Body water dynamics at the onset of egg-laying in the Zebra Finch Taeniopygia guttata

The body water dynamics of egg-laying Zebra Finches Taeniopygia guttata was investigated by isotopic dilution of deuterium oxide or D2O. Two successiv e doses of D2O were administered to each hen, once during a non-reproductiv e period and once at roost on the evening before the first egg was laid. A 2- to 4-week period separated repeated dosing procedures for the 10 hens un der study. During prelaying, water comprised 56.2% (range: 50.1-67.1%) (or 7.82 ml[range: 6.89-9.27 ml]) of hens' body mass whereas at the onset of eg g-laying, hydration of body tissues had increased dramatically to 79.8% (ra nge: 75.0-90.2%) (or 11.92 ml [range: 10.44-12.99 ml]). The water equivalen t of 47.5% of body mass was turned over per day during laying and 40.6% dur ing prelaying. This represented a biological half-lift of the isotope of ap proximately 30 hours for both the prelaying and laying periods. This is the first time that a repeated isotopic dilution technique has been employed i n the same individuals to investigate changes in body water dynamics at the onset of egg-laying. This study indicates that the hen is undergoing large physiological changes in terms of water metabolism in preparation for egg- laying.