| Within a species, the abundance of individuals varies in time and space. This dissertation research examines processes giving rise to patterns of spatial and temporal variation in the abundance of multiple species of forest-breeding migratory songbirds at the Hubbard Brook Experimental Forest (HBEF), New Hampshire, USA. Analyses of time series data for five migratory songbird species reveal a general pattern of negative-density dependence in population growth rates. Additionally, levels of food abundance influenced population growth rates for three of these species. These results indicate that bird density and food abundance are two primary factors influencing population growth rates, and subsequent temporal variation in abundance, of migrant songbirds.; In addition to exhibiting a high degree of temporal variation in abundance, bird species also exhibited a high degree of spatial variation in abundance. Based upon three years of survey data collected at 371 locations across the 3160 ha HBEF, most bird species exhibited intraspecific spatial variation in abundance. Furthermore, patterns of spatial variation in abundance remained stable across years. For five species, spatial variation in abundance was fully explained by spatially varying habitat factors. However, the remaining species continued to exhibit spatial variation in abundance after removing habitat effects, indicating that alternative ecological mechanisms (e.g., conspecific attraction) may be operating to increase local abundance beyond that expected based on habitat associations.; For the black-throated blue warbler (Dendroica caerulescens) one consequence of spatial variation in abundance is that specific locations on the landscape are occupied consistently from year-to-year, while others are occupied intermittently. Consistently occupied sites had more deciduous vegetation and a well-developed dense understory. Warblers were more abundant and arrived earlier in the breeding season at these sites. While there were no differences in average reproductive output across sites with different occupancy rates, these results indicate that the majority of individuals are found, and, subsequently, the majority of young are produced, within relatively few locations on the landscape. Combined, the results indicate that, even within relatively undisturbed, homogenous landscapes, there is significant intraspecific temporal and spatial variation in bird abundance which appears to result from subtle variation in environmental and behavioral factors. |
