The Ecology Book

114

W

hy do some birds lay

more eggs than others?

For example, Blue Tits

lay nine eggs, Northern Flickers six,

and Robins four. In the 1940s,

British ornithologist and evolutionary

ecologist David Lack proposed an

explanation that rapidly gained

support. He argued that the clutch

size (number of eggs laid) was not

controlled by the female’s ability to

lay eggs, since birds can lay many

more eggs than they typically do.

This fact can be demonstrated by

replacement experiments, in which

eggs are removed from a nest; the

bird will re-lay repeatedly to

compensate for the loss.

Instead, Lack said, the number

of eggs laid by any species has

evolved to fit with the food supply

available. In other words, nature

favors clutch sizes that correspond

to the maximum number of young

IN CONTEXT

KEY FIGURE

David Lack (1910 –73)

BEFORE

1930 British geneticist

Ronald Fisher combines

Gregor Mendel’s work on

genetics with Charles

Darwin’s theory of natural

selection, and argues that the

effort spent on reproduction

must be worth the cost.

AFTER

1948 David Lack extends his

theory of optimal clutch size

in birds to include litter size

in mammals.

1954 Lack develops his food

limitation hypothesis further

in The Natural Regulation

of Animal Numbers, to

encompass birds, mammals,

and some insect species.

1982 Tore Slagsvold proposes

the nest predation hypothesis,

which states that clutch size

is related to the likelihood of

the nest being attacked.

Blue Tit nests contain an average of

nine eggs, although the females can lay

many more. David Lack proposed that

the clutch size is determined by the

likely amount of available food.

BIRDS LAY THE NUMBER

OF EGGS THAT PRODUCE

THE OPTIMUM NUMBER

OF OFFSPRING

CLUTCH CONTROL

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