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The role of fat and protein reserves in the annual cycle of the Grey-backed camaroptera in Uganda (Aves: Syhidae)

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Abstract

The annual cycle of Camaroptera brevicaudata was studied close to the equator in Uganda, particular attention being paid to the role of fat and protein reserves. Camaropteras breed at irregular intervals, two or even three times per year, whenever rainfall and increased primary productivity result in a favourable insect food supply. However, there is no consistent relationship between the onset of breeding and the onset of rainfall or any other environmental variable. It is concluded that an adequate protein reserve is the proximate factor controlling the onset of breeding.

During the laying cycle the fat and protein reserves of female camaropteras fall significantly, though not synchronously. The decline of the fat reserve, which occurs on the days when eggs are being ovulated, appears to be related to loss of feeding time caused by searching for calcareous material needed for egg shell production. The decline of the protein reserve occurs almost throughout the laying cycle, but is small in relation to the total protein needed for the formation of a clutch of eggs. It is concluded that it represents a loss from a store of specialized protein needed for egg production, perhaps particular amino-acids that are scarce in the normal diet. A similar small decline in the protein reserve of males may have a similar function in relation to sperm production. Courtship feeding of female camaropteras by the males is most intensive just before and during the laying cycle. It probably contributes to the nutrition of females by helping them attain adequate fat and protein reserves prior to breeding and by compensating for the time lost searching for calcareous material needed for egg shell production.

The moult of camaropteras begins at about the same time each year, the timing probably being determined largely by the timing of the first, post-juvenile, moult. The moult is extremely flexible—it can be interrupted if conditions become suitable for breeding, and its rate can be slowed if it coincides with severe dry conditions. This is an adaptation to an environment in which favourable conditions are irregular and too short to accommodate both breeding and moult.

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