What was done
The authors analyzed the distributions of British birds over a 20-year period of global warming, looking for climate-induced changes in their breeding ranges.

What was learned
It was determined that the northern margins of southerly species' breeding ranges shifted northward by an average of 19 km from 1970 to 1990; while the southern margins of northerly species' breeding ranges did not shift at all, in the mean.  This finding was stated to be similar to results obtained for European butterflies, "for which the northern margins have expanded more than the southern margins have retracted."  Both the bird and butterfly results were attributed to the effects of global warming as experienced in the regions studied.

What it means
One of the great horror stories associated with predictions of CO2-induced global warming is the claim that the warming will be so fast and furious that many species of plants and animals, herbivores in particular, will not be able to migrate poleward in latitude or upward in elevation at a rate commensurate with the rate of warming, leading ultimately to their extinction when climatic conditions conducive to their survival are no longer within their reach.  Contradicting this claim, we have repeatedly cited the experimentally established fact that atmospheric CO2 enrichment typically ameliorates the effects of heat stress in plants and actually causes an upward shift in the temperature at which they function optimally (Idso and Idso, 1994; Idso, 1995).  Hence, when temperature and atmospheric CO2 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.  We suggest that in the case of the butterflies it may be a response to a temperature-induced poleward shift in the northern boundaries of the ranges of the plants upon which they depend for food, plus a response to the combined effects of temperature and CO2 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.  Indeed, the birds may even feed on some of the butterflies!  Be that as it may, and whatever the causes, in the cases of both the birds and the butterflies, the behavior is one of total range expansion in the face of global warming and concomitant atmospheric CO2 increase, which seems to us to be a positive, rather than a negative, response to this dual global change.

References

Idso, S.B.  1995.  CO2 and the Biosphere: The Incredible Legacy of the Industrial Revolution.  Special Publication.  Department of Soil, Water & Climate, University of Minnesota, St. Paul, MN.