Timing of reproduction in a Darwin's finch: temporal opportunism under spatial constraints

In seasonally unpredictable habitats, organisms cannot anticipate and prepare for a regular yearly reproductive period. Therefore, they opportunistically breed at any time of year. Opportunistically breeding birds are thought to maintain an activated reproductive system all year around. However, always being prepared can be costly and we therefore examined whether an opportunistic breeding strategy is realized in birds that are confined to harsh, climatically unpredictable habitats from which they cannot escape. We studied temporal variations in reproductive activity in small ground finches (Geospiza fuliginosa) in the Galapagos archipelago, Ecuador, by determining gonad sizes and plasma luteinizing hormone (LH) concentrations. We compared populations from the arid coasts of St. Fe and St. Cruz islands with birds from the St. Cruz humid highlands. The population from the arid St. Fe island increased gonad sizes and LH concentrations at least four months earlier in a wet (El Nino) year compared to the following dry year, indicating high temporal flexibility and opportunistic breeding. However, birds had completely regressed gonads for about 10 months between breeding events during a period of drought. Long-term changes in environmental factors such as ambient temperature, daylength and light intensity did not appear to influence reproductive activity. Instead, environmental signals more directly connected to the occurrence of rains such as humidity, barometric pressure, or rainfall itself may stimulate reproduction. Our data demonstrate that small ground finches regulate the onset of reproductive activity flexibly like other opportunistic breeders. However, they differ from those by their unusual long-term quiescence of the reproductive system during dry periods, which might minimize costs. Our findings suggest that within the continuum from seasonal to opportunistic breeding strategies, organisms evolutionarily optimize reproductive strategies to match local ecological conditions.