Clutch Size

What is a Clutch?

A clutch is the total number of eggs laid by one bird during one nesting session.

Clutch Sizes differ among Birds

Clutch size, for most species, is determined by heredity. Birds of the same species have about equal-sized clutches. Among species however, there may be substantial variation in clutch size. Some species, like the King Penguin or the Common Murre, lay only one egg.

Others, like the Common Loon lay two eggs. Shorebirds usually lay four eggs. Many songbirds lay between four and six eggs.
Loon eggs Shorebird eggs Songbird eggs

More Factors that Influence Clutch Size

Lesser Scaup eggs In addition to hereditary factors, clutch size is also affected by environmental factors. Food supply is one such factor. When food is abundant, birds lay more eggs. These eggs are from a Lesser Scaup. These ducks usually lay between 8 and 12 eggs. There are 15 eggs in this nest. Food was probably especially plentiful the year she had this clutch.

Egg size also affects clutch size. More energy is required to produce large eggs than small eggs. Birds that lay large eggs thus lay fewer of them. Bird age is another factor that influences clutch size. Older birds have larger clutches than younger birds. Finally, the number of eggs that can be incubated successfully and the predation intensity affect clutch size. Ideally, the bird produces an optimal clutch size. Abnormally large or small clutches tend to produce fewer surviving young.

Geography affects Clutch Size

Among closely-related species, clutch size tends to vary with latitudes. As we move north from the equator, clutches tend to increase in size. Females in the north likely have fewer broods per year than do birds nearer the tropics, so they may increase the number of eggs they lay in a single clutch. Differences in seasonal food abundance as well as the long days available for feeding young are also important factors in this variation in clutch size.

Fixed or Flexible Clutch Sizes

Some birds always lay clutches of the same size while others may have clutches of variable size. A bird that only lays a particular number of eggs at a time is a fixed or determinate layer. A bird that will continue to lay eggs as they are removed from the nest, is a flexible or indeterminate layer. The bird may require the feel of the "proper" number of eggs before it stops laying.

Number of Broods

Birds nesting at higher latitudes tend to produce fewer broods per year. The climate gets colder and harsher faster. Eggs and hatchlings that emerge at sub-optimal times of the year are less likely to survive. In many instances, Alberta birds lay only one clutch per year unless there is a failure of the first clutch. An exception is the introduced House Sparrow, which commonly raises two and sometimes four broods in our short Alberta summer. When a wild species does lay more than one clutch per year, clutch sizes tend to decrease in successive broods.

In the north, there usually is not sufficient time for more than one brood per year, particularly for large species with slower development times. In warm regions, smaller birds often raise two or even three broods per year.

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