Hatching

Thanks to a web cam I’ve been able to keep an eye on a pair of tawny owls that has taken up residence in a nest box situated within a large suburban garden. This particular pair is somewhat early in its timing this year, well ahead of most other tawny owl pairs nesting in the UK. The female laid her first egg in mid-February, her second following a few days later on Saturday 16^th. I know this because during the early days of laying she would vacate the box just as it was getting dark, presumably to hunt, preen and stretch her wings, leaving the eggs clearly visible in the bottom of the nest box.

While our familiar songbirds, like tits and finches, tend to lay one egg per day, tawny owls lay one egg every two to four days, a pattern seen in our other breeding owls. The interval may vary depending on the weather conditions and it is usually longer between the last two eggs than the first two. Tawny owls usually start to incubate with the first egg; this is different from tits and finches, which usually wait until the clutch is complete. This means that tawny owl chicks do not all hatch at the same time, but emerge over a number of days to leave a clear hierarchy of ages. This is what we biologists call asynchronous hatching.

Asynchronous hatching is thought to be an adaptation, allowing owls and other birds of prey (who also show this behaviour) to cope with years when food supplies are poor. If all of the young owlets were the same age and size, then a shortage of food would probably see the loss of the whole brood. Since older chicks always secure food first, a hierarchy of ages ensures that it is the youngest (and weakest) chick that succumbs first when times are tough. This acts to reduce the brood size downwards towards a level that is better supported by the resources that are actually available.

This may seem particularly cruel but, as a strategy to produce as many young as the conditions can support, it is a successful one for the owls to adopt. The strategy is even more obvious in the barn owl, where a larger clutch of eggs tends to be laid and the reduction in brood size can be more dramatic. Being born so early in the year may prove challenging for the young tawny owls. However, if they fledge from the nest successfully they should gain independence at about the time that the abundance of food reaches a decent level, supporting them as they learn to hunt and to fend for themselves.