Why birds have different clutch sizes

The success of individual birds is measured by their contribution to the gene pool determined by the number of surviving offspring they produce. To determine the rate of offspring production, population biologists study fecundity — the number of eggs per nest (clutch size) and nests per year (broods). Following are two examples.

A female bobwhite leads her 12 newly hatched chicks from the nest to a secluded habitat with good cover and abundant food. Because they feed themselves, mom mostly needs to keep them together. After the youngsters have grown and dispersed, she may mate again and bring up another brood. Perhaps she’ll raise 15-20 youngsters in one summer. That’s huge, and she could live another five to 10 years. This reproductive strategy yields great production with minimal investment of parental energy per chick.

On another extreme, consider the Laysan Albatross. It lays a one-egg clutch. The parents spend about two months incubating and more than five months caring for the chick before it fledges. That’s a production of one chick with more than seven months of high-energy attention. Some adults deservedly take the next year off. That means one chick every two years. Laysan Albatrosses can handle that low production because they have few predators and can live for 30 to 40 years.

The most famous Laysan Albatross is a female banded on Midway Island in the Pacific in 1956, when she was at least 6 years old, by ornithologist Chandler Robbins. He captured and re-banded her 46 year later. She is named Wisdom and is the oldest known banded bird. She has nested every year since 2006 with her paramour, Akeakamai, which in the Hawaiian language means “seeker or lover of wisdom.” When Wisdom laid her egg last year, she was at least 68 years old.

The variation in breeding strategies is almost unlimited. Most evolved from ancient birds and are linked to DNA. Some bird species are genetically programmed to lay a certain number of eggs. The Killdeer lays four, most terns lay three, and the Mourning Dove two. These birds are called “determinate layers.” If eggs are either removed or added to the nest experimentally, the female will lay no more than her set number. Why egg production is fixed is not known. Experiments have shown that most birds can produce more eggs than they normally lay and that they can also incubate more eggs than they normally do.

Researchers have a possible explanation for doves and pigeons, which differ from other birds in that they feed newly hatched chicks sloughed esophageal cells full of lipids called “pigeon milk.” This is essential during the first few days of a chick’s life. The youngsters reach their beaks into the oral cavity of the adults — one on each side of the adult’s beak— to get their nourishment. A third chick, if present, is out of luck. After two have fed, the resource is apparently too low to support a third chick.

For most birds, however, clutch size is not the same for all members of a species. Individuals lay a certain number, but the number can vary from one individual to another in the population. This variation in clutch size proves to be important. If eggs are experimentally removed or added to the nests of these birds, the female will either continue to lay until her correct number appears or lay fewer because the added eggs bring the correct number sooner. These birds are called “indeterminate layers.” Why they stop laying at a certain number, or how they know they have reached this number, is unclear.

Observe a pair of songbirds feeding nestlings in your backyard, and you’ll be amazed by the number of feeding trips the adults make. If the number of nestlings is too large or if food abundance is low, the adults can’t keep up the number of feeding trips per chick. Consequently, chicks fledging from smaller clutches tend to be heavier than chicks fledging from larger clutches. And they have a greater chance of surviving the first year.

When food supply is abundant, a large clutch can be an advantage since all of the young can be raised successfully with good chances of survival. When food supply is limited, however, a large clutch can be a disadvantage, as a small amount of food shared by a large number of chicks can reduce survival chances for the entire brood.

Birds have an optimum clutch size, therefore, that will result in the maximum number of surviving chicks. Because food supply varies from year to year, optimal clutch size will also vary. The significance of clutch size variation is now clearer. Whatever the food conditions that develop, some individuals will have a clutch size optimum for that condition, and they should produce youngsters well prepared for survival.

Birds have developed myriad reproductive strategies to maximize the number of surviving offspring. This is another example of the clever and amazing behaviors of birds.

This article from Eldon Greij’s column “Amazing Birds” appeared in the March/April 2019 issue of BirdWatching.