Why do birds flock in winter?

If you live in a temperate climate and pay attention to your local birds, you have probably noticed that their preferences for companionship change with the seasons. In spring, pairs stick close by each other and three’s a crowd—any third-wheel interloper is likely to be chased off in a flurry of angry wingbeats. But in fall and winter, the birds suddenly become community minded, travelling around in flocks of dozens of their fellows. In Chicago in the winter you can find trees liberally decorated with the round orange forms of fluffed-up American Robins, bearing more than a passing resemblance to Christmas tree ornaments. Even in the Bay Area, not generally known for its seasonal variation, huge flocks of quietly chirruping Dark-eyed Juncos make it clear that (mild, occasionally rainy) winter has arrived.

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Not pictured: about fifteen of this robin’s winter friends.

Why flock in winter? Or, why flock only in winter?

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“Rare” birds at the banding station

“Rare” is context-dependent. My Collins Bird Guide lists the Dark-eyed Junco as a “rare vagrant,” but that is, of course, because Collins is a bird guide to Europe. Common birds where you don’t expect to find them are exciting. We have our own rarities at the banding station, birds that may be common in the general region but rarely grace our nets; and although no one else would consider them remotely remarkable, we still get excited.

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Dark-eyed Junco

Juncos flock in huge numbers all around the area, but for whatever reason, they do not like the specific patch of riparian land that the banding station monitors. The banding station catches only around five juncos every year, making them rare by our very particular standards.

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Birds and mirrors, revisited

I wrote about birds and mirrors a while ago, and not much has changed scientifically since then. Most bird species tested have interpreted their own reflections as other individuals, responding either with aggression or courtship. Female pigeons who view their own reflections ovulate, apparently interpreting their reflections as suitable mates. Among birds, only magpies, so far, have been demonstrated to understand that the mirror reflects their own image, although pigeons can be trained to use spatial information from mirrors correctly in the real world.

So why bring this up again? Recently I saw a Yellow-rumped Warbler interacting with its reflection in a car side mirror, and took a video with my phone. Here it is (apologies for the lack of zoom):

At the time I took the video, I didn’t think much of it beyond general amusement. But rewatching it, I began to have some questions.

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Animal accomplishments that have caused my students to break into applause

I’m teaching an Animal Behavior course this semester. The lectures are 80 minutes long and exactly during the sleepiest time of the afternoon; I enjoy the challenge of getting a reaction from the students under these circumstances. Videos of baby animals in peril always get attention (some good ones: marine iguana, barnacle goose, water buffalo), but they’re so reliable it almost feels like cheating.

My students have actually broken into applause during lecture three times so far. One of these will not be discussed in detail (it involved the recitation of poetry), but the other two were in response to two quite different animal accomplishments, which I thought I would share.

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This might be why full moons are spooky

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Photo by Patrick Emerson*

The full moon is pregnant with foreboding interpretations, from the legendary werewolves who are supposed to transform under its malevolently shining face to a recent article about November’s upcoming full “supermoon” that faux-reassures, “despite all the rumors… there is no evidence linking supermoons to natural disasters.”

If you look at a full moon and shiver, you aren’t alone—but you are a bit of a mystery. In humanity’s past, the full moon should have been the safest time of the month, since our nocturnal predators tend to attack most on dark nights. The new moon should be spooky, as your hindbrain—unaware that you no longer live on the African savannah (unless you do!)—looks out for predators slinking in the shadows. This is presumably why the fear of darkness is such a common and instinctual one. But the full moon is bright: it should be comforting.

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The very confused wren and the birdwatcher who noticed

In the spring of 2015, a male House Wren and his mate built their nest inside a nestbox near a honeysuckle. His mate laid her eggs and dutifully incubated them. Then, one morning— cheep! cheep! High-pitched calls and gaping red mouths cried hungry, daddy! and the male wren was off in a paternal tizzy, collecting bugs and delivering them to his new offspring.

It was, maybe, odd that his new offspring weren’t in the nest that he had built. It was, maybe, odd that other, larger birds were also feeding his babies. It might even have been called odd that his mate was still sitting in their nest, atop whole and silent eggs. But— cheep! No time for that! The chicks were hungry!

What this male House Wren was doing, no doubt to the profound irritation of his mate, was feeding the offspring of a pair of Northern Cardinals who had nested in the honeysuckle near his nestbox.

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