Current address. BIOME Group, Department of Animal & Plant Sciences, Sheffield University, Sheffield S10 2TN, UK.
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Investigating geographic variation in clutch size using a natural experiment
Article first published online: 19 AUG 2005
DOI: 10.1111/j.1365-2435.2005.01016.x
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How to Cite
EVANS, K. L., DUNCAN, R. P., BLACKBURN, T. M. and CRICK, H. Q. P. (2005), Investigating geographic variation in clutch size using a natural experiment. Functional Ecology, 19: 616–624. doi: 10.1111/j.1365-2435.2005.01016.x
Publication History
- Issue published online: 19 AUG 2005
- Article first published online: 19 AUG 2005
- Received 1 November 2004; revised 14 March 2005; accepted 31 March 2005
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Keywords:
- Ashmole's hypothesis;
- clutch size–lay date relationships;
- introduced birds;
- latitudinal gradients;
- seasonality
Summary
- 1Clutch sizes generally increase with latitude, and are smaller at southern latitudes compared with equivalent northern ones.
- 2Descriptions of such patterns and attempts to identify their causal mechanisms are complicated as different species, with different ecological traits are often compared in different regions. We reduce such problems by using the introduction of 11 passerine species from the UK to New Zealand as a natural experiment to explore interspecific geographical variation in clutch size.
- 3Nine species have significantly smaller clutches in New Zealand than the UK. Seasonality, measured both by climate and how birds respond to variation in resource availability, is also lower in New Zealand. Comparing across species, the magnitude of clutch size change is unrelated to the magnitude of reduced seasonality that each species experiences.
- 4Such observations are partly compatible with Ashmole's hypothesis that areas with high seasonality have large clutch sizes (higher winter mortality results in a breeding population that is significantly lower than the environment's carrying capacity, and hence in extra resources for rearing chicks). However, additional data on seasonal changes in resource availability and population densities, combined with comparative data on survival and nest predation rates, are required to evaluate fully the mechanisms generating smaller clutches in the southern hemisphere. We discuss the potential determinants of geographical variation in the patterns of temporal variation in clutch size.