Bird Ranges? Shifting North: A Possible Signature of Global Climate Change?
1 Aug, 2007 10:18 am
A recent study published in the journal Conservation Biology, by A. Hitch and P. Leberg, found evidence supporting previous findings that birds? ranges have extended northward into cooler climates following a period of global warming.
The study utilized data from the North American Breeding Bird Survey (BBS). The BBS is a large scale survey of birds in North America during the breeding season, conducted annually over the past forty years by skilled observers on designated roadside routes. The survey is highly standardized and now includes over 3,500 routes in the United States, Northern Mexico and Southern Canada. The information gathered every year makes up one of the largest and longest records of bird activity in North America, and is useful in estimating speciesí breeding distributions. Due to the deep history of the survey, shifts in distributions can be analyzed. Despite this wealth of data, little research has made use of the survey in quantifying effects of global warming.
This analysis, carried out at the University of Louisiana at Lafayette, selected fifty-five bird species from the central and eastern United States on criteria that would allow for variation in natural range to be detected through BBS data. This excludes game species, due to human manipulation of range through release, and species dependant on aquatic habitats, because their populations may not be well represented in a roadside survey. The resulting set of species contains mostly forest birds, which were then separated into twenty-six having southern distributionsand twenty-nine having northern distributions. A comparison of data from the periods of 1967 to 1971 and 1998 to 2002 was performed to identify if there was a significant difference in breeding ranges.
The analysis looked at distributional shifts across all species involved. For the species with southern distributions, an average of the ten northernmost routes for each species was calculated from both periods. The averages from each period were then compared collectively, to determine if there was a directional movement in the boundaries of bird ranges on the whole. The analysis used the southernmost routes for the northern bird species as a control to avoid misinterpreting other influences, which would result in the moving of range boundaries, as the effect of global warming.
The first (cross-species) comparison revealed a significant northward expansion by birds with southern distributions, at an average rate of 2.35 kilometers per year. Comparatively, the northern birds showed no southward range expansion, supporting the impact of temperature increase on this northward trend.
There is a clear trend, seen independently in this study and the study conducted in Great Britain, of bird species distributions expanding northward as global surface temperature increases. Both studies display remarkably similar findings, and when taken together the possible influence of climate change is hard to ignore. But the change in temperature does not necessarily represent an immediate threat to these species; the extent of temperature increase thus far has not forced these species to abandon the southern portions of their ranges. Birdsí ability to move and redistribute allow them flexibility in the presence of climatic shifts. The possible problems facing bird populations in this situation lie in dependence on less mobile habitats, and the habitatís response to a changing climate, but this study does not allow for predictions regarding extinction of any species involved.
Alan T. Hitch and Paul L. Leberg, Breeding Distributions of North American Bird Species Moving North as a Result of Climate Change, Conservation Biology, Volume 21 Issue 2 Page 534-539, April 2007