Why are California birds getting bigger?

Nov 11, 2011 By Joel N. Shurkin
Credit: peterp via flickr

Alfred Hitchcock would have appreciated this twist: The birds in central California are getting bigger.

Researchers at San Francisco State University have found that the on both ends of San Francisco Bay have greater and longer wingspans since Hitchcock made the 1963 horror film, "The Birds," in which birds went mad and attacked a town on the coast.

Researchers said that the reason for the growth is believed to be , but therein lies a mystery: The birds should be getting smaller as the climate warms, not bigger.

San Francisco State Rae Goodman and assistant professor Gretchen LeBuhn used data from 14,735 birds collected since 1971 near the northern tip of the bay at the Palomarin Field Station at Point Reyes National Seashore, and 18,052 birds collected between 1983-2009 at Coyote Creek Field Station, on the southern end of the bay. Each bird was caught, banded with an identification tag, and released after being weighed and measured either by the locally based PRBO or the . Many were recaptured in later years, providing a data base that could track changes. Dozens of species were involved.

"The birds in the study included resident birds as well as migrants, so birds from Alaska or Central America were also showing in increase," Goodman said.

The birds grew on average 2 percent in mass, and the wingspans increased about one-eighth of an inch.

The mystery, however, involves a theory known to as Bergmann's Rule, which generally holds that animals get bigger the farther north they are found. Ravens in Alaska are noticeably larger than ravens in the Lower 48. It applies to mammals as well: moose in the Arctic are much bigger than moose in Maine. The reason, the theory holds, is that larger animals can retain heat better and are therefore better able to survive at . Christian Bergmann, the 19th-century German biologist who came up with the rule, thought it applied only to warm-blooded creatures but scientists have found it applies to some cold-blooded animals as well.

So, as the climate warms, the birds should not be getting bigger.

The only other similar study of North American birds was done by University of Zurich's Josh Van Buskirk in western Pennsylvania. The birds he studied shrunk, obeying the rule.

Researchers from Cambridge University and the University of Tel Aviv, working in England, studied passerines (bullfinches and tits), and also found they were getting smaller just as expected.

"We expected them to show a decrease based on the first few studies," Goodman said of the California birds. "That left us with the puzzle of figuring out what was going on."

In the study, published in the journal Global Change Biology, Goodman and LeBuhn suggested two reasons for this exception.

One is that the birds are storing more fat to counter the greater number of extreme weather events that most climate scientists link to climate change. Bigger birds can survive storms better than can smaller ones so natural selection is choosing those with bigger bodies and wingspans.

The other theory is that the birds are eating a different diet. As the climate warms, plants change as does the population of insects birds might eat. That could contribute to larger birds.

Wesley Hochachka, senior research associate at the Cornell Laboratory of Ornithology, who studied the size of birds in Germany (they also shrank), had a slightly different interpretation. He said another explanation is that changes in temperature had nothing to do with it.

"It's not thermal at all," Hochachka said. Factors such as long droughts brought on by El Nino and the extent of the food supply are the most likely reasons. "Basically, Bergmann's rule is irrelevant."

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More information: Correction (11/12/2011): 6th paragraph: The birds grew on average 2 percent in mass

Source: Inside Science News Service

3 /5 (8 votes)

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User comments : 17

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TheGhostofOtto1923
3 / 5 (4) Nov 11, 2011
Evolution? Just kidding.
xznofile
4 / 5 (1) Nov 11, 2011
how does the size of birds in the Bay area compare to birds elsewhere? It's possible that older specimens were suffering from pesticides coming downriver from the Sacto & San Joaquin Valleys, previous to their ban.
ryggesogn2
2.2 / 5 (10) Nov 11, 2011
"The birds should be getting smaller as the climate warms, not bigger."
Any data that challenges the AGWite faith must be rejected.
ubavontuba
3.7 / 5 (6) Nov 11, 2011
The birds grew on average 0.2 percent in mass, and the wingspans increased about one-eighth of an inch.

...One (suggested reason) is that the birds are storing more fat to counter the greater number of extreme weather events that most climate scientists link to climate change.
Like fat contributes to increasing wingspans? How does that work?

More likely; the birds are maximizing their growth potentials with readily obtainable calories provided by humans.

ryggesogn2
2.1 / 5 (8) Nov 11, 2011
Why do birds grow larger with more food? The local geese have so much food they don't fly away. Same for pigeons. They get fat but do they become physically larger?
Where is the data to show cooler climate is NOT the cause? BTW, even mosquitoes are larger in Alaska.
plaasjaapie
1.7 / 5 (6) Nov 11, 2011
Birds on the central coast here are getting bigger over time. Of course, it's all down to climate change. Good old watermelons. They have about 3 neurons firing dependably. :-D
bluehigh
3 / 5 (6) Nov 11, 2011
In the case of over feeding watermelons (the fruit) with growth hormone - they explode. Exploding birds next- damn climate change!

axemaster
3 / 5 (4) Nov 11, 2011
"The birds should be getting smaller as the climate warms, not bigger."
Any data that challenges the AGWite faith must be rejected.

You realize that you're the one operating on faith right? You BELIEVE that global warming isn't happening, while we KNOW that it is. It's something called "quantitative evidence".
M_N
3 / 5 (4) Nov 11, 2011
The birds grew on average 0.2 percent in mass, and the wingspans increased about one-eighth of an inch.


That's a pretty small increase to bother announcing. Is it even outside the margin of error?
axemaster
5 / 5 (1) Nov 12, 2011
The birds grew on average 0.2 percent in mass, and the wingspans increased about one-eighth of an inch.


That's a pretty small increase to bother announcing. Is it even outside the margin of error?

The sample size probably is sufficient to justify the conclusion, if you consider that the data was taken over several years and using tens of thousands of birds. But yeah, it is a very small change.

used data from 14,735 birds collected since 1971 near the northern tip of the bay at the Palomarin Field Station at Point Reyes National Seashore, and 18,052 birds collected between 1983-2009
Urgelt
5 / 5 (1) Nov 12, 2011
The article didn't mention whether the climate in the San Franciso Bay area is changing, or in what direction. Global warming does not mean all areas will warm; birds will respond to local, not global, conditions.

But of course there are other environmental variables which can come into play. Changing food sources and extreme weather events are just such variables, but it doesn't seem that anyone has advanced evidence for these or other explanations.

Unless scientists nail down the cause(s), we'll have to settle for suspending judgment.
Isaacsname
not rated yet Nov 12, 2011
Perhaps an examination of bone densities would help to provide more evidence for/against. I doubt they'd get larger without needing more support...
ThanderMAX
not rated yet Nov 12, 2011

More likely; the birds are maximizing their growth potentials with readily obtainable calories provided by humans.


Hell, yah. Mc Donald and KFC to be blamed for those obese bird population. Aren't humans also getting fatter ?

So, in 9745 AD, every living creature will be over-weight.
kaasinees
2.3 / 5 (3) Nov 12, 2011
The growth of the wingspan is probably due to increased weight of the bird. More weight = require more air flow for flying.
Pirouette
1 / 5 (1) Nov 12, 2011
In humans, obesity as well as age can cause fat people to lose height due to gravity, while age can produce brittle bones, which is generally caused by poor diet. Perhaps the larger wingspans and body size of these birds are caused by consuming LESS fat and more calcium and protein in their diets, which would produce more muscle. More muscle can also produce a wider girth without much fat. Does the article say the birds had more fat? or more muscle? Fertilizers containing lots of calcium are taken up by grains produced in northern states. The runoff also would contain calcium. Birds eating such grains and seeds would tend to get bigger, rather than eating grains and seeds that haven't been fertilized with these building blocks.
RazorsEdge
1 / 5 (1) Nov 13, 2011
It's just too simple - use bird size as a proxy for thermometers and the conclusion is California is getting colder.
ryggesogn2
1 / 5 (1) Nov 13, 2011
It's just too simple - use bird size as a proxy for thermometers and the conclusion is California is getting colder.

Then all such proxies must be called into question.