In eastern bluebird society, fathers favor their sons over their daughters, with brighter-colored sons winning out over their duller brothers, according to a new study. This favoritism has dire consequences for the less loved chicks, which are more likely to end up as crow food.
In the last 15 years, a lot of work has gone in to investigating parental favoritism in various animals, birds in particular. Most often, researchers look at how parents preferentially provide more food for some of their children — for many bird species, larger chicks are favored.
Hawk and seabird parenting are classic examples of this kind of size favoritism, says Lynn Siefferman, a behavioral ecologist at Appalachian State University in North Carolina. As a kind of insurance policy, females lay eggs on sequential days, rather than all at once. When food is abundant, every child gets fed; when food is scarce, only the biggest chicks (those born first) get to go to bed on a full stomach. "Nobody wants to be the third little hawk," Siefferman told io9.
Siefferman and her colleague Nicole Barrios-Miller figured that parental favoritism should also exist in species with equal-sized chicks. In this case, rather than strategically feeding the chicks differently, the parents may unequally protect them from predators. To test this idea, they decided to take a look at the eastern bluebird (Sialia sialis).
In this species, nest mates are roughly equal in size because they're born at the same time, but they have different plumage colorations, with some chicks being brighter than others. As they mature to the fledgling stage, the chicks start to leave the nest and venture off alone, sometimes spreading far apart from each other. However, they still can't fly very well at this time, so they require food and protection from their parents.
"In our experiments, we wanted to mimic that natural scenario," Siefferman says. "So we used a fake predator and forced the parents to decide which child to save."
The set-up was simple. Siefferman and Barrios-Miller placed two wire cages about 16 feet apart, and dangled an American crow replica above each cage. They then placed one nestling in each cage. In some trials, they put a male in one cage and a female in the other. In other trials, they used two males with experimentally altered plumages (they either dulled or brightened the feathers with markers). To further build the scene, the researchers played audio of crow calls and bluebird nestling cries.
Finally, the team released the parents and watched what they individually did. In male-female trials, fathers more often attacked the fake crows threatening their sons (by dive-bombing them or pecking at their eyes), rather than those looming over their daughters. And in two-male trials, fathers protected their brighter-colored sons instead of their duller-colored sons. Mothers, on the other hand, didn't seem to show any child preference.
The researchers think the fathers are choosing the offspring they believe will be the most reproductively successful. You see, for adult males, color matters. Males that are brightly colored secure the better territories and get better quality mates. In fact, Siefferman says, males that are pitted against brighter males won't even put up a fight — they'll just give up. "If males are paying attention to color as adults, they may be predisposed to pay attention to nestling colors," she says.
The fathers are also assuming that their sons will have more babies than their daughters. For females, producing sex cells is energetically costly, so the number of babies they have is relatively fixed. But males born in an environment where food is plentiful will grow up to be brightly colored "studs" and have lots of babies with multiple females (eastern bluebirds are monogamous in that they raise the kids together, but extra-marital affairs are common). If the bluebirds were instead raised in low-quality conditions, the fathers may well have saved their daughters, as their sons would likely grow up to be "duds" and have relatively few babies.
The reason why mothers don't seem to play favorites may have to do with their different role in rearing fledglings, Siefferman says. Fathers invest more time and energy in feeding and protecting fledglings because mothers are recuperating and taking care of themselves, having already laid another batch of eggs. So the fathers may feel a greater selective pressure to discriminate among fledglings than their mates do.
One caveat to all of this is that the researchers tested the bluebird parents together, rather than separately. Throughout the trials, it was clear that that there was some copying going on between the pairs, so it's not entirely clear-cut which fledglings the mothers would protect if they were alone.
Siefferman and Barrios-Miller had initially tried to test the parents individually, but this only distressed the birds — they spent their time calling and searching for their missing mates, rather than protecting their children. So it seems that even though the eastern bluebird parents care for some offspring more than others, they care about each other a hell of a lot more.
Siefferman and Barrios-Miller published their work recently in the journal Animal Behavior.
Top image via Sandysphotos2009/Wikimedia Commons. Inset image via Elsevier.