鸟类体型变大 是气候变迁还是全球变暖所致

　　Birds are getting bigger to survive harsh storms related to climate change, scientists have suggested.

　　A study spanning 40 years found that birds in central California are significantly larger than they were 25 to 40 years ago.

　　Robins, for example, have increased by an eighth of an inch in wing length and about 0.2 ounces in weight since the 1980s.

　　On average, birds in central California have bulked up between two and five per cent in body weight and wingspan.

　　Researchers suggested this increase in body weight is to protect the birds during increasingly erratic weather as global temperatures rise.

　　The findings, published in Global Change Biology, are at odds with previous research.

　　One ecological benchmark, Bergmann’s rule, states that birds and mammals tend to be larger at higher latitudes, perhaps to conserve body heat.

　　In keeping with this reasoning, birds and mammals would get smaller to adapt to rising global temperatures.

　　The researchers, led by San Francisco State University graduate student Rae Goodman, wrote: “Previous studies from other regions of the world have documented decreases in avian body size and have used Bergmann's rule and increases in mean temperature to explain these shifts."

　　“Because our results do not support this pattern, we propose that rather than responding to increasing mean temperatures, avian body size in central California may be influenced by changing climatic variability.”