What do we know about clutch size?

Clutch sizes differ not only among major taxonomic groups of birds and among species, but also among populations and individuals of the same species. For instance, albatrosses, shearwaters, tropicbirds, and frigatebirds characteristically lay only one egg per clutch. Loons, goatsuckers, most pigeons, and hummingbirds lay two eggs per clutch, and most shorebirds lay four eggs per clutch. With the exception of shorebirds, practically all species that normally lay more than two eggs per clutch show marked variation in clutch size. There are numerous factors that appear to influence the number of eggs in a clutch. They include the following:

• Age of the female. Within populations, the age of a female bird is related to the size of her clutch.
• Temperature and time of season. Cold weather may reduce the size of a clutch; and clutches laid later in the breeding season may contain fewer eggs than clutches laid by the same individual earlier in the season.
• Health of the female. If a female is unhealthy, her clutch size will probably be smaller than if she were in peak physical condition.
• Food availability. Abundant food supplies can mean more eggs per clutch.
• High population density. Females lay fewer eggs per clutch when breeding in colonies or other high population areas. Habitat quality may also affect clutch size.
• Geographic location. On average, within a species, birds lay smaller clutches when breeding at either lower latitudes or higher altitudes.