Most of the birds in Maine are in the process of nesting now. Among the different bird species, there is great variation in many facets of reproduction. For instance, whether one or both parents brood, the type and location of the nest, the size of the eggs.
One striking feature that varies widely among different species is clutch size, the number of eggs laid in a single nesting attempt. Some birds (albatrosses, petrels, some penguins and some terns) have a clutch size of only one. Hummingbirds and doves always have clutch sizes of two. At the other extreme, some ducks and geese, pheasants and rails may have clutch sizes as high as 20 eggs. Our songbirds have clutch sizes varying between two and 12, depending on species.
Why is there such striking variation in clutch size among different species? This question has been the basis of a great deal of research. In this column, I will briefly introduce some of the findings of this body of research.
To begin, we must realize that reproduction is a taxing experience for birds. Birds that undertake a particularly heavy reproductive effort in one year may reduce their chances of surviving to reproduce the next year. Birds have to make a trade-off between producing very large clutches in a short life and moderate-sized clutches over a longer life.
David Lack, a British ornithologist, was one of the first scientists to rigorously study clutch size variation in birds. He came to the conclusion that a female bird should lay the number of eggs that will produce the most fledged, independent offspring.
Let’s consider Lack’s conclusion for three females which lay three, five and seven eggs, respectively. The female and her mate with only three eggs will probably succeed in fledging three young. The pair with five eggs will have to work harder but still might lose only one chick, fledging four young. The pair with seven eggs have bitten off more than they can chew. The demands of the seven hungry chicks are too much for the parents and perhaps four of the seven chicks will die from starvation. So, what is the best clutch size to have for this species? A clutch size of five – a trade-off between ambition and caution. The pair with only three eggs is not ambitious enough; the pair with seven eggs is too ambitious.
Ornithologists have tested Lack’s prediction by altering the clutch sizes of birds. One species that has been studied intensively is the Tree Swallow. These birds frequently nest in nestboxes at high densities, making it easy for an ornithologist to monitor large numbers of nests. Tree Swallows usually lay five eggs. Experiments are done by removing one or two eggs from some nests and placing them in other nests. After the egg-switching, some nests have three, four, five, six and seven eggs. The results largely confirm Lack’s prediction. A clutch size of five is the most productive clutch.
In other species, that have been examined, birds lay one less than the most productive clutch. Scientists believe that these birds are cutting back on their reproductive effort in a single year to increase their chances of having a long life.
Clutch size within a species often shows striking variation with latitude. The typical pattern is that tropical members of a species have lower clutch sizes than members of the same species that live in the temperate zone. For instance, female Northern Flickers that live in the tropics only lay clutches of three or four eggs while flickers in the northern U. S. A. and Canada may have ten or more eggs.
One possible explanation for this variation is the number of predators. The tropics have many more predators that are threats to eggs and nestlings, including snakes, other birds and various mammals. The chances of a nest being found and destroyed by a predator in the tropics is very high. In the temperate zone, the number of predators is much reduced. Many temperate nests are never discovered by predators.
Ornithologists therefore argue that tropical birds should not invest heavily in a single clutch because it is likely to be lost. It’s not prudent to make a large investment in a risky environment. It is much better to have a low clutch size, allowing the parents to live a longer life. With luck, one of the many clutches a female will lay over her lifetime will escape the predators. On the other hand, a temperate bird does not have the same high risk from predators so can lay a bigger clutch. It’s OK to make a large investment if there is little chance of losing it all.
[First published June 27, 2009]