2012: Advent Biology: Feathered visitors

by on December 5, 2012

Every species has its own particular place-its niche- within the complex web of interactions that make up an ecosystem. For some, life is even more complicated: to travel between two places, fit in and flourish in both is quite an achievement, and this is certainly shown by migratory birds.

The Pied Flycatcher is one such long distance traveller. It and other long distance species spend the winter in tropical regions and then return for the summer and Europe or north-west Asia, timing their journey to get the best of the weather and food for the breeding season. For the flycatcher, their main food supply and the bounty they return north for are caterpillars, and they time their return to their breeding grounds to coincide with the peak season for them.

This in itself is quite a feat. The emergence of caterpillars depends on when the eggs are laid, and how long they take to pupate. This timing and their abundance is affected by when their food source plant buds and when its leaves are growing. The birds are cued by the length of daylight to depart from wintering grounds in the warm south, and their migration brings them to us in the north just in time to take advantage of copious numbers of caterpillars to raise chicks on. They’ve evolved to make the most of two worlds, and it’s all about timing.

Unfortunately, this makes it very difficult for them when anything throws the timing off-a mismatch as might occur when the trees bud too early, and then the caterpillars are emerging too early, and by the time the birds get there and have chicks to feed there’s just not enough food around, because the north was ahead of them this year. So some years they rear fewer chicks, but the population has a chance to recover next year, when the timing is right again.

This is where climate change predictions are proving very worrisome for migrants. An overall increase in temperature would mean the mismatch between the birds and their northern food source isn’t just an occasional thing, but happens every year. The Pied Flycatcher has already started to show a response to this: their arrival date in the north is creeping earlier, and for some populations, it’s keeping up with the peak times for caterpillars. For others, it isn’t, and they are already declining. The question is whether the adaptations this and other migratory species make will be fast enough to cope with whatever changes the climate throws at them.

[image from RSPB]

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