THE OUTSIDE STORY - Phoebes: To thine old nest be true
While winter in New England can be stunningly beautiful, with its magical snowfalls and ethereal silences, I must admit that by mid-February the long absence of so many songbirds has me feeling bereft. I miss the vireos; I miss the thrushes and most especially I miss the pair of phoebes who settle into the well-worn nest on the gable end of my house to raise their young.
It's amazing how two tiny beings who weigh no more than a handful of twigs can evoke such strong emotions in me. I am joyful when the pair resurfaces in early spring; moved by their devotion to their shared progeny, and I take pleasure in the companionship they provide one another.
But on some level I know that these are sentimental notions that I am ascribing to behavior that is biologically, not emotionally, driven. The phoebes I see one year aren't the same phoebes that I observed the previous year. Or are they?
It turns out the mating habits of songbirds are far more complicated than I ever knew. No, emotions probably don't play into it (not theirs anyway). But my belief that I am being revisited by the same pair of birds year after year may not be so absurd; while my assumption that all the young that they tend are in fact their young may be inaccurate. As Steven Hagenbuch, a conservation biologist with Audubon Vermont explains, the process of songbird procreation generally follows a set pattern, but within that pattern there are quirks and there are exceptions.
For 90 percent of songbirds, including phoebes, the process goes like this: The male establishes a territory and then courts a female. That pair then build or find a nest, copulate, incubate their eggs, feed each other and their nestlings, and eventually, if all goes well, see off the fledglings. The departure of the young completes a breeding cycle, at which point the paired birds may stay together and produce another brood, or they might seek other mates and start the cycle anew. This, says Hagenbuch, is known as breeding cycle monogamy -- one male, one female together for as long as it takes for a nest of baby birds to make it to independence.
But, he adds, even within this brief period of commitment, things get complicated. Social monogamy describes the shared behaviors outlined above, but the commitment here is not to the relationship between one male and one female -- it's each bird's commitment to the propagation of its own genes.
To that end, both the male and female of a socially monogamous couple may copulate with multiple other partners. In fact, according to Kent McFarland, a conservation biologist with the Vermont Center for Ecostudies, most do. The female will add eggs fertilized by another male to the eggs she tends to with her steady partner. According to a study done by researchers at Purdue University, the first clutch of eggs produced by a pair of Eastern Phoebes will contain about 9 percent of what is referred to as "extrapair young." If the pair stays together and produces a second clutch, the percentage of extrapair eggs increases.
These seeming transgressions do not disrupt the enterprise at hand, namely the raising of young. The original couple will proceed as planned, tending to a batch of nestlings that may be genetically mixed.
As for long-term pair bonds, that kind of fidelity just isn't part of the songbird world. There is nothing to suggest that songbirds look for last year's mate. There is, however, site fidelity, meaning birds will return to the same site where they successfully nested the year before.
Which brings us back to my phoebes.
They are among the species of birds that will seek out the same site for multiple years, says McFarland. And researchers have discovered that, because of this commitment to place, sometimes the same male and the same female that jointly raised a brood or two the year before will find each other again.
So, will last year's phoebes that nested in my yard be among those that re-attach? Maybe, maybe not. Will all the nestlings whose fleshy heads pop up at feeding time be the product of the pair tending them? I'll never know.
Ultimately it doesn't matter. The particulars of my phoebes' mating habits won't change how I feel about them. This spring, as the snow melts and the air warms, I will see the phoebes anew and start my own cycle of welcoming them back, observing their behavior, rooting for their young and then mourning their inevitable departure.
Carolyn Lorié lives with her two rescue dogs and very large cat in Thetford. The illustration for this column was drawn by Adelaide Tyrol. The Outside Story is assigned and edited by Northern Woodlands magazine and sponsored by the Wellborn Ecology Fund of New Hampshire Charitable Foundation: firstname.lastname@example.org.
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