Spring has finally arrived, and birds’ nests all over the country will soon be filling up with eggs and then nestlings. Watch a nest long enough (the Science News staff is partial to the DC Eagle Cam) and you’ll see itty bitty baby birds begging for a meal. But mama birds don’t always reward that begging with food. In some species, like the tree swallow, birds that beg more will get more food. But in others, like the hoopoe, mom ignores who is begging and gives more food to the biggest chicks, researchers have found.
This lack of an overall pattern has confounded ornithologists, but it seems that they may have been missing a key piece of the puzzle. A new study finds that the quality of the birds’ environment determines whether a mama bird can afford to feed all of her kids or if she has to ignore some to make sure the others survive. The study appears March 29 in Nature Communications.
Stuart West of the University of Oxford and colleagues compiled data from 306 studies that looked at 143 bird species. When the birds were living in a good environment — one that had plenty of resources or a high amount of predictability — then mom would feed the chicks that beg the most, which were often the ones that needed the most help. But when the environment was poor in quality or unpredictable, then mama bird responded less to begging.
In those tough times and places, mama birds invested more of their limited resources in chicks that were big or had colorful beaks — signs that they were already doing well. It might seem nicer for mom to give more food to the smaller, weaker chicks that obviously need it (and may be making a bigger racket in hopes of getting more to eat). But if she favored those chicks, then she might not have enough for all of them, and none might survive. So investing her limited food resources in the bigger chicks is a good strategy to ensure at least some of her brood make it to adulthood and have chicks of their own.
It’s also a brutal strategy, but it’s a necessary one. Raising offspring, as any parent knows, is a huge investment of time and resources. For birds, West and his colleagues say, raising a brood places a metabolic demand on a mama bird that is equivalent to a human cycling the Tour de France. And in the animal world, sometimes the only options are to be cruel or risk death.