ONE OF THE great concerns that ornithologists have is that climate change will throw the nesting activities of birds out of sync with the availability of food for the raising of chicks. For one species, the pied flycatcher, a new study shows that some of its clan are proving to be remarkably adaptable.
Upon returning to Europe from their African wintering grounds, the flycatchers time their egg-laying to the short period when juicy caterpillars are most abundant. During the past three decades this caterpillar peak has advanced by three weeks. Pied flycatchers initially had difficulty adjusting, but over time have started laying their eggs earlier to grab the caterpillars. Some, though, are doing a lot more to improve their reproductive chances of success, according to a study in the Journal of Avian Biology led by Christiaan Both of the University of Groningen, in the Netherlands.
Like most bird species, pied flycatchers have long been thought to lay a single clutch of eggs during the breeding season. This was widely considered to be a trait that does not change. Then, in 2007, a Swiss team led by Pierre-Alain Ravussin began to suspect that clutch numbers were flexible. They discovered a female pied flycatcher that immediately produced a second brood with a new male after raising an early set of chicks. Aware of Dr Ravussin’s findings, Dr Both wondered whether this was just a single, odd instance or if second broods might be happening on a larger scale driven by the arrival of earlier springs. So, they collaborated to delve into the data to find out.
The team studied pied-flycatcher populations in the Netherlands and Switzerland that were known to be among the earliest nesting members of the species. In total, they tracked the egg-laying times and hatchling-rearing success of 8,848 breeding pairs in the Netherlands and 1,372 in Switzerland between 1980 and 2018. They found that since 2006, 11 cases of second broods were observed, all of them among the earliest breeders in both populations.
Further studies ruled out that the birds were making up for a failed first attempt at raising chicks or that the second group of nestlings suffered.
With no obvious downside to laying a double clutch, Drs Both and Ravussin conclude that the birds are attempting to double their annual reproductive output. While this behaviour is still rare, they argue that if the tendency is driven by heritable genes (which it may well be) then a succession of early springs could make the strategy much more common.