A highly variable feature of bird reproduction among species is clutch size, the number of eggs laid in a nest. Ruby-throated hummingbirds and mourning doves always have a clutch size of two. herring gulls lay two or three eggs. Most songbirds have clutch sizes in the single digits. For instance, chestnut-sided warbler and black-throated green Warblers lay three to five eggs.
Some birds are more ambitious. The clutch size of ruffed grouse varies from nine to 14. A female wood duck lays six to 16 eggs.
Why the variation? The explanations are complicated but food availability is a major driver. Let’s start with grouse and ducks. These birds have precocial development; the chicks hatch out fully feathered and can begin foraging for their own suppers soon after hatching. The adults provide watchful eyes but are not burdened with feeding hungry babies. So, clutch sizes of precocial birds can be large.
At the other end of the development spectrum, we have altricial birds that hatch out naked and blind. The nestlings are utterly dependent on their parents for food and warmth. Most songbirds, hummingbirds, owls and woodpeckers are good examples.
Because the adults must keep their rapidly growing chicks fed, clutch sizes are smaller than clutches in precocial birds.
But within altricial birds, we see variation among species. How can we explain this variation?
That question was tackled by the British ornithologist, David Lack, over 75 years ago. He studied blue tits and great tits, relatives of our chickadees. These two tit species readily nested in nest boxes that Lack erected in a local woodland in England.
He used an experimental approach to study the success rate of nests with different clutch sizes by taking some eggs from nests and placing those eggs in the nests of other birds. So, he reduced the clutch size of some tits, left others the same and augmented the clutch size of others.
Lack found that the normally observed clutch size was the most productive, resulting in the highest number of fledged young. For great tit, eight eggs is the most common clutch size. Nests with five or six eggs did not tax the parents as much as a clutch of eight eggs. However, starting with fewer eggs led to fewer fledged young. Pairs with ten or eleven chicks simply couldn’t keep up and some of the nestlings starved.
Pairs with eight eggs found the happy medium. Pairs with five or six eggs could have worked harder and those with ten or eleven eggs were taxed beyond their abilities to feed that many young.
Lack’s work is commemorated as Lack’s Hypothesis, claiming that a female bird should lay the number of eggs that will result in the maximum number of fledged young.
Subsequent work has shown that pairs of birds within a species with altricial development vary in their ability to provide for their young. Some birds are simply better at finding food to bring back to nestlings.
In Sweden, Goran Högstedt examined the quality of parenting in magpies, members of the crow family. He had banded a large number of magpies and recorded each pair’s clutch size in the first year of his study. The clutch sizes varied from five to eight.
In the next year, he manipulated clutch sizes to produce sizes of five, six, seven or eight eggs within each clutch size class of the previous year. He found that, for instance, pairs that normally laid five eggs fledged the most young with a clutch of five eggs. Pairs that normally had a clutch of seven did more poorly with five, six or eight eggs. Each pair could assess their own ability to raise young and laid the optimal number of eggs. Brilliant!