Latitude plays a role in how many eggs a bird lays - but why?
Published: July 1, 2005
|Birds of northern latitudes often lay larger clutches than their southern relatives do. The variation has been observed for many species, including hawks, owls, grouse, herons, and songbirds.|
White-crowned Sparrows in Alaska, for example, typically lay four or five eggs, while White-crowns in Oregon and Washington lay three or four, and those in southern California generally lay three. David Lack, a noted British ornithologist, attributed the phenomenon to the ability of adults to feed more young at higher latitudes, where the increased daylight allows more time for feeding.
Anyone who has watched songbirds feed their young knows the large number of feeding trips they make. Many birds feed their young continuously from dawn to dusk. You can assess this phenomenon yourself by counting the number of feeding trips a pair of nesting songbirds make during three 15-minute sampling periods at different times of the day. Be careful not to disturb the adults or the nest. Multiply the sample average by four to estimate the number of feeding trips per hour. Then multiply that by the number of hours of daylight in your area to get the number of feeding trips per day. The total will likely astound you.
Studies show that, for most species, the number of feeding trips increases when more young are in the nest. But there's a limit to the number of feeding trips possible. If clutch size (and subsequent number of nestlings) is too large for the available food supply, the number of feeding trips per nestling can decrease, resulting in less food per nestling and underweight fledglings.
Lack developed his food hypothesis while working with starlings. He found that clutch sizes in his study varied from one to eight, with five being the most common. Starlings that had smaller clutches were successful in rearing young, but the number of fledged young was small. Larger clutches would obviously contribute more young, or would they?
Survival is the goal
Lack reasoned that simply getting the young to fledge wasn't enough; the fledglings also had to be healthy enough to compete and survive. He searched for dead starlings after fledging. Because all nestlings were banded, he could match carcasses to nests with known clutch sizes. He demonstrated that mortality was highest for young from larger clutches, and that the loss outstripped the apparent gain from the larger number of chicks that hatched from the clutches. Five was the optimum clutch size because it resulted in the maximum number of surviving offspring. Similar results were obtained from a study of House Wrens in Ohio, where higher mortality of chicks from larger clutches correlated with smaller nestling weights of those young.
Lack's hypothesis has stood for almost 60 years and still has support, yet it doesn't answer every question. For example, the breeding season is shorter at higher latitudes than at lower latitudes, so it often restricts the number of broods that birds can raise. In fact, many northern birds raise only one brood, while southern birds raise several. Consequently, some would argue, northern birds must lay larger clutches to assure enough offspring to balance mortality. And in fact, severe winters for resident species and losses associated with migration for nonresidents combine to make mortality greater for birds nesting at higher latitudes. Birds nesting at lower latitudes can lay smaller clutches and still balance their losses. Furthermore, the longer breeding season means southern birds have time to raise a second brood or to renest if a nest is destroyed.
A study of Northern Flickers supported the hypothesis that larger clutches are due to increased adult mortality during the winter and reduced competition during brood rearing. Increased clutch size of flickers at higher latitudes correlated with unfavorable conditions during winter, rather than the ability of birds to feed more youngsters during summer (Lack's hypothesis).
Having smaller clutches at lower latitudes continues to the tropics. Tropical songbirds typically lay two eggs, while their temperate North American relatives usually lay from three to six. The non-breeding period in the tropics doesn't contribute to higher adult mortality, and competition during brood rearing is very high. Both factors argue for smaller clutches. High rates of nest destruction in the tropics also favors small clutches as nests can be smaller and therefore less visible and, if destruction occurs, the eggs can be readily replaced in another nest.
Larger clutch sizes in northern regions have been demonstrated for many species. This may result either from the ability of adults to feed more young during the summer or from increased winter mortality of adults (and subsequent reduced competition during the summer), and there might be other explanations. The birds don't care that we don't fully understand. They just continue to lay eggs in numbers that tend to maximize their production - and continue to demonstrate another of their amazing behaviors.