From: Andy Soos, ENN
Published November 1, 2011 03:27 PM

Bigger Birds

Birds tend to be small creatures on average. SF State scientists, working with researchers from PRBO Conservation Science and the San Francisco Bay Bird Observatory who collected the data, found that birds' wings have grown longer and birds are increasing in mass over the last 27 to 40 years. What's making the birds bigger? The researchers think that the trend is due to climate change, but their findings put a twist in the usual thinking about climate change and body size. A well-known ecological rule, called Bergmann's Rule, states that animals tend to be larger at higher latitudes. One reason for this rule might be that larger animals conserve body heat better, allowing them to thrive in the generally colder climate of higher latitudes. The logic would then be that as the climate warms, birds should get smaller. Apparently this is not so.

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The study, published in the journal Global Change Biology, suggests that the connection may not be so simple.

Bergmann's rule is an ecogeographic principle that states that within a broadly distributed genus, species of larger size are found in colder environments, and species of smaller size are found in warmer regions. The rule is named after a nineteenth-century German biologist, Christian Bergmann, who was among the first to describe the pattern in 1847. Bergmann's rule is most often applied to mammals and birds which are endotherms, but some researchers have also found evidence for the rule in studies of ectothermic species. While Bergmann's rule appears to hold true for most mammals and birds, there are exceptions.

There seems to be a tendency for larger-bodied animals to conform more closely than smaller-bodied animals, at least up to certain latitudes, perhaps reflecting a reduced ability to avoid stressful environments by burrowing or other means.

The SF State scientists found the bird wings have grown longer and birds are increasing in mass over the last 27 to 40 years.

Climate change may affect body size in a variety of ways, they note in their paper. For instance, birds might get bigger as they store more fat to ride out severe weather events, which are expected to be more common under global climate change. Climate change could also alter a region's plant growth, which may eventually lead to changes in a bird's diet that affect its size.

LeBuhn said she was completely surprised to find that the central California birds were growing larger over time.

The bird data come from two long-term banding stations in central California, where a wide variety of birds are captured, banded about the leg with an identification tag, and weighed and measured before being released. Many of the same birds were captured each year, allowing the researchers at the sites to build up a unique database that could be used to track changes among the birds over several decades.

After testing and discarding a number of other explanations, Goodman and her colleagues were confident that climate change was behind the longer wings and bigger bodies in most of the birds. The birds may be responding to climate-related changes in plant growth or increased climate variability in central California, the researchers suggest in the paper.

The findings offer a glimpse at the potent effects of climate change across a wide range of species, LeBuhn said. "Even over a pretty short period of time, we've documented changes in important traits like body size, where we don't expect to see much flexibility.

Climate change may affect body size in a variety of ways, they note in their paper. For instance, birds might get bigger as they store more fat to ride out severe weather events, which are expected to be more common under global climate change. Climate change could also alter a region's plant growth, which may eventually lead to changes in a bird's diet that affect its size.

For further information: http://www.physorg.com/news/2011-10-bigger-birds-central-california-courtesy.html

Photo: http://nehemiahproject.org/blog/wp-content/uploads/2010/08/the-birds.jpg

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