It’s the quiet time of the year.
The snakes and turtles are hibernating safely underground, the beautiful warblers have made their journey south, and the trails are filled with the calls of woodpeckers and sparrows.
Every winter, the white-throated sparrows that breed in Canada, fly south to winter in the temperate zones of the U.S. A trip to Mason Farm will reveal hundreds of white-throated sparrows noisily erupting with their “Oh sweet Canada” call from the underbrush at every curve of the trail. When you see other birders on the trail, they generally sigh when you ask them what they have seen. They tick off a list of woodpeckers and then as an afterthought mention the huge flocks of white-throated sparrows. On lucky days, a fox sparrow may be hidden in the bunch.
We take these northern migrants for granted, and mainly enjoy them just because they represent a species that has enjoyed success at a time when much of our native wildlife is threatened. But even the small and unnoticed creatures have a story to tell.
The white-throated sparrow comes in two morphs. They all have a crisp, well-defined white throat, and beautiful yellow lores, but some of these passerines possess white and black head stripes, while the alternate morph has tan and black head stripes. The white stripes are slightly more attractive, in that the color pattern is a bit bolder and well defined.
The curious fact is that genetics and behaviors are often linked, and the behaviors of the two color morphs are dramatically different. Males of the white-striped group are more aggressive in their bids for territory than their tan-striped brethren. They sing more often to solicit mates, and intrude upon the territories of other birds. They are also philanderers. They spend very little time aiding their mates in guarding the nest, and feeding the nestlings, as they are on the prowl for new mates.
Tan-striped males are far more domestic by nature. They are quieter, remain close to the nest, are monogamous, and do a very good job of caring for their offspring.
The story is even more complex when you delve a bit deeper. One may wonder why a female bird would take on a good looking ne’er do well, but there is far more to consider. In almost every instance, white-striped sparrows will mate with birds of the opposite stripe. This disassortive mating assures the evolutionary success of both morphs.
So, you may ask, is there a specific set of traits associated with females of each color morph? The tan-striped females are also far more domestic. They tend to be monogamous, and give careful attention to their offspring. The white-striped females are careless egg dumpers, who might deposit eggs in another bird’s nest for a stranger to brood, and are known while attending a nest to occasionally sneak off on a designated partner to mate with an outside bird. With their own partner they are also more likely to solicit copulation.
So there you have it. White stripes are irresponsible cheaters … the philanderers and slatterns of the bird world!
When you put aside all the anthropomorphic slurs against these birds, you see a mixed pattern to maximize species fitness. Both males and females are constantly making trade-offs between investing in parental effort, or investing in reproductive effort. Will reduced parental effort be costly in respect to the survival of the young?
Many species of birds have evolved to have a reliable strategy for species success. The great horned owl male is actively involved in the parenting of the nestlings by guarding the nest, and hunting for almost all of the food for the nestlings and female. Wood duck drakes have no involvement with the wood duck hen after mating, and sotted sandpiper females always abandon their eggs to the care of the male of the species so that she can lay more eggs with other sandpiper males. This strategy makes particular sense for a species that experience a high rate of predation. What amazes and fascinates me is the fact that behavioral traits are so clearly linked to the actual genetic make-up of the bird.
For each morph of the white-throated sparrow, there appears to be evidence of success. Tan males rarely get the chance for any dalliance because they are so busy tending their mate and their nestlings. The reward for this behavior is a low incidence of cuckolding because he is guarding his mate carefully from intruders. Conversely, the tan females (even when busy with nestlings) are more likely to find extra partners when their outgoing Lothario is gadding about. The advantage for the male white stripe is the possibility of parenting more offspring with other females. Both strategies are successful.
Of course, all this talk about birds and their various life strategies has me thinking about how this applies to humans. Have you ever gone out to dinner with the couple that exhibits this tan stripe/white stripe behavior? Oh, she is a delightful woman, but he is such a pill . or he is so kind and outgoing, where did he find that stick in the mud?
Perhaps our choices are just driven by evolution, and reproductive strategy. Does the happy extrovert need some reliable but dreary partner to turn in the tax forms on time? And how about the cheating spouse with the long-suffering partner who stays home caring for the children?
Imagine if these behaviors were driven entirely by our chromosomes, and not the result of any conscious choice on our part. I suppose the white stripes in our midst are already contemplating the perfection of this excuse. “It’s not my fault, it’s my destiny.”
Mary Sonis is a naturalist, photographer and writer in Carrboro. You can reach her at firstname.lastname@example.org