BREEDING STATEGIES: CLUTCH SIZE
Birds lay eggs in sets, or clutches. Clutch size is the number of eggs laid in one nesting attempt; it varies across species, from a single egg to as many as 20. In part, this is determined by evolutionary history; all albatrosses lay just one egg, hummingbirds consistently lay two, while partridges can lay 10–15 or more. But this is not always the case. Clutch size also can vary within species, within populations, and even for an individual from year to year, based on such variables as habitat, latitude, altitude, nest type, size of a nesting colony, food availability, and body size and health of the mother. Such trade-offs and the choices involved are a critical part of a bird’s breeding strategies at all levels.
Members of a Waved or Galapagos Albatross pair take several days for each turn to incubate their single egg until it finally hatches, two months later.
CLUTCHES LARGE OR SMALL
Broadly speaking, natural selection leads birds to lay as many eggs as they can successfully rear. For example, birds that breed in northern latitudes tend to have larger clutches while related birds from the tropics lay fewer eggs; although temperate regions have a shorter breeding season, there is plenty of food during that time to feed more chicks. Birds that lay multiple clutches each year may have fewer eggs in their final clutches; this may be due to reduced resources in the health of the mother but also could be related to the availability of food late in the breeding season. Likewise, birds with precocial young (those that leave the nest soon after hatching and can feed themselves thereafter) lay more eggs per clutch than do birds with altricial young (those that require prolonged brooding and care). Parents with altricial young may find more success with a smaller brood.
Most small birds lay one egg each day until their clutch is complete; others, typically large birds, lay only one egg or lay consecutive eggs two to three days apart. Female kiwis, whose eggs weigh some 25 percent of their body weight, may go weeks between laying the first and second egg.
SURVIVAL OF THE FITTEST
Parent birds can control to some extent the rate and timing of egg development to influence the success of their breeding attempt. Some start to incubate each egg on the day when it is laid, resulting in asynchronous hatching; the chicks hatch over a series of days, and the first-hatched chicks are larger and more dominant. In Cattle Egrets, asynchronous hatching, and the resulting size and dominance hierarchy of the chicks, is further enhanced by the mother depositing different amounts of testosterone into the egg yolk: the first two eggs receive nearly twice as much as the last, third-laid egg. The result is that in seasons with poor food supplies, the hungry, aggressive, and large first chicks attack, peck, and drive off the nest the small, timid last chick, assuring that the resulting brood of just two chicks receives sufficient food from the parents to fledge successfully.
In contrast, synchronous hatching takes place when the parents wait for all the eggs to be laid and start incubation on the last day, so that each egg hatches within hours of the others. This might assure equal chances of survival to more of the young. Mallard embryos take additional steps to assure simultaneous hatching; they listen to maternal calls and vocalize to one another, communicating their developmental state and, thus, the estimated time of hatching.
Great Tits, despite their small size, lay large clutches of eggs. They are an example of a bird of northern latitudes where larger clutch sizes are more common.