Bird Occupancy and Nesting Success in Relation to Habitat Structure in the Cross Timbers Oak Savanna of Kansas

Abstract

Oak savanna, once widespread across central North America, has functionally vanished from most of its range due to land conversion or fire suppression and subsequent afforestation. My objective was to quantify avian habitat associations and nest success across a gradient from open-canopy oak savanna to closed-canopy, afforested conditions in the Cross Timbers region of southeastern Kansas during the typical songbird breeding season. Species-specific site occupancy probabilities and daily nest survival rates were modeled against vegetative variables along the habitat gradient. Occupancy for 14 species was strongly associated with vegetative variables, such as landscape-level canopy cover and point-count-scale tree density, tree canopy cover, and shrub density. Savanna-associated species included Northern Bobwhite (Colinus virginianus), Bewick’s Wren (Thryomanes bewickii), Northern Mockingbird (Mimus polyglottos), Field Sparrow (Spizella pusilla), Dickcissel (Spiza americana), and Orchard Oriole (Icterus spurius). Arboreal habitat structure had less of an effect on daily nest survival rate. Daily nest survival showed positive trends with increasing shrub density for Brown Thrasher (Toxostoma rufum) and Northern Mockingbird. Daily nest survival of Mourning Dove (Zenaida macroura) was negatively, but weakly, associated with increasing canopy cover. Daily nest survival of Yellow-billed Cuckoos (Coccyzus americanus) was unrelated to any habitat variable. Several of the species I found to be associated with savanna are of conservation concern in Midwestern states. Local occurrences of these species might benefit from reductions in tree density within otherwise closed-canopy forest. Keywords: