No, not that President! Thousands of people are captivated by the live video stream of a pair of bald eagles, named Mr. President and The First Lady, nesting on top of a Tulip Poplar tree at the U.S. National Arboretum. The reality peek into the life of a pair of breeding eagles, together with new research just published in the journal Nature Communications, show how parents decide which of their hungry chick gets fed. Begging is important, but sometimes begging is ignored and the parents feed their favorite. Now we know why . . .
We watched the pair of eagles stoically keeping two eggs warm beneath their downy feathers through the rain, snow, and wind, of early spring. Then we delighted when each chick cracked out of its shell–the miracle of new life–and wobbled naked and feebly in apparent bewilderment before their parent’s yellow beak poking raw fish flesh down their gullets. Soon both chicks were begging for food, but the older one dominated in the sibling rivalry for seafood meals of fresh fish plucked by their parent’s talons from the Anacostia and Potomac Rivers.
We admire the exhaustive labor of the attentive parents providing for their hungry offspring. For most species, including humans, the production of offspring is the most energetically demanding stage of an animal’s life. According to a recent study, raising a brood successfully puts a metabolic demand on breeding birds that is equivalent to a human cycling in the Tour de France.
With many mouths to feed in the nest, which one gets the treat? The noisy wheel gets the grease, right? Not necessarily. In one quarter of all bird species studied, the parents ignore their most demanding chicks that beg. Why?
Even though they beg ferociously in hunger, the parents of some birds ignore their most demanding chicks and feed their favorite–the biggest one. The parents give this fat chick the food even though it doesn’t beg for it. How can this odd preference to give in to begging in some bird nests but ignore the most demanding chicks in others be understood?
A research team led by Zoologist Stuart West at the University of Oxford, studied 143 species of birds to find out, and they found a pattern. Whether parents respond to their offspring’s begging or ignore it, all depends–not on the family dynamics–but instead on ecological circumstances. The researchers found that in predictable and bountiful environments, chicks in worse condition beg more and their parents respond to the begging by giving the most demanding child more food. That’s apparently what’s happening in the nest of Mr. President and the First Lady, which is situated nearby two great rivers teaming with plentiful fish. But in poor environments, or in environments where food is unpredictable, the researchers found that parents ignored the begging chick. Instead they favored their chick that was bigger or had physical features that they favor as signs of quality. (The prettier one.)
This harsh unequal treatment seems to make sense in poor environments, because the strategy of the parents seems to be to put most effort into their offspring that is most likely to prosper. In a harsh environment, raising one offspring successfully is better than having both offspring fail to thrive, so they put the greatest effort into assuring that the bird that has the most advantages will survive.
In this interesting study of communication and behavior in birds, it is difficult not to draw parallels to people. Children of the well-off are notorious for being demanding and spoiled, but children in families living in hard times don’t beg, and their parents may lavish over a favorite child. There is no scientific basis for drawing that parallel between birds and people, but the parallel does help to remember the findings of this new research on why birds sometimes favor their most demanding chicks, but in other ecological conditions ignore them in favor of the prettier one.
photo/video credit: © 2016 American Eagle Foundation, EAGLES.ORG.