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Timing and proximate causes of mortality in wild bird populations: testing Ashmole’s hypothesis

May 1, 2012
  1. Fecundity in birds is widely recognized to increase with latitude across diverse phylogenetic groups and regions, yet the causes of this variation remain enigmatic.
  2. Ashmole’s hypothesis is one of the most broadly accepted explanations for this pattern. This hypothesis suggests that increasing seasonality leads to increasing overwinter mortality due to resource scarcity during the lean season (e.g., winter) in higher latitude climates. This mortality is then thought to yield increased per-capita resources for breeding that allow larger clutch sizes at high latitudes. Support for this hypothesis has been based on indirect tests, whereas the underlying mechanisms and assumptions remain poorly explored.
  3. We used a meta-analysis of over 150 published studies to test two underlying and critical assumptions of Ashmole’s hypothesis: first, that ad ult mortality is greatest during the season of greatest resource scarcity, and second, t hat most mortality is caused by starvation.
  4. We found that the lean season (winter) was generally not the season of greatest mortality. Instead, spring or summer was most frequently the season of greatest mortality. Moreover, monthly survival rates were not explained by monthly productivity, again opposing predictions from Ashmole’s hypothesis. Finally, predation, rather than starvation, was the most frequent proximate cause o f mortality.
  5. Our results do not support the mechanistic predictions of Ashmole‘s hypothesis, and suggest alternative explanations of latitudinal variation in clutch size should remain under consideration. Our meta-analysis also highlights a paucity of data available on the timing and causes of mortality in many bird populations, particularly tropical bird populations, despite the clear theoretical and empirical importance of such data.


Citation Information

Publication Year 2012
Title Timing and proximate causes of mortality in wild bird populations: testing Ashmole’s hypothesis
Authors Daniel C. Barton, Thomas E. Martin
Publication Type Thesis
Index ID 70040648
Record Source USGS Publications Warehouse
USGS Organization Coop Res Unit Seattle
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