Opportunism at work: habitat predictability affects reproductive readiness in free-living zebra finches

1. Opportunistic breeding (i.e. breeding whenever favourable conditions occur, regardless of interval or periodicity) is a strategy often adopted by animals breeding in habitats where food resources important for offspring survival are unpredictable. 2. One putative adaptation for this breeding strategy is the maintenance of a partially activated reproductive system year-round. Breeding readiness is assumed to be the default state and reproduction is therefore suppressed only when environmental conditions become inhibitory. 3. We tested this assumption in an opportunistic archetype, the Australian zebra finch Taeniopygia guttata, by comparing reproductive activation and suppression in free-living zebra finches in two climatically different habitats: (1) an unpredictable habitat of arid central Australia, where breeding is closely tied to aperiodic rainfall and can occur during any month of the year, and (2) a more predictably seasonal habitat in southern Australia, where breeding occurs during approximately the same months each year. We quantified reproductive activation by measuring gonad size and circulating gonadotrophin concentration during the breeding and nonbreeding seasons in the predictable habitat and within a nonbreeding period during drought in the unpredictable habitat. 4. We found reproductive readiness changed consistently between the breeding and nonbreeding state in the predictable habitat, but more nonbreeding birds in the unpredictable habitat maintained an activated reproductive system even though they had poorer body condition. We found no evidence for suppression of reproductive activity by stress hormones (i.e. elevated glucocorticoids). Furthermore, birds were able to activate the reproductive axis quickly when removed from inhibitory field conditions and responded equally well to various stimulatory cues.