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Sparrows

    MORNING FLIGHT

    Not Just Birds of a Feather

    In the off-season, mixed flocks of disparate birds work together instead of apart

    By

    Winter is not easy if you’re a bird. The abundant food supply you relied on all summer is gone, you use a lot of energy keeping warm, and without leaves on the trees, you have fewer places to hide when predators appear.

    Birds in the northeastern U.S. tend to flock together over the lean months — not just with members of their own species, but with a diverse array of other birds. It’s not uncommon to see chickadees, woodpeckers, kinglets, nuthatches, and warblers feeding in one flock. It may sound surprising that these birds would choose to forage with their would-be competitors. But research has revealed that these mixed flocks help keep their members alive.

    Mixed flocks are made up of two kinds of birds. There are “nuclear” species, which form the core of the flock, and “satellite” or “attendant” species, which gather around the nuclear species and follow them in their foraging. In the forests of Cape Cod, our nuclear species are typically chickadees and titmice, while common satellite species include nuthatches, woodpeckers, warblers, sparrows, and thrushes.

    Scientists describe mixed feeding flocks as being composed of “nuclear” species like black-capped chickadees (top left) and tufted titmice (top right) and “satellite” species like white-breasted nuthatches (bottom left) and golden-crowned kinglets (bottom right) (Photos courtesy Simon Pierre Barrette, Rhododentrites, and Barbara Schelkle, all Wikimedia Commons)

    The primary theory to explain why birds would flock up is to avoid predators. The idea is that a bird on its own must spend a lot of time and energy looking out for predators, while birds in a flock can have certain birds serve as lookouts while others forage. Furthermore, nuclear species like chickadees tend to be more vigilant and quicker to sound the alarm if a predator appears. Perhaps satellite species join these flocks to benefit from the watchful eyes of the nuclear species.

    One of the best experiments to test this theory took place in Ohio, where the winter-feeding flocks closely resemble those on Cape Cod. In this experiment, two scientists from Ohio State University studied the mixed feeding flocks in fragmented patches of woodland surrounded by farm fields — essentially, islands of forest. These flocks consisted of chickadees and titmice as nuclear species and white-breasted nuthatches, downy woodpeckers, and red-bellied woodpeckers as satellite species.

    In half of these patches, the scientists caught and removed all the chickadees and titmice, leaving half the flocks with no nuclear species. They then watched the remaining satellite birds over the course of the winter to see how they behaved.

    They found that, in the patches without nuclear species, the nuthatches and woodpeckers spent much more time and energy watching for predators. This, in turn, led to them eating less and choosing food sources only in parts of the woodlots that were sheltered from the wind in order to save energy.

    For the nuthatches, the loss of nuclear species was exceptionally dangerous — nuthatches in woodlots without chickadees and titmice tended to be in poorer health and had a lower chance of survival than those in plots with chickadees and nuthatches in them. In essence, flocking with watchful chickadees gave these birds safety from predators, which gave them more time to look for food and therefore helped them survive the winter.

    But this exchange doesn’t seem to go both ways. A meta-analysis of studies on mixed species flocks led by researchers from the Indian Institute of Science found that, according to the available research, only satellite species get the benefits of extra foraging time. The nuclear species are just as vigilant no matter who they’re flocking with. So why do nuclear species like chickadees tolerate hangers-on?

    A study in the woods of Tennessee, where chickadees are the nuclear species, helped answer this question. A group of researchers attracted birds to an elevated platform bird feeder, waited for a flock to form around the feeder, removed all the seed, and hung a new feeder beneath it. They then watched to see how long it took the birds to find the new feeder.

    The researchers found that flocks with more species in them tended to find the food faster, allowing all members of the flock to access the new food more efficiently. This finding led the researchers to suggest that nuclear species benefit from being in more diverse flocks because the satellite species find food sources that they might overlook, which allows nuclear species to access a wider variety of food thanks to the different feeding strategies of the satellite species. This dynamic is particularly useful in winter months when food is less abundant.

    To see a mixed species flock, then, is to peer into one of the most complex mutually beneficial systems in the animal kingdom. Satellite species point nuclear species toward new food sources, and nuclear species keep an eye out for predators so satellite species can relax and feed more. Satellite species might get the better end of this deal, but the nuclear species are clearly benefiting enough to maintain the system.

    Most fascinating of all is that we’ve barely scratched the surface of how these flocks operate, especially when it comes to their communication. Woodpeckers and chickadees are as distantly related as we are from mice, after all. But somehow, they communicate well enough to maintain their flock, twittering to each other as they search for whatever meager food sources they can find in order to survive an inhospitable winter.

    MORNING FLIGHT

    The Unusual Sex Life of the White-Throated Sparrow

    In October, a bird with four sexes comes to winter with us

    By

    For birders, fall is not just one season. The whole period from July to December is characterized by southbound migration, sure. But really, a birder’s fall is a string of many smaller overlapping seasons — periods characterized by the birds that migrate during them: shorebirds, warblers, ducks.

    Around the beginning of October is sparrow season, when shrubby edges and weedy fields fill with these brown, streaky birds through the back half of the fall.

    Compared to warblers, the main migrant songbird for the early part of the fall, sparrows are pretty unobtrusive. But the season still offers some surprises. Within the flocks of song, swamp, and chipping sparrows, you might be lucky enough to find some rarer birds, like vesper and clay-colored, both visitors from points west.

    A white-striped morph of the white-throated sparrow. (Photo by Simon Pierre Barrette/CC-SA 3.0)

    Among the most numerous arrivals are the white-throated sparrows. These chunky brownish birds have a distinctive facial pattern: white throat, gray cheek, black eyeline, a yellow spot near the bill, and an array of five black-and-white stripes on the head. They appear on the Outer Cape in late September and overwinter here, not leaving until the end of May.

    Sparrows might strike you as ordinary. But you’d be hard pressed to find a more fascinating species on Cape Cod — at least when it comes to this bird’s sex life.

    I referred to the stripes on top of the white-throated sparrows’ heads as black and white, but this isn’t really true: about half of these sparrows have tan stripes instead of white ones, giving them a more muted, messy appearance overall. These were once thought to be young birds. We now know, however, that these birds are no older or younger than their white-striped counterparts: they are simply an alternate color morph.

    Color morphs are found in a variety of the birds of Cape Cod: eastern screech-owls may have gray or red plumage, for instance, or snow geese may have all-white bodies or black bodies with white heads.

    The genetics behind the two-color morphs of the white-throated sparrow lie in a pair of chromosomes labeled chromosome 2. In tan-striped birds, the chromosomes are identical, but in white-striped birds, one of the pair has a section in the middle that is inverted — as if you pulled a section out of the middle of a deck of cards, flipped it upside down, and stuck it right back in.

    These genetics determine more than mere stripe color. On their breeding grounds in the boreal forests, white-striped individuals are much more aggressive than tan-striped birds. White-striped males sing more often, are more likely to encroach on other birds’ territories, and try to mate with more females. White-striped females, similarly, are nearly twice as likely to try to mate with males as their tan-striped counterparts. Perhaps unsurprisingly, this focus on mating makes white-striped birds neglectful parents.

    Compare this to the tan-striped birds, whose males invest a lot of energy in protecting their mates and raising their young.

    Both strategies are successful: the low-investment white-stripes and the high-investment tan-stripes both produce ample young and appear with about equal frequency in the population of these sparrows.

    You might think that these two morphs would somewhat self-segregate: tan-striped males would mate with tan-striped females and vice-versa, preferring mates who share their parenting strategies. But it turns out that, for white-throated sparrows, opposites attract. With remarkable consistency, tan-striped females mate with white-striped males, and white-striped females mate with tan-striped males.

    A tan-striped morph of the white-throated sparrow. (Photo by N. Lewis/NPS)

    Eliana Tuttle discovered this strange mating system. She studied white-throated sparrows from the early 1990s until her death in 2016, including Ph.D. work at SUNY’s Cranberry Lake Biological Station and at Indiana State University, where she was a biology professor. What she found was that 98.5 percent of these sparrows tend to mate with birds that have opposite stripe patterns.

    It seems evolution rewards this system, too. Birds that are born from white-white or tan-tan parents show signs of being underfed and likely suffer from a glut of recessive genes. The offspring of different colored pairs, meanwhile, tend to be much healthier.

    Laboratory experiments have examined how this phenomenon works. When both white- and tan-striped females are presented with a choice between white- and tan-striped males, both varieties will choose the tan-striped males. Similarly, both types of males appear to be more attracted to the white-striped females.

    The scientists behind these experiments suggest that the more aggressive white-striped females push away and outcompete the more timid tan-striped females to win the loyal tan-striped males. This leaves the tan-striped females to mate with the brash, careless white-striped males. In short, the dominant females choose the good fathers, while the meeker females are stuck with the deadbeat dads.

    When explained in these terms, the whole arrangement seems simple. These aren’t just different behavioral types, though. There’s more to it than that. It turns out that white- and tan-striped birds are genetically very different, and their offspring will suffer if they don’t mate a certain way. The inversion essentially turns chromosome pair 2 into a second set of sex chromosomes, and white- and tan-striping into a second sexual binary.

    In other words: these birds are the only animal on the planet known to have four sexes.

    We are spoiled on Cape Cod to have a bird like in such abundance. By the end of October, seemingly every patch of brush and overgrown field will be loaded with white-throated sparrows, white- and tan-striped alike. The next time you come across a flock of these birds, take a moment to stop and appreciate the random chance and evolutionary power that created such a unique work of nature.

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