Habitat use of regenerating clearcuts by mature-forest birds during the post-breeding period
Andrew C. Vitz, MS
Advisor: Amanda Rodewald
Declines of mature-forest birds have stimulated numerous studies of breeding and wintering ecology. However, until recently, little attention has been given to the post-breeding period, which spans from completion of nesting until commencement of migration (ca. 2-3 months). Recently, several mature-forest breeders have been documented moving from their breeding territories into early successional forests during the post-breeding period. Reasons for these shifts are largely uninvestigated and remain unclear. Post-breeding birds must avoid predation and starvation while accumulating fat reserves to facilitate migration. Moreover, post-breeding birds are especially vulnerable because juveniles are inexperienced at foraging and evading predators, while adults undergo their pre-basic molt inhibiting flight. Mortality rates can be extremely high for post-breeding birds and a paucity of post-breeding habitat may inflate mortality rates. In this thesis, I examined the extent to which (1) post-breeding habitat use by mature-forest birds was associated with microhabitat characteristics (structural and food resources), (2) patch area (i.e., small vs. large clearcuts), and (3) habitat edge.
I studied post-breeding birds at 12 regenerating hardwood clearcut sites (3-7 years post harvest). Sites were located within Athens, Vinton, Jackson, and Gallia counties in southeast Ohio, which is found within the Ohio Hills physiographic province. Sites were equally divided between small (4-9 ha) and large (12-18 ha) stands. I used constant effort mist-netting to document mature-forest bird abundance between June 15 and August 16 in 2002 and 2003. Nine nets were arranged systematically at each site, and each site was sampled approximately once per week with a total of 9 visits per year. Hatch-year (HY) individuals and after-hatch-year (AHY) birds showing a wrinkled brood patch, flight feather molt, or extensive body molt (>25% of body) indicating completion of nesting were identified as post-breeding individuals. At each net, I sampled fruit (3x per season), arthropod (3x in 2002, 2x in 2003), and vegetation characteristics (once during study).
In total I captured 1614 post-breeding mature-forest birds of 30 species with 7331 net hours. The most common species captured in descending order were Ovenbird (Seiurus aurocapillus), Worm-eating Warbler (Helmitheros vermivorous), Red-eyed Vireo (Vireo olivaceus), Hooded Warbler (Wilsonia citrina), Scarlet Tanager (Piranga olivacea), and Wood Thrush (Hylocichla mustelina). The majority of captures consisted of hatch-year individuals (%).
For analysis I selected 6 of the most common species and 3 guilds (all mature-forest birds, hatch-year birds, after-hatch-year birds). Microhabitat features expected to affect post-breeding habitat use were examined using an information theoretic approach. Seven a priori models were created and subsequently ranked using the food (fruit) and cover (low vegetation and average canopy height) variables for each of the 3 guilds and species of interest. Hierarchical linear models using Proc Mixed (SAS) were used to calculate Akaike’s Information Criterion (AIC) values to rank models. Capture rates of Scarlet Tanager were positively related to fruit abundance, and best explained by the model containing fruit abundance. In general, capture rates, for other species and guilds of interest, were best explained by models composed of vegetation structure. For all guilds and species of interest (except Red-eyed Vireo and Scarlet Tanager), an inverse relationship existed between capture rates and low vegetation (stem and leaves < 3 m tall) and a positive relationship with average canopy height. Overall, habitat selection by post-breeding birds seems guided by both food and cover variables.
Capture rates of the 3 guilds and 6 species of interest also were compared between large and small stands and distance from the mature forest edge. I used a split-plot analysis of variance to evaluate all mature forest-breeders where the whole-plot (site) consisted of 3 split-plot units (20, 50, 80 m from edge). To examine responses of the 3 guilds and 6 species of interest, I used a multivariate analysis of variance (MANOVA) to test for differences in capture rates between stand size and another to test for differences with distance from the edge. A posteriori univariate analysis of variance identified which guilds and species showed the strongest responses to both stand size and distance to the edge. As a whole, mature-forest post-breeders were captured more in small stands than in large stands. This relationship was strongest for hatch-year individuals, Scarlet Tanager, Wood Thrush, and Hooded Warbler. Interestingly, captures of mature-forest post-breeders increased with distance to the mature forest edge. Causes of the apparent area and edge sensitivity are unclear. Microhabitat characteristics (i.e., fruit, arthropods, low vegetation structure, average canopy height) did not significantly differ with stand size nor distance from the edge. Nevertheless, managers aiming to improve value of clearcutting to mature-forest birds during the post-breeding period may need to consider size and shape of clearcuts.
The post-breeding period may be an important phase in the avian life cycle because mortality rates can be extremely high, and this vulnerability may be partly ameliorated by habitat (e.g., dense cover). This study is (1) the first to evaluate the influence of microhabitat features on habitat use within regenerating clearcuts and (2) to assess area and edge sensitivity of post-breeding birds. These results demonstrate that mature-forest post-breeding birds are a diverse and abundant component of bird communities in early successional forests, and that post-breeding habitat selection is a function of both vegetation structure and fruit resources. Consistent with breeding ecology of mature-forest birds, post-breeding birds appear to be sensitive to edge and area, which may warrant special management consideration. My findings are important because they both elucidate the ecology of the poorly understood post-breeding period of migratory songbirds and identify habitat features that can be manipulated by wildlife biologists trying to improve value of early successional forests to a wider bird community.