Date of Award
Doctor of Philosophy
Ecology, Evolution, and Organismal Biology
Ecology and Evolutionary Biology
Rolf R. Koford
David L. Otis
Loss of habitat is one of the primary factors affecting population declines of grassland birds, and recovery efforts have focused on increasing the amount of grassland habitat in the landscape. Assessing the value of habitat restorations for grassland birds is an essential component of grassland bird conservation. We compared grassland bird habitat use, reproductive success, nestling growth rates, nestling baseline corticosterone, and blood glucose levels among restored grasslands planted with seed mixes of varying plant species diversity. In addition, we tested the ability of a conspecific song playback system to attract Henslow's sparrows to previously unoccupied restored habitat. We selected 4 grassland planting types, cool-season, warm-season (newly planted and mature), and high diversity for our study to encompass the range of planting mixtures typically available to land managers. The most common bird species we encountered were Bobolink (Dolichonyx oryzivorus), Common Yellowthroat (Geothlypis trichas), Red-winged Blackbird (Agelaius phoeniceus), and Sedge Wren (Cistothorus platensis). Bird densities overall were not consistently higher or lower in any one of the 4 planting types. Bobolink densities, however, were higher in Cool-season fields than in any of the other field types. Models of the relationships between bird density/bird species richness and habitat characteristics revealed that both vegetation characteristics and food resources were important in explaining grassland bird densities. Our habitat models showed that different species, even those within a species category, were influenced by different habitat characteristics. Given the importance of cool-season, non-native grass plantings for Bobolinks in our study, consideration must be given to the impact that elimination of these plantings may have on the future of Bobolink populations. Red-winged Blackbird nest survival was influenced by year, visual obstruction, and variation in the availability of invertebrate food resources throughout the nesting season. We found evidence for a difference in daily nest survival among planting types (P=0.06). Specifically, estimated nest survival was more than twice as high in mature warm-season fields (36%) than in cool-season fields (14%). Red-winged Blackbird nestling size at fledging differed among grassland planting type. Male nestlings were larger than females at fledging with regard to mass, wing, and tarsus measurements. Nestling growth rates did not differ either among grassland restoration planting types or between sexes. Invertebrate food resource availability did not appear to affect growth rates of nestling Red-winged Blackbirds. Red-winged nestling baseline corticosterone levels were lower in the warm-season planting type than in either cool-season or high diversity plantings. We found no evidence of differences in baseline corticosterone levels or blood glucose levels between males and females. We found no relationship between baseline corticosterone levels and body mass or between baseline corticosterone levels and blood glucose for male or female nestlings. Blood glucose levels in male nestlings had a significant positive relationship with nestling mass, but not in female nestlings. Management activity and brood size had positive relationships with baseline corticosterone, suggesting that more intensive management activity and larger brood sizes were related to increased stress levels. Nestling age and temperature during the nestling period were positively related to blood glucose levels. We successfully attracted Heslow's Sparrows to 3 of 7 treatment plots using conspecific song playbacks and we found no Henslow's Sparrows in control plots. The addition of social cues using playback systems in restored grassland habitats may aid conservation efforts of Henslow's Sparrows to available habitat. We conclude a variety of planting types and management strategies may be necessary to successfully conserve grassland birds.
Jennifer Amy Vogel
Vogel, Jennifer Amy, "Grassland bird response to enhanced vegetation diversity in restoration plantings in the Spring Run Complex of northwestern Iowa" (2011). Graduate Theses and Dissertations. 10324.