Postfledging Survival and Habitat Use of Spotted Towhees (Pipilo maculatus) in an Urban Park

摘要

Habitat fragmentation, and the resulting increase in edge habitat, has important effects on birds, including the increased probability of nest predation, changes in habitat structure, and the increased presence of non-native plant species. It is critical to understand the effects of fragmentation at all stages of the avian life cycle, including the often overlooked postfledging period. Because much of juvenile mortality occurs during the immediate postfledging period, and juvenile mortality contributes substantially to population dynamics, it is necessary to understand if fledgling survival is reduced in edge habitats and if fledglingsu27 survival is influenced by their habitat use. During 2008 and 2009 I radio-tracked 52 fledgling Spotted Towhees (Pipilo maculatus) during the 30-day postfledging period in a 24-ha urban park near Portland, Oregon. Thirty-six fledglings (69%) survived the 27-day tracking period (an estimated 62.1% survived the entire 30-day postfledging period). At least 9 of 16 predation events were attributable to domestic cats (Felis domesticus) or Western Screech-owls (Megascops kennicottii). Although fledglings were more likely to be found near edges than the park interior, fledglings located closer to park edges had a higher probability of dying. However, I found that towhee nests were more likely to be found near edges, nests near edges produced more fledglings, and nestlings near edges were heavier. I used a STELLA-based stochastic model of nest success and fledgling survival to show that the benefits initially gained by nesting near edges were reversed during the postfledging period. The number of fledglings per nest that survived to the end of the 30-day postfledging period was significantly lower near edges than in the park interior. This apparent preference for nesting near edges, paired with higher fledgling mortality near edges, is consistent with the idea that edges are ecological traps. Fledgling habitat was significantly more structurally dense and had a greater abundance of non-native plant species, particularly Himalayan Blackberry (Rubus armeniacus), than nest habitat. Towhees avoided English Ivy (Hedera helix) for both nesting and care of fledglings. However, fledgling survival was not related to vegetation characteristics, which suggests that increased fledgling mortality near edges was a direct result of increased predator abundance or predation near edges, and was not an artifact of changes in habitat near edges. My results help to establish that fledgling survival and the unique habitat requirements of fledglings should be considered along with nest success and nest habitat when examining the effects of habitat fragmentation on bird populations. More broadly, this study has important implications for conservation, as it exemplifies how phenomena such as ecological traps created by anthropogenic changes in the environment can be overlooked if only one life history stage is studied.