Reduced clutch-size is correlated with increased nest predation in exotic Turdus thrushes

A fundamental prediction of life-history theory is that individuals should reduce their reproductive investment per breeding attempt when the risk of nest predation is high. We tested this trade-off in two species of exotic Turdus thrushes in New Zealand (Common Blackbird (T. merula) and Song Thrush (T. philomelos)). Differences in nest survival were estimated between two habitats (horticultural and agricultural) and among four replicate horticultural sites. Overall, we identified shared patterns of nest survival within a habitat but a significant interaction with different habitats. Critically, as predicted by life-history theory, we found that clutch-size consistently and positively co-varied with site-specific rates of nest survival. Although site-specific difference in habitat and variation in female quality cannot be ruled out as explanations for this pattern, our results support the hypothesis that females can manipulate their reproductive effort across different predation regimes. Future experimental work is required to test these alternate hypotheses explicitly, and to demonstrate the behavioural cues that might lead to variable levels of reproductive effort and trade-offs of maternal resources.