Variation in clutch size, egg size variability and reproductive output in the Desert Finch (Rhodospiza obsoleta)

We studied the degree of variability and repeatability of egg size in clutches in the context of clutch size and reproductive output of a double-brooded Desert Finch population (Rhodopsiza obsoleta). The within-clutch coefficient of variation (CV) was significantly greater in first (mean=1.92, 95% CL: 1.54–2.30) than in second broods (mean=1.24, 95% CL 0.94–1.55). Repeatability estimate for egg volume was smaller for first (0.51, 95% CL: 0.26–0.77) than for second broods (0.74, 95% CL: 0.52–0.90). Simultaneously, mean clutch size was bigger for first (4.61, 95% CL: 4.31–4.92) than second broods (3.69, 95% CL: 3.40–3.98). However, the number of nestlings and the number of fledglings were not different between first and second broods. The observed differences in clutch size and the within-clutch variation for egg size between first and second broods lead to a similar overall breeding output. These results point to a possible strategy applied by females during the two egg-laying periods in the breeding season, and depend on the fast-changing and extreme environmental conditions in arid habitats.