Local and landscape effects of industrial forestry on the reproductive activity of forest songbirds in northwestern New Brunswick, Canada

Abstract

In the northern hardwood forest of New Brunswick, industrial forest management affects within-stand vegetation and the landscape structure of the forest mosaic. Understanding the effects of industrial forestry on songbird populations requires the investigation of songbird reproductive success, in addition to abundance, on a landscape scale (i.e., in a mosaic of forest stands). I present a method to efficiently assess the reproductive success of multiple songbird species across a large area (e.g., > 300 ha). The method uses systematic playbacks of a recording of black-capped chickadee (Poecile atricapilla) mobbing calls to attract species to an observer. Once birds are attracted to the observer, reproduction-related activities can be recorded to infer successful pairing, hatching, or fledging. Trends in reproductive activity of two focal species obtained using this method were similar to their actual reproductive success as measured using intensive nest monitoring in the same locations. I studied the mobbing response of forest birds at different times of the year and in the presence or absence of potential avian predators. Mean duration of mobbing time was significantly longer when confronted with potential predators, but mobbing intensity was not significantly different. Mobbing group size and overall intensity were greatest early in the breeding season before the initiation of egg laying. I concluded that the variability in avian anti-predator mobbing is based on the proximate (individual safety) and ultimate (safety of offspring) risks of participation. I explored the relationship between reproductive activity and songbird abundance using Spearman correlation coefficients. Reproductive activity and songbird abundance were not significantly correlated in 56% of the comparisons (9 of 16). The lack of a consistent relationship emphasizes the importance of studying fitness parameters in addition to abundance or density. I then used canonical correspondence analysis to assess the relationship between forest management and reproductive activity of eight species of forest birds (Vireonidae, Paridae, and Parulidae) in three study grids of varying silvicultural intensity. I predicted landscape effects would become significant as silvicultural intensity increased. Reproductive activity, local vegetation, and landscape structure data were collected on one 6 x 8 systematic grid and two 8 x 8 systematic grids with stations spaced 250 m apart. Basal area of American beech (Fagus grandifolia) and sugar maple (Acer saccharum) within 100 m of the sample locations were the most significant variables associated with reproductive activity on two of the three study grids (most and least intensively managed). In the third study grid (moderately managed), the amount of tolerant hardwood forest within 1 km of the sampling locations was most significant. The relatively minor effect of landscape structure probably reflects the high proportion of suitable habitat remaining in the study area. Landscape effects on songbird populations may become more important if the proportion of suitable habitat declines.