The Nutritional Demands of Egg Production in Female Zebra Finches (Taeniopygia guttata)

Egg production is a demanding process for female birds. A number of studies have shown that body condition declines during breeding and reserves of lipid and protein are depleted. The aims of this study were to measure the relative contributions of exogenous and endogenous nutrients to the formation of eggs by captive female Zebra Finches and to investigate the transfer of material from the body reserves to the developing eggs. Analysis of eggs revealed that there was little change in egg size or composition between the eggs of a clutch. On average the eggs contain 58.3mg of lipid, 134.8mg of protein and 17.8mg of calcium. The amino acid composition of egg proteins was similar to that in domestic hens. The mean clutch size for the colony was 5.2 +/- 0.9 eggs. The onset of ovarian development was detected at around Day -4 of the laying cycle (ovulation of first egg on Day 0). Similarly, the oviduct grows rapidly from Day -4 to Day -1, then it declines in weight as the clutch is laid. Taking the above information into account the investment of protein and lipid in the clutch was calculated. Demand for egg nutrients increases rapidly from Day -4. Peak protein demand occurs on Day 1 of the cycle and for lipid on Day 0, after this demand gradually falls as the eggs are laid. The consumption and the nutritional value of seed was measured during the period of egg formation in an attempt to estimate the use of exogenous nutrients. There was no measurable increase in seed consumption by breeding pairs of Zebra Finches from day to day of the laying cycle. However, a marked increase in the consumption of cuttlefish bone was recorded. Comparing the composition of a clutch of five eggs and the food consumed at the time of their formation it was clear that the diet could not meet demand for egg protein or amino acids. There is a possibility that during egg formation there is an increase in digestive efficiency to liberate more nutrients from the diet. However, this would lead to relatively little protein becoming available. In terms of energy available the diet could go some way towards satisfying demands for lipid. Calcium for the eggshells could be supplied from the diet alone. The body reserves of protein and lipid were investigated during the period of egg formation. The lean dry weight and lipid of the pectoral muscle, ovary, oviduct and total carcass was measured, together with the dry weight of the leg muscles, heart, liver, gut and gizzard. There was a decline in the lean dry weight of the pectoral muscles equivalent to 15% of the protein in a five-egg clutch. The total carcass lean dry weight declined also by an amount equal to 76.8% of the protein in a five-egg clutch. The timing of this decline closely matched the demand for egg protein. Other organs, except the heart, followed a similar pattern of decline across the laying period. There is a decline in body lipid by 61% of the amount found in females at the start of the laying cycle. This amounts to much more than the lipid content of a clutch of five eggs. The bulk of this lipid is lost from lipid depots. Intramuscular lipid declines but the amount involved is relatively insignificant. Ash weight of the carcass showed no significant change and the calcium content of the ash was the same in post and pre-breeding females. In considering the budgeting of nutrients for the eggs; Body reserves of protein decline by an amount equivalent to 74.6% of the total reproductive requirement (eggs plus oviduct). If there is an increase in digestive efficiency of the order seen in a previous study then up to 15.2% of protein could come from the diet. The remainder could be made available by a decrease in female activity that would free protein from metabolism for reproduction. It is possible that the diet can make a significant contribution to lipid needed for the eggs. In addition, the body reserves of lipid fall by an amount much greater than that found in the eggs. This surplus of lipid indicates that as well providing for the eggs themselves the lipid reserves may act as an energetic buffer to offset the extra demands of egg production. Calcium is likely to be obtained entirely from the diet. There was no evidence to suggest reserves were used. The pectoral muscle was investigated in more detail. Direct measurement of the protein content of sarcoplasmic and myofibrillar fractions of muscle revealed a similar pattern of decline in both fractions to that seen in lean dry weight. Also, the total amount of measured protein lost by the pectoral muscle was close to the loss of lean dry weight, indicating that lean dry weight is a good indirect measure of protein in muscle. Gel filtration analysis of the sarcoplasmic fraction revealed three proteins, two of which were tentatively identified as myoglobin and haemoglobin. The remainder, of high molecular weight appeared to be responsible for the bulk of the decline in sarcoplasmic protein. Isotope labelled methionine was used to provide evidence that protein from the body reserves is transferred directly to the developing eggs. There was a significant difference in the isotope content of the pectoral muscle and oviduct between breeding and non-breeding females suggesting a higher turn-over of protein in the breeding birds