In birds, destruction of a clutch by predators (or removal by humans, for example the California Condor breeding program) results in double-clutching. The technique is used to double the production of a species' eggs, in the California Condor case, specifically to increase population size. The act of putting one's hand in a nest to remove eggs is known as "dipping the clutch".
Clutch size differs greatly between species, sometimes even within the same genus. It may also differ within the same species due to many factors including habitat, health, nutrition, predation pressures, and time of year. Clutch size variation can also reflect variation in optimal reproduction effort. Long-lived species tend to have smaller clutch sizes than short-lived species (see also r/K selection theory). The evolution of optimal clutch size is also driven by other factors, such as parent-offspring conflict.
Clutch size recorded in ornithological field notes may or may not include lost or broken eggs.
Mallard (Anas platyrhynchos), very large clutch or possibly from two females
Great Black-backed Gull (Larus marinus), small clutch
Masked Lapwing (Vanellus miles), typical clutch
Common Moorhen (Gallinula chloropus), small clutch
Lesser Spotted Eagle (Aquila pomarina), typical clutch
Feral Pigeon (Columba livia domestica), typical clutch
European Starling (Sturnus vulgaris), typical clutch
European Goldfinch (Carduelis carduelis), large clutch
Northern dusky salamander (Desmognathus fuscus), typical egg clutch
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