Last year most people, including politicians, finally realised that climate change in the form of global warming is affecting our planet. However, it does not only affect us humans in the form of increased dramatic weather and lower crop yields due to draught or flooding. Climate change is likewise affecting plants and animals. Birds for instance use temperature as a cue for many life cycle decisions. Now an international research group headed by the Swedish ornithologist Anders Hedenstr

Although many previous studies have shown changes in the date of egg-laying or migration in a range of different birds, they have usually focused on one aspect only. However, climate change is likely to affect several aspects of the birds’ behaviour. The research group therefore incorporated several factors into their models. They developed two different models. In the first simple model they assumed that climate change affect the overall flight speed during migration. Flight speed is determined by energy reserves, which again depends on foraging success prior to migration and at stopovers during migration. In the model they assume that global warming results in more food along the way due to earlier spring arrival. In general this results in the birds arriving earlier at the breeding site, which again advances egg-laying. The effect, however, will be small in larger birds that already maximises food intake. 

In the second more advanced model, the length of stopovers depends on the energy consumption at the stopovers and the future expected fuelling opportunities. That is the birds in this model use an optimal strategy. Here the model assumes an earlier rise in food availability in the stopovers, but followed by a later decline. This model also predicts an earlier start of spring migration and start of egg-laying, although moulting and fall migration seems unaffected.

The models show that climate change is likely to disrupt the annual routines in migrating birds and lead to conflict between environmental cues and the birds’ inner clock. For birds that show high behavioural plasticity and a high capacity for changing their routines, climate change should have a relatively small impact on their survival. However, for birds unable to adjust fast, enough the consequences will be dire.

A. Hedenström, Z. Barta, B. Helm, A. I. Houston, J. M. McNamara and N. Jonzén (2007). Migration speed and scheduling of annual events by migrating birds in relation to climate change. Climate Research 35: 79-91.