Small birds in temperate zones increase body mass in winter (Lehikoinen, 1987). We investigated daily and annual variation of body mass and fat reserves of locally-reared Australian zebra finches Taeniopygia guttata, by exposing them to different photoperiods, feeding periods, and temperatures.
Experiments with long and short photoperiods and long and short feeding periods, showed that long photoperiods increased body mass and fat reserves of the zebra finches, and readiness to breed, independently of the actual feeding period.
Furthermore, the zebra finches in indoor aviaries with constant temperature (22-24°C) and in outdoor aviaries with ambient temperature, both exposed to the natural daylength changes of Bielefeld, Germany (52 °N), had high dawn body mass in summer (12.9 and 12.0-12.4g, respectively) and low in winter (10.7 and 11.1 g, respectively). Thirty to sixty percent of these mass changes were related to changes in fat reserves, so that the finches had only 0.1-0.2g of metabolizable fat reserves in short photoperiods (or in winter), which increased up to 1.5g in long photoperiods (or in summer).
Indoor finches consumed more seeds in summer than in winter (3.3 vs. 2.7g/day), while outdoor finches consumed 4-5g of seeds per day throughout the year, which probably represented the limit of energy intake for a 11-13g bird (Kirkwood, 1983). Nightly mass loss, increasing from 0.7g in summer up to 2.0g in winter, was highly positively correlated with night length, not influenced by ambient temperature. Foraging before dawn and after dusk, roosting with well-filled crops, and decreasing body mass and fat reserves, seem to be adaptations of zebra finches for survival in winter. The summer fattening probably accelerates reproduction in this opportunistic breeder, by allocating more time to reproductive behaviour and more endogenous nutrients to egg-formation.