What comes first, the zebra finch or the egg: temperature-dependent reproductive, physiological and behavioural plasticity in egg-laying zebra finches

Avian reproduction is generally timed to synchronize chick-rearing with periods of increased food abundance. Consequently, the energetically demanding period of egg production may coincide with periods of lower food availability, fluctuating temperature and more unstable weather. Little is known about the physiological mechanisms underlying temperature-induced variation in egg production. We therefore examined the influence of low ambient temperature (7 degrees C vs 21 degrees C) on reproductive output (e.g. egg mass, clutch size, laying interval, laying rate), daily food consumption and lipid variables in zebra finches Taeniopygia guttata. When faced with egg production at 7 degrees C, laying zebra finches increased energy intake by 12.67 kJ day(-1), and were thus able to maintain body condition (e.g. body mass, fat and muscle score) and circulating triacylglyceride at levels comparable to those at 21 degrees C. However, when producing eggs at 7 degrees C, females took longer to initiate egg laying (6.5 vs 6.1 days at 21 degrees C), and ultimately laid fewer eggs (5.5 vs 6.0 eggs) at a slower rate (0.90 eggs day(-1) vs 0.95 eggs day(-1)). These temperature-related declines in reproductive output were accompanied by decreases in modal (from 36.6 at 21 degrees C to 24.3 nm at 7 degrees C) and median very low density lipoprotein (VLDL) particle diameter (from 29.6 to 26.4 nm) and in the proportion of VLDL particles that were capable of passing through the pores in the ovary to access the developing ovarian follicles (i.e. particles with diameters between 25 and 44 nm; from 45.90% to 32.55%). However, variation in reproductive output was not related to any static concentration or structural measure of VLDL. Therefore, other temperature-dependent mechanisms must be involved in the physiological processes that regulate reproductive output of passerine birds at low ambient temperatures.