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Photo © Al Tuttle

What is clutch size and why do we study it?

A clutch is the total eggs a bird lays per each nesting attempt. Some birds have more than one nesting attempt per year.  

Clutch sizes differ not only among major taxonomic groups of birds and among species, but even within an individual. 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. Many factors appear to influence the number of eggs in a clutch. They include:

  • 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. Remember, she needs a lot of energy to produce each egg.
  • 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.
  • Geographic location. On average, within a species, birds lay smaller clutches when breeding at either lower latitudes or higher altitudes.

The topic of clutch size variation has been a source of fascination for generations of bird biologists. We know there is variation in clutch size both within, and among, species. We want to know more about what factors might influence this variation in clutch size. For example, within a given species, does the number of eggs a female lays per clutch vary with latitude? With altitude? Why do birds that are colonial or nest at relatively high densities often lay fewer eggs than their solitary-nesting relatives? Why do small species tend to have larger clutches than large species? It is thought that birds lay about the number of eggs that will produce the maximum number of surviving offspring over the parents’ reproductive lifetime, but how do they regulate this? What ecological factors determine this number for different birds?

Increasing our understanding of the biological and ecological factors that affect clutch size allows scientists and wildlife managers to make better-informed decisions about bird conservation.

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Cornell Lab of Ornithology

Cornell Lab of Ornithology