Are younger siblings at a competetive disadvantage to their older brothers or sisters? What wisdom can a bird study provide to us about human sibling relationships?
Common wisdom holds that the first-laid birds in a clutch have a better chance of surviving to leave the nest. But Keith Sockman, an assistant biology professor in UNC’s College of Arts and Sciences, has discovered that first-laid eggs are, in fact, the least likely to hatch at all. His findings, based on studying a population of Lincoln’s sparrows in a remote stretch of Colorado’s San Juan Mountains, were published in the March 12,2008 issue of PLoS ONE. "I believe this is the first study to follow siblings from laying through fledging and demonstrate that the effect of laying order on hatching is very different from its effect post-hatching," said Sockman.
It is well-known that because the youngest hatchlings are too small to compete with their stronger brood-mates for the food provided by their parents, they often die. This pattern is often repeated in other animals, from beetles to marsupials to humans. But these observations have so far not allowed for whathappens to eggs before they hatch.
Female Lincoln’s sparrows usually produce three to five eggs, laying one egg a day. Monitoring the birds for three breeding seasons, Sockman and his researchers noticed that the mothers did not start incubating the eggs right away, since they had other things claiming their attention, such as foraging for food. Sockman thinks this makes it probable that first-laid eggs won’t hatch at all, besides helping ensure that a greater number of healthy, strong birds will hatch and mature into young birds.
"At these elevations, conditions can be fairly harsh even during the summer when Lincoln’s sparrows breed," said Sockman. "It’s often freezing at night, which is hard on an un-incubated egg, while daytime temperatures are warm enough to foster the growth of harmful microbes. As a result, since the mother sparrow isn’t keeping them at the most optimal incubating temperature from day one, first-laid eggs can be exposed to environmental conditions that lower the chance those embryos will ever see the world outside their shell."
"If the female did start incubating all her eggs as soon as she laid them, it would increase the probability they’d all hatch. But it would also give a huge head start to those first-laid eggs and the chicks that emerge from them, putting their younger siblings at even more of a competitive disadvantage once they begin battling for food and their mother’s attention," said Sockman.
"It may also reduce the number of eggs she is capable of laying.The mother’s careful balancing of this trade-off enables her to end up with three or four relatively equally robust offspring, instead of one or two strong hatchlings and several "runts of the litter."
Sockman plans to examine what, if any repercussions laying order has once young birds reach adulthood. "The severely competitive environment in the nest may have consequences on the individual’s ability to compete for resources and mates the following year when it is reproductively mature," said Sockman.
Keep these findings in mind as you raise your own young!