Influence of light and temperature on abundance of swallow nests
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Habitat parameters affecting survival and reproduction can be enhanced or degraded by human activities including disturbance and development. While development of human-made structures can obviously reduce a species’ survival and reproduction through loss of habitat, human structures might also promote population growth by providing nesting and roosting habitat. My study examined the overlap (spatially and temporally) of Cliff Swallows (Petrochelidon pyrrhonota) and Cave Swallows (P. fulva) during nesting season and seasonal use of five bridges in Central Texas by both species. The five sites are: B1-Colorado River, B2- Plum Creek, B3- Blanco Bridge, B4- Guadalupe River, and B5- Blanco State Park. Specifically I examined seasonal use of bridges by both species of swallow and spatial isolation of nests at nest sites based on thermal and ambient light properties. For both years of this study, Cliff and Cave Swallows were present during our surveys; while numbers were variable between years and among bridges, Cliff Swallows were the dominant species present. In contrast, Cave Swallows were recorded only at two of the five sites: B2-Plum Creek, and B5- Blanco State Park during both years. I found no interaction (F = 0.901, P = 0.493) between bridges and data loggers for mean temperature (°C) but the three bridges (B2, B3, B5) differed (F = 15.104, P <0.001) in mean temperature with B2 being warmer than B3 and B5. For mean light (Lux), I found a interaction (F = 63.75, P <0.001) between bridge and data logger with interior spans of all bridges receiving less light than the outer spans and bridges differing in overall ambient light; in order of decreasing light: B3, B2 and B5. Cave Swallows were found only within the interior spans of bridges (i.e. darker areas) and at the two bridges that received the less light. However, Cave Swallows did not appear to be influenced by temperature because they occupied one the warmest (B2) and coolest (B5) bridges. Based on my results, Cave Swallows are selecting bridge sites that are relatively dark but do not appear to be influenced by temperature at the nest site. Future studies are warranted to continue investigating the nest site selection of Cave Swallows as they continue to expand their range into the south western United States.