Song Post and Foraging Site Characteristics of Breeding Varied Thrushes in Northwestern California

Abstract

We used a hierarchical approach to describe habitat characteristics of song posts and foraging sites used by Varied Thrushes (Ixoreus naevius) in coastal redwood (Sequoia sempervirens) forests of northwestern California. We measured mesohabitat (0.04-ha circular plots) and microhabitat (0.5-m radius) scale attributes centered on occupied and random song posts and foraging locations at four study sites during March–August of 1994 and 1995. Ninety-five percent of song posts were in trees or snags. Male thrushes used song posts with low foliage density near the top of large conifers (microhabitat scale), located on steeper slopes, surrounded by a high density of trees, and centered in drainages closer to water (mesohabitat scale) as compared to random locations. Varied Thrushes foraged predominantly on the ground early in the breeding season, then subsequently included fruit in their diet after the young had fledged. Although many variables were correlated with ground foraging locations, microhabitat foliage density had the greatest explanatory power, indicating thrushes selected foraging locations primarily at the microhabitat scale, and emphasizing the importance of measuring habitat characteristics at the appropriate spatial scale. Abrupt forest edges, such as those produced by clearcuts, may reduce habitat suitability for Varied Thrushes possibly explaining their absence from small forest fragments during the breeding season.

Ecological and Economic Services Provided by Birds on Jamaican Blue Mountain Coffee Farms

“…To avoid including collinear variables in regression models (Legendre & Legendre 1998), we entered all 4 dependent bird variables, the difference between control and exclosure plants in number of berries infested by borers ( CBB ), and all 12 independent vegetation com-plexity variables into a Spearman's rank correlation matrix (Greaves et al 2006). We first chose the variable with the highest Spearman's rank correlation coefficient for all dependent variables and then eliminated all independent variables that were collinear (r > 0.60) with the selected variable (Beck & George 2000). We repeated this process with the remaining independent variables until we established a subset, which was then entered into a forward-stepwise regression (α = 0.15 to enter; Anderson et al 2000).…”

The Influence of Perch Height Selection on Vocalization of Pied Triller, Lalage nigra (Aves: Campephagidae) in Suburban Landscapes, Sarawak

“…Call type A in this study however shows a positive correlation for all parameters except duration of vocal output. Calls emitted by the bird tend to increase in minF, maxF, domF, amp and number of syllables with the increase in perch height as to improve interactions with conspecifics (Beck & George, 2000).…”

“…Songs with higher minimum and dominant frequency are more favourable as they help to project the song louder and further away (Luther & Baptista, 2010;Magalhães Tolentino et al, 2018). Furthermore, emitting songs tend to make the bird to be more conspicuous (Beck & George, 2000) as the duration of vocal output is longer compared to calls. Therefore, emitting songs increases the probability of predation by cats (Møller, 2011) or raptors (Beck & George, 2000).…”

“…Furthermore, emitting songs tend to make the bird to be more conspicuous (Beck & George, 2000) as the duration of vocal output is longer compared to calls. Therefore, emitting songs increases the probability of predation by cats (Møller, 2011) or raptors (Beck & George, 2000).…”

Drivers of winter population cycles in the Varied Thrush (Ixoreus naevius)

“…Varied Thrush is a boreal forest species that undergoes considerable variability in winter abundance but is neither a small mammal specialist nor a typical boreal seed-eating species. Instead, its main diet during the breeding season consists of ground-dwelling arthropods (Beck and George 2000), switching to fruits, berries, and nuts, including acorns, in the winter (Beal 1915;Hagar 1960;George 2020). Consequently, the factors affecting winter abundance are not obvious.…”