Demographic characteristics of a Grasshopper Sparrow population in a highly fragmented landscape of western New York state
2003
We studied the breeding biology, site fidelity, and dispersal of Grasshopper Sparrows (Ammodramus savannarum) from 1996 to 2000 in a fragmented landscape in western New York State. Ten fields (1.8–13.2 ha) contained territorial male Grasshopper Sparrows during the study; total territorial males in the study area varied between 31 and 19 birds. In 1996, eight fields were occupied; five extinctions and two colonizations occurred between 1997–2000. Fields that suffered extinctions were smaller than fields in which subpopulations persisted or colonizations occurred. Adult return rates (0.33 vs. 0.16), nest success (0.59 vs. 0.25) and average number of fledglings/female/year (2.3 vs. 1.3) tended to be higher in fields ≥8 ha. Estimates of λ, the finite rate of increase, were 0.23 for small fields and 0.46 for large fields. Although sample sizes were small, our data suggest that return rates and productivity were greater in large than in small habitat patches. However, even the larger habitat patches in our study area appeared to function as population sinks, suggesting that the Grasshopper Sparrow population is unlikely to persist without immigration. Survival prospects for our study population are poor, given its demographic characteristics and the fragmented nature and continuing loss of grassland habitat. Our results suggest that conservation efforts in the Northeast should focus on protecting large patches of continuous grassland habitat.
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