Population Dynamics of Willow Flycatchers in the Sierra Nevada
AuthorMathewson, Heather Alexis
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In the western United States, Willow Flycatcher (Empidonax traillii) populations have declined, and the magnitude of these declines varies across the species range likely because of ecological differences among subspecies and variation in biotic and abiotic factors. Three subspecies of Willow Flycatchers breed in California one is the federally endangered southwestern subspecies and the two subspecies to the north are California state endangered species. Once common throughout central and northern California these populations now breed exclusively in montane meadows in the Sierra Nevada and Cascades. Surveys in the 1980s and 1990s indicated that populations were declining in the south and central Sierra Nevada. My research focused on how multiple factors influenced reproductive performance of flycatchers and the implications of these estimates on population change. I examined these relationships in a low-density population of Willow Flycatchers breeding in the central Sierra Nevada from 1997-2008. Beginning in 2003 I also monitored two higher density flycatcher populations in the north Sierra Nevada to provide comparative estimates of reproductive performance. I grouped study sites into four regions based on geographic separation; however, I collectively refer to them as the central or north regions based on my comparative approach. The central and north regions differ in topography, vegetative characteristics, and land-use practices. This research addressed the associations among weather, timing of the breeding season, predation and brood parasitism rates, and meadow and nest-scale characteristics and, in turn, how these affected reproductive investment, fecundity, site fidelity, and potential for population growth. Flycatchers breeding in the central regions initiated nesting on average a week later than populations in the north and nesting was further delayed in years with late-season storms and low temperatures. Flycatchers in both regions ceased renesting attempts by the third week in July, and this pattern was consistent across all years regardless of nest initiation date. Thus, each day delay in the onset of nesting reduced a female's ability to renest. The effect that renesting probability has on fecundity is directly related to predation rates and females in the central region were not only limited by the length of the season by also by higher predation rates than the northern populations. Reproductive success has strong implications on immigration and emigration processes in a region. Our results concurred with other studies on Willow Flycatchers that reduced reproductive performance encourages breeding dispersal from an area and also may reduce emigration if flycatchers use social cues to determine territory settlement in subsequent years. Furthermore, high natal philopatry contributed to annual site occupancy and considerably reduced fecundity would disrupt this dynamic. I documented a declining trend in flycatcher numbers in the two central populations over the twelve years of this study. Sensitivity analysis of population change models indicated that population growth in this area depended on increases in mean annual fecundity estimates; maximum survival estimates could not compensate for the consistently low fecundity estimates currently observed in the central Sierra Nevada population. Results of this study indicated that nest survival rates in the central region were affected by weather patterns and meadow characteristics differently from populations in the north. Within the central study regions spring snowfall and a one-year lag affect of winter snowfall influenced predation rates on flycatchers likely through top-down effects of predator-prey dynamics. While these fluctuations occur naturally, nest survival rates in the central region may be further suppressed alterations to the riparian habitat that they rely on for breeding. I found a trend towards decreased nest survival with distance from forest edges in large meadows where the majority of breeding flycatchers are in the central regions. Alternatively, edge effects predicted survival in Warner Valley with lower survival close to the forest. Large meadows may act to concentrate nest predators because of the availability of resources. Small mammals are common nest predators in my study sites and they rely on the cover of riparian shrubs for protection from their predators. Thus, the proportion of shrubs available for movement and foraging will increase probabilities of encountering nests within the shrub matrix. In small meadows shrubs were denser and provided increased contiguous cover. Compared to the population in the north, the results of this study implied that reproductive success of flycatchers in the central Sierra Nevada was limited by length of the breeding season and nest predation, and that these factors were influenced by environmental stochasticity, population dynamics, and interactions between meadow-area effects and riparian shrub density. Improvements to the riparian ecosystem through restoration of natural hydrologic regimes and protection of riparian shrubs will improve habitat conditions for breeding flycatchers. Dense riparian shrubs will reduce predation pressure by influencing foraging strategies of predators and increased annual fecundity can buffer against the effects of unfavorable weather conditions.