Partial cutting of woodlots in an agriculture-dominated landscape: effects on forest bird communities

We studied the short-term effects of partial cutting on the forest bird communities of mixed maple forests in an agriculture-dominated landscape in southwestern Ontario. Blocks that had been recently harvested were grouped according to provincial silvicultural guidelines (standard and heavy cuts) and compared with blocks that had been uncut for at least 24 years (old cuts). We found significant differences in forest bird community structure between standard and heavy cuts and between heavy and old cuts, but not between standard and old cuts. Heavy cuts had more species and more individuals than old cuts, the result primarily of greater numbers of early-successional species. Brown creeper (Certhia americana Bonap.) was the only species to show a significant negative response to harvesting and was the best indicator of old cuts, while indigo bunting (Passerina cyanea L.), brown-headed cowbird (Molothrus ater Bodd.), chestnut-sided warbler (Dendroica pensylvanica L.), American redstart (Setophaga ruticilla L.), and great crested flycatcher (Myiarchus crinitus L.) were all significant indicators of heavy cuts. Our research suggests that it is possible to protect native bird communities in southwestern Ontario by using the single-tree selection system to meet the minimum basal area targets and harvest intensities recommended in provincial silvicultural guidelines.

Extrapair Paternity Increases Variability in Male Reproductive Success in the Chestnut-Sided Warbler (Dendroica Pensylvanica), A Socially Monogamous Songbird

A monogamous mating system that includes extrapair fertilization can potentially generate higher variability in male reproductive success than monogamy without extrapair fertilization. That increased variability could provide a correspondingly higher opportunity for sexual selection and, thus, for the origin and persistence of sexual dimorphism in monogamous species. To determine whether extrapair fertilization enhanced the opportunity for sexual selection in a sexually dimorphic, monogamous bird species, we used microsatellite DNA typing to assess the prevalence of extrapair fertilization and its effect on variation in male reproductive success in a population of Chestnut-sided Warblers (Dendroica pensylvanica). We found that the level of extrapair fertilization in our study population was at the upper end of the range reported for bird populations (47% of nestlings had extrapair fathers; 61% of broods contained extrapair offspring). We also discovered that almost all extrapair offspring were sired by paired males resident on nearby territories. In addition, we found that variation in male reproductive success was substantially higher than variation in female reproductive success, and that extrapair fertilizations made a significant contribution to variation in male reproductive success. Together, those findings suggest that extrapair fertilization creates an opportunity for sexual selection on male traits in this population.

Food-chain chemistry, reproductive success, and foraging behaviour of songbirds in acidified maple forests of central Ontario

We examined the breeding success of two insectivorous bird species, the resident black-capped chickadee (Parus atricapillus) and the Neotropical migrant chestnut-sided warbler (Dendroica pensylvanica), in relation to food-chain chemistry at a healthy and a declining forest site in central Ontario, Canada. The health of sugar maples (Acer saccharum) was poorer and the pH of both soil and throughfall was lower at the declining site than at the healthy site. The calcium and magnesium concentrations and the calcium:aluminum ratio in the soil were also lower at the declining site than at the healthy site. The calcium, magnesium, and phosphorus concentrations and the calcium:aluminum ratio in sugar maple foliage were lower at the declining site than at the healthy site, indicating extensive acid leaching of the soil and foliage at the declining site. Caterpillars (Geometridae) at the declining site had a lower calcium concentration and beetles had a lower magnesium concentration than these groups at the healthy site. The calcium:aluminum ratios in the tissues of insects in both groups were lower at the declining site than at the healthy site, paralleling the foliar and soil chemical differences. There were, however, no differences in clutch or brood sizes, or percent hatch, for either bird species between the healthy and declining sites. Chickadees consistently foraged lower in the canopy at the declining site, a possible compensatory behaviourial response to canopy dieback and (or) lower food quality in the upper canopy.

Indirect effects of conifer release alternatives on songbird populations in northwestern Ontario

The effects of conifer-release alternatives on songbird densities (determined by territory mapping, mist netting, and banding) in four young (four to seven year old) spruce plantations were examined. Twenty to 38 (block dependent) species bred on each treatment block. Post-treatment data revealed no major treatment-related changes in breeding bird species composition. The mean density of all birds decreased from 6.9 pairs ha−1 pre-treatment, to 6.3 pairs ha−1 in the first post-treatment growing season (P > 0.05). Mean densities of the 11 most common species increased by 0.35 ha−1 on the control plots during the first post-treatment growing season, but decreased on treated plots by 1.1 ha−1 (brush saw), 1.6 ha−1 (Silvana Selective), 0.14 ha−1 (Release®) and 0.72 ha−1 (Vision®). Following the conifer-release treatments, Chestnut-sided Warbler (Dendroica pensylvanica) had lower (P < 0.05) mean densities on the brush saw- and Silvana Selective-treated plots than on the control plots and fewer (P < 0.05) female birds were captured in the first post-treatment year. Key words: brush saw, Chestnut-sided Warbler, conifer-release, Fallingsnow Ecosystem Project, glyphosate, mist netting, Silvana Selective, songbirds, territory mapping, triclopyr

Use of song types by the Chestnut-sided Warbler: evidence for both intra- and inter-sexual functions

Males of many wood warbler (Parulinae) species use different song types in different contexts, yet the exact functions of the two main song type categories remain unclear. We studied the use of songs by both experimental (males whose mate had been removed from the territory) and control male Chestnut-sided Warblers (Dendroica pensylvanica) during the dawn hour and midmorning throughout the breeding season. Unpaired males sang more accented-ending songs and fewer unaccented-ending songs than paired males during all observation periods. Accented-ending songs appeared to be used primarily in the absence of intrasexual stimuli, and the percentage of unaccented-ending songs that was used during the nesting cycle appeared to fluctuate directly with the intensity of defense by the male of both his female and his territory. During courtship the male sang accented-ending songs on those infrequent occasions when he did sing in the immediate presence of his female, regardless of her location and the presence or absence of other males. These critical observations seem most consistent with the conclusion that the accented-ending songs are primarily intersexual. The unaccented- and accented-ending categories of song types appear to be used mainly as intra- and inter-sexual messages, respectively.

A comparison of foliage discrimination learning in a specialist and a generalist species of migrant wood warbler (Aves: Parulidae)

The Bay-breasted Warbler (Dendroica castanea) is more generalized in its foraging behavior than the Chestnut-sided Warbler (Dendroica pensylvanica) where the two species winter together in Panama. I tested the hypothesis that the Bay-breasted Warbler would learned to discriminate between foliage types of differing profitability more rapidly than the more specialized Chestnut-sided Warbler. The performance of the two species in a laboratory discrimination experiment involving two foliage types was similar. Both species learned the foliage discrimination rapidly, in an average of 22 trials. Extinction occurred at a similarly rapid rate (17 trials) in both species as well. Differences in apparent foraging plasticity between the two species may be based primarily on their different responses to novelty, rather than differences in learning abilities. None of the warblers showed any tendency to attempt to solve the problem based on the irrelevant spatial cues prior to acquiring the correct response. Visual discrimination problems based on habitat cues are more relevant to insectivorous birds than are the more commonly administered spatial learning problems.