CLIMATE CHANGE AND BREEDING BIRDS

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CLIMATE CHANGE AND BREEDING BIRDS HUMPHREY Q. P. CRICK Our climate is changing and there is compelling evidence that animals and plants have already been affected. Future scenarios of climate change suggest that these changes are likely to continue and will move us into uncharted waters. Here I would like to describe what we know about how climate change is affecting breeding birds.

We are particularly lucky in the UK in that there has always been a great passion for birds. Ever since Gilbert White published his Natural History of Selborne, there has been a keen amateur interest in these obvious, colourful and interesting parts of our natural world. The natural historians had a great penchant for making written observations of what they saw and in the 19th Century natural history was a common pastime for the relatively well-off. In the 20th and 21st Centuries, this became less exclusive, leading to the development of the serious birdwatcher, who often takes part in systematic surveys and monitoring. These surveys provide an unparalleled record of how the fortunes of our native birds are faring and are quite simply the envy of the World. They are now providing very valuable information for the tracking of the impacts of climate change. Without such historical information we would not be able to tell whether the developing trends are real or not and whether they are related to the climate or not. They provide some of the best evidence to help convince decision-makers of the reality of climate change and that it is already having impacts that we can measure today.

So, in this article, I will draw on such information to describe how climate change is affecting breeding birds in a variety of ways.

Timing of breeding The British Trust for Ornithology (BTO) has, since 1933, been organising standardised surveys for birdwatchers to gather systematic data on birds in the UK. One of its long-running schemes is the Nest Record Scheme, started in 1939. Volunteers simply have to find nests of birds and make repeat visits to ascertain clutch and brood sizes and whether or not the nest succeeded or failed. This has been very popular over the years and we are lucky enough now to have a dataset that covers 1Â3 million individual nest histories for more than 220 species. Some thousand nest recorders each year send in about 30,000 Nest Record Cards and these are the gold-dust that has shown important changes to our birds.

In 1996, as part of the annual monitoring that the BTO undertakes as part of its partnership with the UK Government’s wildlife advisor, the Joint Nature Conservation Committee, I noticed that a large proportion of species were showing a strange trend towards earlier nesting. When I looked closer at this I found that 20 of 65 species (i.e. about a third) were showing statistically significant trends towards earlier laying. The average advancement for these 20 species, over the 25 years from 1971–1995, was 9 days, but ranging up to 17 days. Furthermore, there seemed to be no rhyme or reason behind the species affected: there were early- (e.g. Magpie), mid- (Chiffchaff) and lateseason (Corn Bunting) nesters; there were waterbirds (Oystercatcher), resident insect-eaters (Dipper), migrants (Tree Pipit), Crows (Carrion Crow) and seedeaters (Greenfinch).

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I then went and looked in more detail at species for which we had sufficient sample sizes going back over 57 years. Working with Tim Sparks at the Centre for Ecology and Hydrology, we were able to show that many species actually showed a trend towards later laying between 1940 and 1970 that reversed between 1970 and 2000. This was matched by a trend in spring temperatures that had become cooler and then warmer over the same time period. What we were able to show was that for some species the trends in laying dates could be explained entirely by the trends in temperatures and that if such warming continued then further advances in laying dates could be expected.

Breeding Performance Having established that birds are migrating and breeding earlier, we now want to know what impacts climate change might have on their breeding performance.

There are straightforward examples of how weather affects birds, for example in warmer springs, birds such as Pied Flycatchers lay larger eggs that promote better hatching success because larger eggs cool less easily when the parents are away. Studies in Europe have shown that warmer temperatures leading to earlier nesting seasons have allowed some species such as Blue Tits to become more often double brooded – they can actually fit two broods into the season instead of just one.

A remarkable study by Nicholas Aebischer and colleagues looked at the entire food-chain of Kittiwakes in the North Sea. He found parallel trends in the abundance of phytoplankton, zooplankton and herring and in Kittiwake breeding performance that were all related to the major weather patterns.

These effects have been particularly evident in recent years and especially in 2005 when there have been widespread and near-complete breeding failures of Kittiwakes, Guillemots and Arctic Terns linked to warmer sea temperatures, reductions in plankton and reductions in sandeels. Although seabirds are relatively long-lived, compete breeding failures, if repeated are likely to affect their population sizes eventually.

Population Size and Distribution Populations can be strongly affected by the weather, for example when very prolonged cold weather reduces populations of resident birds such as Herons and Wrens. However, populations usually recover over subsequent years to regain their former size. Populations are likely to then remain at a relatively constant level unless food supplies (or some other important resource) increase.

Climate warming is likely to affect most birds in one way or another.

There will be new colonists from southern Europe, birds such as Black Kite, Cattle Egret, Hoopoe and Bee-eater may become established. Among birds already in Britain, there is a range of likely winners. Generalists, such as crows, gulls and pigeons are likely to do well, as are less-welcome exotics such as Rose-ringed Parakeets. Birds currently restricted to the south, such as Cirl Bunting, Dartford Warbler, Cetti’s Warbler and Golden Oriole might be able to expand northwards. In addition some of our more common species that are vulnerable to cold winter weather may prosper, such as Wren and Kingfisher.

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Among the potential losers, we should be most worried about the high latitude and high altitude species. These literally have nowhere else to go, so Dotterel and Snow Bunting on the high tops and Red-necked Phalarope and Greenshank in the far north may lose all suitable habitat. Species that are specialists of particular habitats, particularly those likely to be affected by sealevel rise, are vulnerable, for example Bitterns. Trans-Saharan migrants may suffer if their crossings become too arduous due to drought and increased desertification. Seabirds may suffer badly if the fish stocks they rely on to breed move elsewhere.

Hard evidence of distributional change due to climate change is currently lacking. There is suggestive information, such as the spread of Little Egrets around our coasts and the spread of Dartford Warblers, but the scientific linkage to climate warming has yet to be undertaken. Chris Thomas and Jack Lennon from Leeds University looked at the BTO breeding bird atlases undertaken in 1968–72 and 1988–91 and showed that the northern margins of southern species had shifted northwards, on average, by 19 km between the two atlases, despite only a small amount of climate change between those two periods.

There has been a substantial effort put into the statistical modelling of climate change impacts on the distributions of birds. This involves trying to establish how the distributions of birds are related to current climate and then seeing how things would change with different scenarios of climate change.

Birds such as Willow Tit, for example would lose a substantial area of currently suitable land in the south of England and would have to redistribute itself much more up into Scotland over the next 50 years. Whether it would be able to do so in our currently fragmented landscape, dissected by large areas of farmland, is another question.

Long-term consequences This is the area in which we know least. There are likely to be a whole range of impacts that we can’t even guess at, given that we will be moving into unknown territory, climate-wise. An example that has already been demonstrated is that birds may become out of synch with their food supplies.

In an oak wood, bud burst is already advancing, as is the hatching of caterpillars that feed on the leaves and the laying of the Great Tits that will feed on the caterpillars. However, under very warm spring conditions, the caterpillars can halve their developmental time (from 56 down to 23 days).

Whereas the Great Tit, sitting on her eggs, has to incubate them for a fixed period – she can’t reduce it. As a result, the later-nesting Great Tits have chicks in the nest when the availability of caterpillars is declining. Scientists have made observations to show that the later-breeding Great Tits are getting fewer and fewer young into the next generation than the earlier birds. So here we have an example of natural selection in action – caused by climate change.

It is the unexpected that is the really worrying aspect of climate change.

We don’t know when natural systems will suddenly flip into a different mode.

There is a whole range of potential complexities that may be affected in the future in ways that we can’t imagine at the moment – each species interacts

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with prey, predators, parasites, competitors and resources. This is where constant vigilance is necessary and where naturalists are at a premium. It is only through the coordinated observations of skilful naturalists, working with professional scientists, that we can hope to achieve the essential monitoring of our countryside.

Humphrey Q. P. Crick British Trust for Ornithology Thetford Norfolk IP24 2PU

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