Cooperative Fish and Wildlife Research Units Program: Montana Cooperative Wildlife Research Unit
Education, Research and Technical Assistance for Managing Our Natural Resources

Avian life history theory predicts that high adult survival and populations near carrying capacity favour reduced clutch size and increased investment per offspring to increase juvenile survival. Consistent with this hypothesis, south temperate and tropical birds are commonly longer-lived and have smaller clutch sizes. Yet, comparative analyses of juvenile survival are largely lacking. We measured post-fledging survival for eight south-temperate passerines in South Africa, and combined our measurements with estimates from a review of published studies from around the world to test two predictions: 1) survival through the first few weeks post-fledging increases with adult survival and reduced brood size, and 2) south temperate and tropical species exhibit higher post-fledging survival than north temperate species associated with higher adult survival and smaller brood sizes. Using post-fledging survival estimates for 73 passerine species, we found that variation in post-fledging survival across species was explained by region and not by adult survival or brood size. South temperate species had higher post-fledging survival than both north temperate and tropical species. Post-fledging survival did not differ between north temperate and tropical species, but tropical species were under-represented. Our results highlight that adult and juvenile mortality do not simply covary, but they both differ between the northern and southern temperate regions. Given the importance of age-specific mortality to life history evolution, understanding these geographic patterns of mortality is important.