Effets des changements climatiques sur les oiseaux

À travers l'évolution de leur longue histoire, les oiseaux ont su s'adapter aux changements climatiques et nul doute qu'ils sauront le faire encore. Toutefois, les changements climatiques actuels et prévus sont nettement différents de ceux qu'ils ont réussi à vaincre jusqu'ici.

En effet, ces nouvelles perturbations sont beaucoup plus rapides et elles surviennent au même moment où une autre série d'agressions environnementales, pertes ou fragmentations d'habitats, contaminations chimiques, s'abat sur les oiseaux et le reste du monde de la biodiversité.

Les effets combinés de ces attaques humaines pourraient se révéler insoutenables pour les occupants de ces habitats naturels.




























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Impacts of Climate Change
on Birds

Tony Diamond
University of New Brunswick, Fredericton

April 2003

ost of the birds that breed in Atlantic Canada are migratory, spending brief summers here and migrating through the rapidly-changing weather of spring and autumn. They winter far away from the harsh cold endured by more hardy resident species such as chickadees and jays.

(Photo: Dorothy McFarlane)

Black-capped chickadee, adapted to harsh Canadian winters

Climate affects where birds live (their geographical distribution); most species occupy a limited range of the earth's surface. Migratory species occupy several different areas of the planet in the course of their lives, while residents are confined to a much narrower range of climatic conditions. The climates experienced by birds following these alternative strategies are of course quite different, and so are the adaptations needed to survive and reproduce.

Birds are affected directly by climate, for example through their tolerance for a range of temperature or rainfall. They are also affected indirectly, through the effects of climate on their habitat and food supply. It is often difficult to separate direct and indirect effects, but in the case of North American songbirds, we know that about half the winter distribution is determined by the availability of suitable habitat and half by January temperatures. Thus as winters become warmer, bird distributions are likely to change, but only as fast as their habitats can respond to the changing climate. So to fully understand or predict how birds will respond to climate change, we need to study not only how they themselves respond to climate, but how the plants and other organisms that make up their habitats will respond. For example, if our climate becomes unsuitable for Trembling Aspens (as some have suggested) the woodpeckers, chickadees, nuthatches and other cavity-nesting birds for which this species provides most nest sites will either have to find an equally suitable species to nest in, or will decline in numbers.

Bicknell's Thrush, a threatened species that nests in high-altitude forest in north-eastern U.S., southern Quebec, northern New Brunswick and Cape Breton, may lose much of its habitat if warming reduces available high altitude coniferous forest and birds can't successfully move further north to similar habitats. This species winters in a very few locations on Caribbean islands where its habitat is at risk from deforestation and the spread of agriculture into mountain forests.

(photo: Dan Busby)

Bicknell's thrush, a high-altitude species at risk

Many of the changes we expect to see in birds as a result of changing climate are quite subtle. One of the commonest seen in other parts of the world is for birds to start nesting earlier; in many species it seems that the signal to lay the first egg is one of temperature, so if springs become warmer, birds breed earlier. In North America, Tree Swallows show signs of this effect, breeding on average about 9 days earlier now that they did 30 years ago (though this effect is not seen in Atlantic Canada). In Britain, many species show this effect, in some cases nesting as much as 18 days earlier than 30 or 40 years ago. Birds use temperature as a cue to begin nesting partly because it is often a good predictor of the timing of food supplies for the young. Most small birds feed their young on insects (even if the adults feed on seeds or berries), and need to time their egg laying so that the chicks hatch out when insects are abundant. If the emergence of insects does not respond to the same temperature cues as the birds do, then their nesting may become mis-timed in relation to their food supply, and they will breed less successfully until they can make the appropriate evolutionary adjustment to their timing of breeding.

Migration is a critical phase of the life-cycle in many species. One of the most important local examples is the Semi-palmated Sandpiper, which breeds in the Arctic and winters in northern South America; en route, most of the world's population of this species migrates through the upper Bay of Fundy in late July and early August. There they fatten up for the long flight non-stop across the western Atlantic Ocean. To make this flight successfully, they need to take off with a tail wind, and the weather systems that produce such winds must occur at the right time to coincide with the availability of their food (mud-shrimps) and the stage of the sandpipers' annual cycle. Their nesting grounds (arctic tundra), wintering grounds (tropical mudflats) and migration habitat (temperate mudflats) are all susceptible to the rise in sea level that is already occurring as a result of climate change. This is a species about which we know quite a lot; there are many others about which we know much less but which are presumably equally vulnerable to a variety of aspects of climate change.

Some birds will undoubtedly benefit from changing climates. Recent range expansions into our region of Mourning Doves and House Finches, for example, may be attributable at least in part to the warmer winters of the 1990s. Birds have been adapting to changing climates throughout their long evolutionary history, and will no doubt continue to do so. Yet current and predicted climate changes differ in two important ways from earlier, natural ones; they are much faster, and they are taking place in addition to a battery of other environmental changes we have inflicted on birds and the rest of the world's biodiversity, such as habitat loss and fragmentation, and chemical contamination (including pesticides and pollution). The combined effects of these manmade onslaughts on natural systems may prove too much for some of their habitats.