Date of Award

8-2008

Level of Access Assigned by Author

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Ecology and Environmental Sciences

Advisor

William E. Glanz

Second Committee Member

Thomas P. Hodgman

Third Committee Member

Rebecca L. Holberton

Abstract

The Rusty Blackbird (Euphagus carolinus) is a poorly understood wetland-breeding songbird that has experienced an 85–95% population decline since the mid 20th century. Although anecdotal evidence suggests that Rusty Blackbirds are “loosely colonial” and breed in bogs, fens, beaver-modified wetlands, and wooded swamps of the boreal forest. Ornithologists have few or no data with which to comprehend the species’ habitat use, nesting success, and social organization or spatial use of the landscape. To quantify habitat selection, I modeled Rusty Blackbird habitat occupancy and detectability (the probability of detecting a Rusty Blackbird) as functions of wetland site variables and sampling variables, respectively (Chapter 2). Furthermore, I studied nest site selection and nest survival for 35 Rusty Blackbird nests in northern Maine and northeastern Vermont (Chapter 3). Finally, I calculated 95% kernel density estimates of home range size for 13 radio-tagged individuals, eight of which were part of the first “loose colony” reported in New England (Chapter 4). Four variables predicted wetlands occupied by Rusty Blackbirds best: puddles (shallow, unconnected to flowing water and often ephemeral), wetland area > 0.5 ha, a coniferous upland, and the area of forest less than 5 m tall within 1 km buffers. This analytical description of Rusty Blackbird breeding habitat, along with my estimates of mean wetland occupancy (0.37 ± 0.12 SE) and detectability (0.19 ± 0.05 SE), can improve the efficiency of efforts to monitor the species. Nest success was 61.6% (n=32) overall, 32.7% (n=20) in regenerating clear-cuts and 100% (n=12) in nests not in regenerating clear-cuts. Compared to non-colonial individuals, birds at the colony had lower nesting success (22.4% vs. 66.2%), and larger home ranges (71.1 ha vs. 15.2 ha). The best a priori predictor of nest mortality (the abundance of firs < 3” diameter at breast height) and the best predictor of nest site selection (the abundance of firs 2-3 m tall) were highly correlated, suggesting that Rusty Blackbirds used maladaptive cues to select nest sites. Furthermore, both the aforementioned measures of pole-stage FIR abundance were excellent predictors of regenerating clear-cuts less than 20 years old, which was ultimately the best predictor of nest mortality. All failed nests were located in regenerating clear-cuts, and 73% of failed nests were predated. When anthropogenic changes create habitat that appears suitable, yet when chosen, ultimately results in decreased fitness, animals become caught in an “ecological trap”. I hypothesize that regenerating clear-cuts are an ecological trap for Rusty Blackbirds, because although they mimic natural nesting habitat (e.g. stunted conifers in bogs), the inevitable fragmentation increases nest predation risk. To reduce nest predation, I recommend that forest practitioners maintain at least a 100 m buffer around wetlands suitable for (or occupied by) Rusty Blackbirds, and that they delineate wetlands during May and June when ephemeral wetlands are present. Given the steep decline of this species and the need for effective management actions, it is critical to understand how social organization, spatial use and reproductive success vary under different timber management schemes.

Download

Share