The Great Race
Breeding is one of the most fundamental of avian behaviors. While there are a variety of strategies employed, they all converge at the same central point, the urge to procreate, the need to perpetuate “self.”
Most North American species will nest a single time during a given breeding season. There are many species that will renest if necessary, i.e., they will lay a replacement clutch if the eggs or hatchlings in their first clutch are destroyed. The feasibility of renesting depends on factors such as the length of the nesting season and the point at which the first clutch was lost, the time it takes for eggs to hatch and young to mature, and the availability of food.
There are also a surprising number of species that will nest a second time in the same breeding season if the opportunity presents itself. This in part accounts for the abundance of members of nuisance species such as house sparrows and European starlings. But second broods have also been documented in wood ducks and various of the swallows, thrushes, wrens, sparrows, buntings, and blackbirds. It has also been recently reported that there are a small number of species that migrate north in the Spring to nest, and then stage a second breeding season in Mexico during their Autumnal migration south. It has been postulated that declines in yellow-billed cuckoo populations may be due to loss of habitat in northwestern Mexico, impacting the birds during their second nesting attempts.
While searching the foothills for fall migrants this afternoon I stumbled across several nests filled with young barn swallows, a species known for second nesting. Will these birds mature before the first frosts kill the insects on which they rely? The leaves on the trees in the hills above the nests have already begun to show the first changes of color. It is only a matter of days until the warmth of summer will be little more than memory.