Global Warming and Shifts in British Bird Ranges Global Warming and Shifts in British Bird Ranges Reference Thomas, C.D. and Lennon, J.J. 1999.
Birds extend their ranges northwards.
Both the bird and butterfly results were attributed to the effects of global warming as experienced in the regions studied. Hence, when temperature and atmospheric CO 2 concentration rise together, as they have been doing over the past century or so, there should be little to no thermal impetus for poleward migration at the warm edge of a plant's range, although there would be an opportunity for significant poleward expansion at the cold edge of its range. It is interesting that this type of behavior is precisely what we see in the cases of birds and butterflies here described. CO 2 increases that act to maintain the stability of the southern boundaries of their host plants' ranges. In the case of birds, the explanation may be more complex, as they reside further up the food chain than do butterflies.