Bird-habitat relationships in southern Iowa forests
Recent avian population declines emphasize the need to quantify populations of at-risk species, assess bird community diversity, and better understand the habitat characteristics associated with bird population densities and community diversity. In response to bird declines, the Iowa Department of Natural Resources has established Bird Conservation Areas (BCAs) and has listed many at-risk bird species as Species of Greatest Conservation Need (SGCN). This study focuses primarily on Iowa forest birds, one of many groups of birds with declining populations. The goals of this study were to 1) estimate densities of breeding avian SGCNs and other species of management interest, 2) determine relationships between breeding bird densities and habitat metrics, and 3) quantify bird diversity and determine relationships between bird diversity and habitat metrics. Our study took place in three primarily forested Bird Conservation Areas in south-central Iowa. To estimate density, we used point counts over a grid of 493 points visited twice each breeding season from 2016 to 2019 and hierarchical distance sampling (HDS) models. To estimate relationships between bird density and habitat, we incorporated 13 habitat covariates over a range of spatial scales into HDS models for 10 species of conservation and management concern. To estimate bird diversity and determine bird diversity-habitat relationships, we estimated species richness by summing occupancy probabilities from HDS models for 77 total species and 24 SGCN and used multiple regression to compare estimates with 13 habitat metrics over a range of spatial scales. The five SGCN with the greatest estimated densities (in descending order) were Eastern Wood-Pewee (Contopus virens), Ovenbird (Seiurus aurocapilla), Common Yellowthroat (Geothlypis trichas), Acadian Flycatcher (Empidonax virescens), and Scarlet Tanager (Piranga olivacea), with mean estimated densities ranging from 0.195 – 0.698 birds/ha. Median estimated overall species richness within a 100-m point count radius was 20.4, and median estimated SGCN species richness was 4.0. Densities of Red-headed Woodpecker (Melanerpes erythrocephalus), Acadian Flycatcher, Wood Thrush (Hylochichla mustelina), Ovenbird (Seiurus aurocapilla), Common Yellowthroat, and Scarlet Tanager (Piranga olivacea) were positively associated with landscape scale forest cover at either a 1 km or 10 km scale, while Field Sparrow densities were negatively associated with landscape-scale forest cover. Landscape-scale forest cover within 10 km was also positively associated with overall species richness and SGCN species richness. Densities of forest birds such as Eastern Wood-Pewee, Acadian Flycatcher, Wood Thrush, Ovenbird, and Scarlet Tanager were positively associated with distance to forest edge, while densities of Field Sparrow and Common Yellowthroat, two species of edges and open areas, had a negative association with distance to forest edge. Edges and forest interiors (~800 m from edge) both had relatively high species richness overall and for SGCN; SGCN species richness was greatest in the forest interior. Bird-habitat relationships at smaller spatial scales were less consistent. For example, leaf litter ground cover was positively associated with densities for five of ten individual species analyzed, but negatively associated with Common Yellowthroat densities, overall species richness, and SGCN species richness. Our results suggest that intact interior forest away from edges is especially beneficial to SGCN and the bird community as a whole, but that the maintenance of some edge and open habitat is also needed for some species.