Boreal Forest Birds Research Articles

Territorial Responses of Boreal Forest Birds to Habitat Gaps

We used playback trials to determine whether birds will cross treeless gaps to respond to simulated territorial intruders. We evaluated the effect of gap width on responses by five forest bird species. We found that for forest specialists such as the Swainson's Thrush (Catharus ustulatus), Golden-crowned Kinglet (Regulus satrapa), and the Black-throated Green Warbler (Dendroica virens), the probability of crossing gaps decreased sharply with gaps 25-40 m wide. By contrast, control trials showed no significant decrease in their probability of response up to 100 m through continuous stands. Habitat generalists such as the White-throated Sparrow (Zonotrichia albicollis) and the Dark-eyed Junco (Junco hyemalis) were more prone to cross treeless gaps than forest specialists. Playback studies provide a new tool for understanding birds' responses to microscale habitat discontinuities.

Stochasticity in Bird Habitat Selection: Year-to-Year Changes in Territory Locations in a Boreal Forest Bird Assemblage

We studied the degree of apparently stochastic variation in the location of the breeding territories of 17 species of birds in a 36-ha study plot in a heterogeneous coniferous forest in southern Finland over six consecutive years; 'apparent stochasticity' was defined as the sum effect of factors that are contingent relative to structural attributes of the habitats as they are measured by human observers. The data were collected by territory mapping, with 50 x 50 m spatial resolution, and the analysis involved three steps: (1) calculation of frequency of use of each 50 x 50 m grid cell in the plot by each species over the six years; (2) construction of logistic regression models (using GLIM) to describe the habitat attributes of cells favoured by each species in the plot; and (3) exploration of stochasticity by analysing how much random models need to be constrained to generate frequency distributions of grid cell occupancy that resemble those observed - for this purpose we used both binomial probabilities and Monte Carlo simulations. For a majority of the species a sufficient constraint was avoidance of such grid cells not used by the birds during the six years at all: the fit between expectations and observations improved in the binomial model, and improved markedly in the simulation model which also included an adjacency constraint and a weak preference constraint on the 'centroid' of the simulated territories. Six of the 17 species showed stronger preference for particular grid cells than predicted by the avoidance models; four of them presumably for nest site and nesting/foraging substrate (Dendrocopos major, Ficedula hypoleuca, Parus major, Certhia familiaris), and two for a particular habitat (Phylloscopus trochilus, Regulus regulus). Four of these species occupied in years with low population size predominantly such grid cells that were occupied in the other years, in accordance with the 'buffer effect' idea. Overall, however, in 11 of the 17 species the territory locations seemed to vary stochastically from year to year once the grid cells not used at all during the study years were excluded. We discuss theoretical and methodological implications of this conclusion for bird habitat relationships.

Riparian Forest Strips as Habitat for Breeding Birds in Boreal Forest

Riparian forest strips are usually protected from logging for their buffer effect on aquatic habitats. However, their value to terrestrial wildlife is unknown. From 1989 to 1992, we compared bird abundance and species composition in 5 experimental riparian forest strips (20-m, 40-m, 60-m, and control [>300 m wide], intact strips, and 20-m-wide thinned strips), in boreal balsam fir (Abies balsamea) stands, for 3 years following clear-cutting. Bird densities increased 30-70% (P 0.05) thereafter to approximately pretreatment levels. The 20- and 40-m-wide riparian strips had highest mean bird densities, but also the fastest (P < 0.05) decreases thereafter. By the third year after clear-cutting, forest-dwelling species were less (P = 0.01) abundant than ubiquitous species in the 20-m strips. The golden-crowned kinglet (Regulus satrapa), Swainson's thrush (Catharus ustulatus), blackpoll warbler (Dendroica striata), and black-throated green warbler (D. virens) became nearly absent in 20-m strips. The removal of 33% of the trees in some 20-m strips resulted in a <20% decline of bird densities, a moderate effect that combined with the greater effect of strip narrowness. There was evidence that 60-m-wide strips are required for forest-dwelling birds. Bird populations may continue to decline in strips before regeneration of adjacent clear-cuts provides suitable habitat for forest-dwelling species

Predation on Ground-Nesting Birds' Nests in Relation to Predator Densities and Habitat Edge

To study which predators are responsible for nest-losses among ground-nesting birds in boreal forest and how predators utilize different habitats, especially forest/farmland edge, artificial nests were exposed to predators in central Sweden in 1981. A setup which forced nest-robbers to leave foot-prints showed that different avian and mammalian species robbed nests in relation to their relative densities. Among both avian and mammalian nest-robbers there were habitat specialists and generalists. Predators appeared not to develop a memory for the experimental nests. In forest, birds were relatively more common than mammals as nest robbers than in farmland. Differences in the role of different predator species as nest robbers in forest vs. farmland habitats reflected their choice of habitat. No predator seemed to have developed specific nest-robbing skills and loss of individual nests was considered as a random event. Predation rates were higher when nests could be detected from a distance. However, the relevance of the results for real nests remains largely unknown. Variations in predation pressure in relation to different types of ecotones is discussed in view of the recent debate on how patterns of predation become altered as the size of patches of pristine environments are reduced as a consequence of human influences. It is suggested that the main factor affecting the rate of predation in patchy environments is the steepness of productivity gradients between an habitat island and the surrounding matrix rather than patch size itself.