Begging: the question
My wife and I have a bird feeder outside our kitchen window. Yesterday I saw an adult male cardinal feeding some of the seed to an immature cardinal not much smaller than he was. I guess it's hard to say "no", but should he have?
This week's paper, "The adaptive value of parental responsiveness to nestling begging" by Uri Gordzinski and Arnon Lotem, published online in Proceedings of the Royal Society, may have answered this question.
Their experiments used pairs of sparrow siblings raised by humans. They simulated either a "responsive parent" (giving more food to whichever of the two birds "begged" more), or a "nonresponsive parent" (picking which bird got more food at random).
The weight gain of the baby birds (averaged over the two siblings) was 2.8 g with the simulated responsive parent, but only 2.2 g with nonresponsive (random) feeding. The difference appeared to be due to wasted food: with random feeding, twice as much food that was supplied was not eaten, presumably because it was given to a bird who was not begging because not hungry. The authors suggest that responding to begging also reduces the chance that a nestling will miss being fed often enough to affect its growth, which could happen with random feeding.
So kin selection would favor responsive rather than nonresponsive behavior. Genes for responding to begging are common because baby birds are more likely to survive if their parents respond to begging -- and the babies have a 50:50 chance of inheriting the same genes.
The same issue has an article looking at the effects of grandparents on the reproductive success of their children. In contrast to grandmothers, grandfathers that live longer don't end up with more grandchildren, at least in Finland. This seems like something that might vary among societies. But what if your grandfather is a cardinal?