Nutritional constraints on egg production in birds.

The production of eggs can make a major nutritional demand on a female bird. The scale of the required investment varies between species, depending on the size of each egg and the number of eggs laid. For large species, where egg weight is a small proportion of female body mass, the investment in the egg is small, but in some passerines the female will lay a clutch that weighs more than her own body weight. This requires a substantial investment of both energy and nutrients. Robbins (1981) estimated that the daily cost of egg production to wild birds varied between species from 37 % to 216 % of normal daily energy metabolism and from 86 % to 230 % of daily protein requirements. There is an extensive literature on the nutrients required for egg formation in domestic poultry and how these are obtained. But we know little about this process in wild birds. The findings on egg production from poultry research have little relevance to wild birds for the obvious reason that domestic poultry have been selectively bred to lay about one egg every day for the whole of their productive life. To sustain this remarkable scale of egg production it is obvious that the daily food intake must provide all nutritional requirements for egg formation. Body reserves could not make any substantial contribution, but can act as a temporary storage pool over a 24 h cycle to regulate delivery of ingested nutrients, as is known to occur for the Ca used in shell formation (Scott et al. 1982). In wild birds, egg production is an unusual, and critically important, event in the annual cycle of the bird. The resources for egg formation could come from three routes: through increased food intake, the use of body reserves, or metabolic changes in the female to permit a re-allocation of resources from body maintenance to egg formation. The relative importance of these various routes has still not been determined for any species of wild bird (Walsberg, 1983; Houston et al. 199%). The present paper first reviews whether there is any evidence that nutrient availability, and particularly essential amino acids, might constrain egg production in wild birds. It then considers whether some birds may use endogenous reserves to assist with the provision of limiting nutrients. Finally it considers the implications of these findings for the captive breeding of some bird species.