First Egg Least Likely to Hatch
A study of Lincoln's sparrows found that being the oldest is not always an advantage.
It is a well-documented fact that being born just a day or so later often sets the stage for a situation whereby youngest hatchlings die. That’s because they’re too small to compete against their feistier brood-mates for the limited resources provided by their parents. Such competitive disparities caused by hatching or birth order can be found in other animals – from beetles to marsupials to humans – which sometimes produce their young in series, then rear the resulting offspring simultaneously....
Female Lincoln’s sparrows lay one egg per day, usually producing three to five eggs in total. While carefully observing and tracking the tiny birds for three breeding seasons, Sockman and his team of researchers noticed that typically, mothers do not settle down and start incubating the eggs right away, since they still have other concerns during the laying cycle, such as foraging for food....
“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 study was published in PLoS ONE, so the full article is available for those who would like to read more.
