Roger Moseby, of Wanaka, asks :-
A hen lays an egg a day. Where do the eggs laid by seagulls, sparrows, and other birds go?
Sheena Townsend, a zoologist at the University of Otago, responded.
Chickens, and many other birds, lay over a number of days, adding to their clutch of eggs until the nest is complete. How a bird determines when to stop laying can be divided into two main categories. A hen continues laying because chickens are indeterminate layers. This means that if an egg is lost due to predation, it will be replaced, until a full clutch is achieved. Our breakfast table benefits from this reproductive strategy, but if allowed, a broody hen will lay 8 to12 eggs and begin incubating.
Many gulls are also indeterminate layers and may be able to compensate for some lost eggs though not to the extremes we see in domestic chickens. Other birds, like House Sparrows, cannot compensate for lost eggs and will begin to incubate even if only a partial clutch remains. These birds are known as determinate layers, laying a limited and pre-determined number of eggs each time they nest. Whichever strategy is used, once a full clutch is reached, laying stops and incubation begins.
Whether determinate or indeterminate layers, many birds will re-nest if an entire clutch is lost early in the breeding season. Conservation managers have used this to help struggling species and perhaps the world’s most famous example is close to home. The Chatham Island’s highly endangered Black Robin lays small clutches of one to three eggs. In 1980, only one female remained. Managers encouraged Old Blue to lay more eggs each breeding season by removing her first clutch entirely. Females of another species, the Chatham Island Tomtit, were “tricked” into incubating these eggs, a practice known as cross-fostering. Old Blue re-nested, greatly increasing the number of successfully hatched Black Robins during a critical stage of the species’ recovery.