Abstract
The objectives of this study were to distinguish demographic source and sink habitats of Red-winged Blackbirds (Agelaius phoeniceus) and to determine the factors influencing source–sink status prior to fledging. In Boulder County, Colorado, I monitored 591 nests between 1995 and 1997 in breeding habitats that were either natural (wetlands, tallgrass prairie) or anthropogenic (hayfields, roadside ditches). To determine source–sink status, I calculated the productivity necessary to offset mortality. Predation was the main determinant of source–sink status, while brood parasitism had virtually no impact on this population. Anthropogenic habitats consistently functioned as sinks throughout the three-year study. Sources occurred only in natural habitats, and source sites had fewer buildings nearby than did sinks. In addition, climate variability may have influenced reproduction by influencing egg-laying dates. In 1995, possible source habitats included tallgrass prairie and wetlands. In 1996, tallgrass prairie...
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