Habitat loss and fragmentation have been identified as important factors in the decline of grassland bird populations. However, population declines are apparent even in prairie ecosystems that remain relatively intact suggesting that additional factors are involved. The degradation of breeding habitat may be one such factor, but few studies have examined habitat-specific demography of grassland birds, and thus little is known of how changes in breeding habitat may be related to population declines. We addressed this question by comparing reproductive success of Chestnut-collared Longspurs (Calcarius ornatus) in patches of native prairie and in monocultures of crested wheatgrass (Agropyron cristatum), a grass introduced from Asia. Using recently developed methods for estimating nest survival rate, we found that, independent of habitat type, daily nest survival generally declined from egg laying to fledging. We also found a positive effect of clutch size on nest survival rate, which we interpreted as evidence for individual heterogeneity in nest survival. Finally, we found that the odds of a nest surviving a given day were 17% lower in the exotic habitat, and that nestlings grew more slowly, and had a smaller final mass in the exotic habitat. Despite having lower reproductive success in the exotic habitat, we found no evidence that Chestnut-collared Longspurs preferred to nest in the native habitat. Our results show that the introduction and spread of a commonly planted exotic grass has adverse fitness consequences for a grassland bird, and highlight the importance of maintaining native prairie.
© 2005, University of California Press. See the original published article in JSTOR.