Big Bird

bird big Big BirdIn a bit of a surprise, the average size of some bird species has gotten bigger in central California over the last few decades. The finding runs counter to predictions that a warming climate would tend to cause creatures to downsize – and highlights the sometimes convoluted effects that climate change can have.

Between 1971 and 2010, birds banded at two sites near San Francisco Bay and the Point Reyes National Seashore.have gotten bigger in two ways, a research team reports in Global Change Biology: The birds’ wings have grown longer at a rate of 0.02% to 0.08% per year, and they’ve gotten, on average, heavier too.

The trend – distilled from measurements of nearly 33,000 birds — “completely surprised” San Francisco State University biologist Gretchen LeBuhn, who conducted the study with then-graduate student Rae Goodman. “It’s one of those moments where you ask, ‘what’s happening here?’”

The results were unexpected because a well-known ecological rule, called Bergmann’s Rule, states that animals tend to be larger in colder climates. In part, that’s because larger animals conserve body heat better. Under Bergmann’s Rule, some scientists have predicted that animals would get smaller as the Earth warmed up.

The California findings, however, show that the connection may not be so simple. That’s because climate change can affect body size in a variety of ways, the authors write. 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 and affect its size. Other research has suggested that a more variable climate can also select for birds with bigger body sizes, perhaps because larger animals can ride out fluctuations better. After testing and discarding a number of other explanations, Goodman and her colleagues became confident that climate change was behind the changes they say. The birds may be responding to climate-related changes in plant growth or increased climate variability in central California, they suggest.

The findings offer a glimpse at the potent effects of climate change across a wide range of species, LeBuhn says. “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.” And the results gave LeBuhn “a little more hope that these birds are able to respond — hopefully in time — to changes in climate.” — David Malakoff | November 1, 2011

Source: Rae E. Goodman et al. (2011). “Avian body size changes and climate change: warming or increasing variability?” Global Change Biology DOI: 10.1111/j.1365-2486.2011.02538.x

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