Tuesday cont’d: Woodpecker brains, cat vs. catbird, and the dark side of brightness

 By Hugh Powell
August 5, 2008
Keep birds safe by keeping cats indoors American Ornithologists Keep birds safe by keeping cats indoors. Photo by L. Sanchez/Wikipedia.
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10:00 a.m. Hans Winkler, of the Austrian Academy of Sciences, asks why do woodpeckers have such large brains? (I hadn’t actually noticed this myself, but Winkler has the measurements to back it up – he poured “steel micro-spheres” into skull cavities of woodpeckers and other birds to make sure.)

The answer might have to do with woodpeckers’ demanding feeding skills – they have to remember the locations of good insect patches and master the art of delving inside a tree trunk using the equivalent of a pair of chopsticks and a retractable tape measure. But my favorite hypothesis is one Winkler grabbed from Alzheimer’s research: big brains may be a safeguard against memory loss. If your life consists of pounding your head against hard objects, a bigger brain might just mean you have some functioning cells left over in your old age.

11:30 a.m. There’s a whole symposium going on today about what cats are doing to bird populations. Everywhere from suburbs to desert islands, when cats get outside, they start killing wild animals. They can’t help it – they’re elite predators (and it doesn’t hurt that their owners give them food and shelter, keeping them in tip-top stalking condition.)

Pete Marra, of the Smithsonian Migratory Bird Center, followed Gray Catbirds around three neighborhoods in Washington, D.C., and Maryland. His team, including a network of nearly 300 citizen-science volunteers, put tiny tracking devices on about 100 young catbirds just as they were leaving the nest. In a cat-free neighborhood in Bethesda, 65 percent of these nestlings survived their first two months in the outdoors. But in two DC suburbs with high cat populations, roughly six out of every 10 fledglings died over the same period. Most were eaten by cats – fieldworkers often caught the cats in the act, Marra said.

Deaths among the just-out-of-the-nest crowd add up. Standard ways of gauging a population’s health (whether it’s increasing or decreasing) base their estimates on how often nests are successful – but they ignore the differences in a young bird’s survival prospects. When Marra factored in the effect of cats, catbird populations that had appeared to be increasing were revealed to be declining.

The take-home message is clear: Keep cats indoors. But it’s remarkably hard to persuade cat lovers to take that step, “Even for our volunteers,” Marra said. “They ask us about feeding birds, or planting shrubs for them. But then we’re out in their yard catching catbirds, and their cats are walking around underfoot.”

12:00 noon. American Goldfinches get their blinding yellow plumage from yellow pigments called “carotenoids” contained in foods they eat during spring. Female goldfinches love it, but new evidence suggests the head-turning looks come at a cost. In experiments conducted at Auburn University, researchers fed goldfinches on a high carotenoid diet (but within natural limits) and measured effects on the birds’ livers and muscles. The birds’ livers – which process the carotenoid compounds before they enter the blood stream – seemed unaffected by the influx of pigments. But the flight muscles told a different story. A marker enzyme indicated the muscle tissues were under stress after a month of the diet, and the effects lasted two months after the diet ended. Actual tests of the birds’ flying ability showed a similar decline. The results of this clever experiment suggest that the brightest males may be essentially poisoning themselves in their race to win mates.

Comments

  • Re woodpecker brain size: Without further details of the measurement procedure, I am inclined to wonder if cranial cavity size is directly proportional to brain size, or if some species may have more “packing material” to cushion the brain within the cranium.

  • With regard to the catbirds, I know 65% is more than 60%, but should that difference really be of much concern? I’m not trying to minimize the situation, just trying to get a more full understanding of it.

  • Wonderful ideas about the big-brains of woodpeckers. Makes sense!

    I think there was some confusion with the stats on the cat predation. If 65% of the catbird fledglings survived, then 35% did not. In the case of the cat-infested area, only 40% survived (since 60% perished due to the cats).

    I think the way the numbers were displayed perhaps led to a bit of misunderstanding.

    Keep cat indoors is an important and difficult message to get people to follow.

    ~Birdfreak

  • Hugh

    thanks to Birdfreak for pointing out the stats. You’re right, that’s 65 percent survival vs 40 percent survival. That’s the difference that changes Marra’s estimate of the population’s overall prospects from increasing to declining. Sorry to Jamal and anyone else who was confused by the way I worded it.

  • Hugh

    @Randolph: I tracked down Dr. Winkler today and asked him your question. He said there is essentially no packing material inside the woodpecker’s cranium. In fact, he said, the force on a woodpecker’s brain when the bird is pecking has been greatly exaggerated. He pointed out that the brain’s overall mass is quite small, so it has much less inertia than ours does. Also, he said the brain sits above the beak rather than in line with it, so the majority of force passes underneath the brain. And finally, woodpecker brains are oriented in the skull slightly differently from ours, so the crucial frontal lobes don’t take the brunt of the force. Hope this helps, and thanks for your comment.

  • NaksLampsypepen

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