the smaller species are more sensitive to annual temperature differences and their effects on food-supplies (although their shorter generation times would also favour more rapid genetic change than is not possible in large longer- lived species).

    Inter-annual var iability in the arrival dates of short distance migrants generally showed a correlation with spring temperatures in the breeding locality, but such correlations were less obvious in longdistance migrants . Moreover, where it has been investigated, weather along the migration corridor often shows a better relationship with arrival dates than does weather at the arrival location.

     Spring weather has not changed everywhere in the same way. Correspondingly, the degree of change in arrival dates in breeding areas varies across Europe, with arrival dates in most areas getting earlier as spring temperatures increase, but later in those areas with decreasing spring temperatures. In the Mediterranean region, springs are now cooler than in the past, which may slowing the return of longdistance migrants from tropical Africa to the mid- and higher latitudes of Europe.

    Most species still arrive on their breeding grounds earlier in warm springs than in cool springs Three explanations may account for the fact that more short-distance migrants than long-distancemigrants now arrive earlier in spring and in closer correlation to temperatures on breeding areas. First, a stronger endogenous control of migration in long-distance migrants might inhibit a rapid reaction to a changing environment. Short-distance migrants are typically more flexible (facultative) in their response, and more able to alter their behaviour in relation to prevailing conditions. Secondly, the closer a species winters to its breeding areas, the more closely correlated are the day-to-day weather changes in the two areas, enabling short-distance migrants to react more rapidly and appropr iately. Thirdly, weather is more var iable ear ly in the spr ing, when most short distance migrants arrive in their breeding areas, than it is later in the spring, when most long-distance migrants arrive.

In most species, males arrive in breeding areas before females, and studies of first arr ival dates

typically concern only males. But the two sexes may not necessarily respond in the same way to

climate change. A long-term study of arr ival dates of male and female Barn Swallows Hirundo rustica in Denmark revealed that only males responded to c limate amelioration dur ing migration . Therefore, even though males arrived ear lier there was change in mean nesting date, because females arr ived no ear lier than they did 30 years previous ly.

Earlier arr ival on the breeding grounds could be brought about by (a) increases in the speed of

spring migration, (b) earlier departure from wintering areas, (c) over-winter ing closer to the breeding

grounds, or (d) combinations of these possibilities. More rapid progress in warm than cold springs has been recorded in many migrants from the dates they pass through successive observation sites in different years. Only facultative responses could account for the year-to-year variation in arrival dates seen in many migrants, but this need not exc lude the possibility of genetic change in response to longer-term environmental trends, such as climate warming. Moreover, a long-distance migrant, the Garden Warbler (Sylvia borin), and a short-distant migrant, the Blackcap (Sylvia atricapilla) bred in captivity, showed no difference in heritability of migration dates.

AUTUMN DATES.

Overall, changes in autumn migration dates over recent decades have been fewer and more

variable, than changes in spring dates. Two patterns have emerged, involving either ear lier or later departure over the years. In some singlebrooded populations, earlier arrival is followed by earlier breeding and moult, and, subsequently, earlier departure. In such populations, the timing of successive events through the summer, from arrival, egg-laying, hatching, f ledging, moult and autumn migration, are correlated with spring temperatures, and show little or no relationship with the prevailing autumn temperature. An earlier spring arrival pulls the whole cyc le forward to give an earlier autumn. At Rybachi on the southern Baltic coast, warming in the 1960s and 1980s led to significantly earlier mean dates in spring passage, breeding and autumn passage. Conversely, colder springs during the 1970s caused a shift towards later spring passage, breeding and autumn migration. These changes occurred in both shortdistance and long-distance migrants. Most migrants at Rybachy came from northern breeding areas that provided time for only one brood.

Similar relationships were found for s ingle-brooded long-distance migrants passing through the

Swiss Alps in autumn .The long-distance migrants may have benefited from an earlier crossing of the Sahara before its seasonal dry period. In contrast, shorter distance migrants passing over the Alps and wintering north of the Sahara mostly showed a later autumn passage. These are mostly passerine species that can raise more than one brood per year, so could better take advantage of a longer season by remaining longer in their breeding areas. Further south and west in Europe, where individuals can make up to two or three breeding attempts in the same season, departure dates of passerines have tended to get later as local temperatures have risen but it is not known whether this has been associated with a lengthening of the breeding.

Changes in the length of migration routes

A)    SHORTENING OF MIGRATION ROUTES

So called migration “short-stopping” has occurred in many species as more food has become

available at higher latitudes in the winter ing range, either through human activities or c limate change.

Several North American populations of Canada Geese (Branta Canadensis) have responded in this way to agricultural changes or to the creation of waterfowl refuges where food is provided  and Common Cranes (Grus grus) in Europe. Other species of waterfowl have Shortened their migrations, apparently in response to warmer winters, as open water has become available nearer the breeding areas. This is manifest by increased numbers wintering in northern and eastern parts of Europe, and declining numbers of the same species wintering in the south and west.

Other species of waterfowl have shortened their migrations in apparent response to reduced

disturbance and predation, as sanctuaries have been established in areas previously open to hunting.

Examples of migratory short-stopping in raptors include Sharp-shinned Hawks (Accipiter striatus) and Merlins (Falco columbarius) in parts of North America. For both species an increased dependence upon bird-feeder birds and suburban birds seems to be respons ible for the change in migration behaviour.

Shortened migrations are also ref lected in the changing distr ibutions of r ing recover ies of many other species. Similar ly, among 30 species of short-distance or partial migrants breeding in Germany, a tendency towards winter ing at higher latitudes was found in ten species, and at lower latitudes in three species, although r inging recover ies are affected by changes in human land use and hunting, as well as in climate . More and more European migrants that former ly wintered entirely in tropical and southern Afr ica are now over-winter ing in small but increasing numbers in the Mediterranean. Examples include the Yellow Wagtail (Motacilla flava), House Martin (Delichonurbica), Osprey (Pandion haliaetus), Lesser Kestrel (Falco naumanni), and White Stork (Ciconia ciconia).

In some regions irruptive migrations have become less frequent than former ly, presumably

because the birds have become less numerous or, more often, remain in their breeding areas yearround.

Comparing the nineteenth with the twentieth centuries, the P ine Grosbeak (Pinicola

enucleator) has become a much less frequent visitor to the middle latitudes of Europe. No noticeable invasions of Scandinavian Great Tits (Parus major) and Blue Tits (Parus caeruleus) to Britain have occurred since 1977 and no big invasions of Great Spotted Woodpeckers (Dendrocopos major) since 1974. In Germany, invasions of Blue Tits, Waxwings (Bombicilla garrulous) and Redpolls (Carduelis flammea) have also become less frequent . On the other hand, Two-barred Crossbills (Loxia leucoptera) have appeared in Fennoscandia in increasing numbers and frequency, possibly associated with the increased planting of larch (Larix spp.) outside their natural range. Likewise, in eastern North Amer ica, Evening Grosbeaks (Hesperiphona vespertina) have become less numerous, and their invas ions less frequent, than previously. This may be associated with reduced outbreaks of Spruce Budworm (Choristoneura fumiferana), a favoured summer food, and with increased winter bird feeding by householders.

Other types of change have also occurred. For example, like many other birds that does not start

to breed until they are two or more years old, young White Storks (Ciconia ciconia) remain in “winter quarters” through their first summer, or migrate only part way towards breeding areas. In recent decades, second-summer birds, whose predecessors used to remain in Afr ica, have returned in increasing numbers to southern Europe to pass the summer. The mean distance of recoveries of second-summer birds from their natal sites in north Germany was 2,517 km in 1923-75 (N = 120), reducing to 720 km in 1978-96 (Fiedler 2001).

B)    LENGTHENING OF MIGRATION ROUTES

In species that have expanded their breeding areas to higher latitudes yet have retained the same

wintering areas, extension of migration routes has occurred. Northern hemisphere examples include:

(1) Black-winged Stilt (Himantopus himantopus) which is expanding its breeding range northward(France, Ukraine, Russia) but still winters south of 40°N latitude; (2) European Bee-eater (Merops apiaster) which has expanded northwards in almost all central European countr ies, yet still winters entirely in Africa south of the Sahara; (3) Citr ine Wagtail (Motacilla citreola) which is expanding its breeding range from Asia westward into Europe, but still winters in India and southeast Asia . The intra-European routes have increased by up to 1,000 km. These examples represent the kind of changes that must have occurred in many spec ies after each glaciation, when ice receded, and plants and animals spread from lower to higher latitudes.

Most Red-breasted Goose (Branta ruficollis) now over-winter in Romania-Bulgaria, some 300-

600 km further from their breeding areas than in the 1950s, as former wintering s ites in Azerbaijan have been altered by land-use changes. In even ear lier times, the species was found in winter even further from its breeding areas, being depicted in the art of ancient Egypt. Thus over recorded history this species has both shortened and lengthened its migration routes. Such changes in the length of migrations could initially involve only facultative responses to local conditions, but as migrations lengthen over time, some genetic change seems likely, as they would require changes to regulatory mechanisms.

In some other species, greater proportions of ring recoveries are now being obtained from the

distant parts of migration routes than formerly, but it is hard to tell whether this is due to altered

migration behaviour, or to increased opportunities for recoveries along the routes.

In particular, over recent decades hunting has declined much more in the northern and mid latitudes of Europe than further south. This could affect the migratory behaviour of hunted species, or the distribution of their ring recoveries.

CHANGES IN MIGRATORY HABITS

MIGRATORY TO SEDENTARY

At many latitudes many populations of birds have become more sedentary recently. For example,

prior to 1940, the Lesser Black-backed Gull (Larus fuscus) was almost entirely migratory in Britain, with only a few individuals remaining year-round. Today, large numbers of all age-groups stay for the winter, feeding mainly on refuse dumps which have increased the winter food-supply. Asimilar change has occurred among Herr ing Gulls Larus argentatus in Denmark.

Another example is the Eurasian Blackbird Turdus merula, in which the British and mid European populations have become progressively more sedentary during the last two centuries, as winters have mellowed. In both Europe and North America, many seed-eaters are now wintering further north in their breeding range, in association with the provision of suitable food at garden feeders. Winter feeding turned a Great Tit Parus major population from migratory to sedentary in the Finnish city of Oulo near the Arctic Circle. Among many other short-distance and medium-distance migrants, increasing numbers of individuals now winter in areas where they once were wholly migratory, these species developing into typical partial migrants.

Some such changes could be genetic in nature, others facultative. Their net effect is to expand the winter avifauna of many high- latitude areas.

SEDENTARY TO MIGRATORY

Examples of changes from sedentary to migratory behaviour are less evident, and are generally

associated with an extension of breeding range into higher latitudes. For example, the European Serin (Serinus serinus) was once restricted to the south of Europe where it is sedentary, but in the early 20^th century it spread north, where it became migratory. In more recent years, with milder winters, this migratory population has become partially resident. Likewise, since the 19th century, many bird species have spread north in Fenno-Scandia, including the Northern Lapwing (Vanellus vanellus), Starling (Sturnus vulgaris), Eurasian Blackbird (Turdus merula) and Dunnock (Prunella modularis). In newly colonised breeding areas they are essentially migratory, whereas further south they are partial migrants or sedentary.

CHANGES IN MIGRATORY DIRECTIONS

A well known example of recent change in migratory direction involves the Blackcap (Sylvia

atricapilla), a species that is now wintering in increasing numbers in the British Islands. Changes in the direction of migration, leading to the adoption of new wintering areas, also were recorded in several species in the last century. For example, Little Egrets (Egretta garzetta) breeding in southern France migrated southward, some crossing the Sahara to winter in the Afrotropics. Beginning in the 1970s, increasing numbers began to migrate northwest to winter in northern France, southern Britain and Ireland (Mar ion et al. 2000). Some later became resident in these areas, and from the 1990s started to breed there. Similarly, Lesser Black-backed Gulls (Larus fuscus) from Europe have begun increasingly to winter on the coasts of eastern North America, with records from Nova Scotia to Florida, a change which requires a much stronger westerly component in the directional preferences.

Almost certainly, such marked directional changes have involved genetic changes, as confirmed for the Blackcap by breeding and direction-testing in captivity.

A different type of change is shown by those northern hemisphere species introduced to the

southern hemisphere, which have reversed the direction of their spring and autumn journeys,

respectively, so that they continue to winter in lower rather than in higher latitudes. This is true, for example, for the European Goldfinch (Carduelis carduelisb) and others introduced from Europe to New Zealand in the 19th century, and also for the White Stork (Ciconia ciconia) which colonised South Africa naturally in the 1930s, and now migrates north to over-winter in Zaire and Rwanda.

CONSEQUENCES OF THESE CHANGES

Generally, warmer climates would lead to an increase in the number of residents populations in

Europe, first as already sedentary populations increase, second as obligate and facultative partial migrants become more sedentary, and third, to a limited extent, as some populations of complete migrants also become sedentary. At the same time, long-distance migrants would shorten their migratory movements. As a consequence of such changes the phenomenon of migration, itself, would be at risk. Specific predictions include:

· Greater survivorship among resident populations in high-latitude areas.

· Increased competition between long-distance migrants and res idents on the breeding grounds.

· Increasing r isk of ecological mismatches between migratory birds and their food-supplies more

probable among long-distance migrants.

· Changes to migratory directions and the choice of new, closer winter quarters.

· A reduction in the migratory distance to the winter quarters,

· Increasingly delayed departure times.

Aquatic birds

Aquatic birds should be included since they depend upon wetlands changes to which are

expected to occur as a result of c limate change. So, several changes are expected in relation to

abundance and distr ibution of waterfowl and waders. As in raptors, aquatic spec ies have been object of major attention by ornithologists and sc ientists for some time and we now possess long datasets regarding their distributions and abundances changes. Examples of aquatic birds that should be monitored inc lude the Greylag Goose (Anser anser) and the Ruff (Philomachus pugnax),

Seabirds

Because climate change is expected to affect sea levels especially in the Mediterranean basin,

monitoring seabird species is highly recommended. Actually there are several programs followingmigration in this group of birds that must be maintained and coordinated at continental scale.

Songbirds

There is a long tradition of work on song birds throughout Europe. Different species of song birds depend upon different habitats and prey bases and 4-6 species of these birds should be identified forfocused monitoring.

To be effective, all these monitoring efforts, and the resulting data, must be communicate to the

scientific community for their use and research, providing a good way to interchange new findings.

The recent experience in the first international meeting on Bird Migration and Global Change, hosted in Algeciras, indicates the value of regular ly scheduled international meetings to favour the interchange of new ideas and recent findings, as well as to coordinate monitoring efforts.

DISCUSSION

Most of the work cited in this report focuses on particular species or suites of similar species, and

it is difficult to determine what proportion of an avifauna’s migration habits, other than arrival times, have changed in recent decades. Over the past 50 years, climate changes have been more marked in some regions than in others, and studies reporting changes in migratory behaviour were more likely to be published than those finding no change. However, among the bird species that breed in Britain, 73 provided enough ring recoveries from a sufficiently long per iod to look for changes in the lengths and directions of migratory movements. Of these, 51 (70%) of these species showed no significant change in either respect during the 20th Century, in 15 species movements became shorter, in five species they had become longer, and in two species movements changed in complex ways. The 22 species that evidenced change were significantly more than the four expected on a significance level of 5%. These species included song birds, raptors, waders, waterfowl, and seabirds. Similarly, of 30 species that breed in Germany, and provide enough ring recoveries, eight species showed decreasing mean recovery distances with time, whereas five species showed increasing mean recovery distances. Again the numbers that showed change were significantly greater than the two expected at a significance level of 5%. Such studies confirm that changes in the migration behaviour of birds have been common over the last several decades.

These observations, together with selection experiments on captive birds, serve to confirm that

migration is a dynamic phenomenon, subject to continual change in response to prevailing conditions.

Some aspects, such as an abrupt change in the direction of migration, imply rapid evolutionary shifts, whereas may represent either genetic or facultative responses to changing conditions. Overall, it seems reasonable to assume that both genetic and facultative responses are likely to be involved, with birds responding initially by facultative means, and, eventually, genetically, as natural selection comes into play. Facultative responses are relatively limited (though variable in extent between species), and if environmental conditions continue to change in the same direction, such responses eventually become inadequate to deal with the new conditions. Only genetic change may enable the population to respond appropriately to conditions beyond the previous range.

Although all major aspects of migratory behaviour have been shown to have heritable components, mainly through artificial selection and cross-breeding in captivity, genetic change is not easily demonstrated in wild populations. The assumption is that, if individuals taken from the wild in different years or from different regions express behavioural differences when held under identical controlled conditions, these differences are likely to have a genetic basis. This conclusion is strengthened if the trend is maintained in captive-bred offspring from these individuals, unaffected by parental effects or experience in the wild. Such a test has been made with Blackcaps (Sylvia atricapilla) randomly collected as nestlings from south Germany and hand-raised each year over a 13- year period. In successive samples of birds, the amount of autumn migratory activity was found to decline, towards a later onset and reduced intensity (less activity per night). This was precisely the result expected if the population had responded genetically to ameliorating environmental conditions, so at least in this species later departure and shorter migration may partly represent a genetic response resulting from natural selection.

Occasionally, a wild population under study has unexpectedly provided evidence for genetic change in some aspect of migration, as in the effect of unusually severe weather on the arrival and departure dates of the swallows. Indications of genetic change in other aspects of migratory behaviour also can be gained from long-term studies of wild bird populations, but these studies are not without problems, and findings can often be interpreted in different ways. Moreover, apart from arrival dates, reliable information on migratory traits is hard to collect from free-living birds. In any population the rate of evolutionary change is limited by: (1) the amount of genetic variation within the population at the time; (2) the strength and consistency of the selection pressure; and (3) the extent to which selection on one trait causes parallel changes in others, which could be beneficial or detrimental. Genetic variance is often reduced in populations that have suffered recent numerical declines in which much of the variance was lost (genetic bottlenecks). Such variance can be increased again by immigration and gene flow from another population, or in the longer term by mutation and other means.

Immigration can also have deleterious effects if it breaks up locally-adapted gene complexes, and makes the local population less well adapted to local conditions.

Single selection events, such as spring storms, can cause rapid genetic change in the arrival dates of populations, but reversed selection pressures could rapidly reverse the situation, and change arrival dates back to their original state. Selection pressures must act consistently in the same direction over several generations if they are to have any more than temporary effects on the genetic composition of a population. Most selection probably acts to stabilise the gene pools of populations rather than to change them. Moreover, most migratory traits (notably incidence, intensity and timing) are part of a syndrome of co-adapted traits, so selection on one trait is likely to have strong simultaneous effects on the others. If this is disadvantageous in the new conditions, it may take many generations of selection to dissociate the beneficial traits from the detrimental ones before evolutionary change can occur. Evolutionary change may thus be rapid or slow, depending on the circumstances. An important aspect of global warming is that temperatures have increased more in some regions

than others, and more at some times of year than others. The timing of spring migration could be

influenced by weather conditions along the whole migration route, whereas the timing of egg- laying depends of conditions on breeding areas. Any discrepancy between conditions en route and in breeding area can worsen the mismatch between breeding and food supply. Moreover, in the breeding areas themselves, birds may respond more or less rapidly than their food organisms to climatic changes, so that birds cease to arrive and breed at the optimal time. An apparent example is provided by Pied Flycatchers (Ficedula hypoleuca) breeding in the Nether lands, where climate change has advanced the food supply on which breeding depends, but spring migration has not advanced sufficiently to allow the birds to make best use of this food supply, as they did in the past . The birds thereby suffered reduced breeding success, and in areas with the biggest “ecological mismatch,” population levels declined by about 90% over a 20-year period. Such mismatches can only be rectified in the long term by changes in the genetic controlling mechanism, so that migration is triggered at an earlier date with respect to prevailing conditions. The longer the migratory journey, the less likely is weather in the breeding and wintering areas to be correlated. Long-distance migrants would have little if any indication on their wintering areas regarding how spring is developing on the breeding ground. Their departure dates from wintering areas are triggered by a photo-periodically timed endogenous rhythm, evolved through natural selection, which ensures that they arrive on breeding areas at an appropriate date (with minor variation according to prevailing conditions). Only by evolution acting on this endogenous control mechanism is the trigger date for departure likely to be changed. In this situation, the selection pressure to migrate earlier is applied in the breeding area, but the action to accomplish an earlier arrival occurs weeks before in the wintering area, hundreds or thousands of kilometres away. Changing this control mechanism may be a relatively slow process, perhaps explaining why the arrival dates of long-distance migrants are less well correlated with temperatures on breeding areas than are the arrival dates of short-distance migrants, wintering nearer to breeding areas. Another mismatch was found in  the American Robins (Turdus migratorius) that breed at high elevations in the Rocky Mountains of Colorado and whose spring arrival dates advanced by two weeks over a 20-year period. At the same time, winter snow-fall increased and took longer to melt, producing a mismatch between arrival dates and the exposure of bare ground feeding areas.

These examples raise the general point that the photoperiodic responses of many birds, through

which their annual cycles are often timed, may become less reliable predictors of seasonal change in food supplies, as climate change alters the phenology of their food supplies. This is not a new problem, as it is faced by all birds as they expand their breeding ranges into different regions, but it will take time for them to adjust genetically to new situations, during which time they could perform less well than usual (though not necessarily with effects on population levels).

CONCLUSION

Having served as reliable indicators of environmental change for centuries, birds now indicate that global warming has set in motion a powerful chain of effects in ecosystems worldwide. As this report shows, robust evidence demonstrates that climate change is affecting birds’ behaviour -- with some migratory birds even failing to migrate at all. Furthermore, new research reveals a trend of escalating impacts that already impairs some birds’ ability to reproduce or even survive, findings which indicate that a march toward a major bird extinction may be underway. Looking to the future, the report includes projections of major population declines for many bird species and high rates of extinction in some zones.

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