Bird Communities

(Hartley and Hunter) – Responses of the forest bird community to gap-harvests were studied from 1995-1998.  Depending on the timing of the initial harvest, avian communities in each research area were censused up to 1-3 years before and 1‑3 years after harvest. No significant changes in community composition, species richness, or density of bird populations were detected. Few pioneer bird species were captured. This lack of response may result from the small size of the treatment areas, from the relatively narrow range of the gap sizes created (110 – 2170 m2), or from the use of retention trees in the harvested gaps.  This pattern may change greatly after the next gap-expansion harvests in 2005-2008.

The effect of gap harvests treatments on bird communities was examined from 1995-98.  How treatments affected breeding songbird abundance, richness, and site-fidelity over four consecutive summers, including 1-3 years before and after each site was harvested, were examined.  The ability to detect treatment effects was limited by the small number of replicates, but power analyses indicated that given the experimental design and observed variability, there was a high (>80%) probability to detect 20-30% differences in overall abundance among treatment groups.  There was no evidence that gap harvests caused changes of this magnitude, or affected densities of individual species, avian richness, or which species were most abundant before versus after treatment.  Annual variations in densities were much stronger than differences between treatment groups.

Of 96 male Hermit Thrushes and 74 male Ovenbirds captured within sites, an average of 62% and 28% of respective males were recaptured annually.  Of these, 90% of male Hermit Thrushes and 94% of male Ovenbirds were recaptured on the same site in successive years, regardless of the site’s treatment status.  However, there was a significantly higher tendency for Hermit Thrush to disperse to new sites if they were previously captured on treated versus control sites.

Based on the results, it was argued that annual fluctuations in bird densities may be driven largely by predator (i.e., red squirrel, Tamiasciurus hudsonicus) population dynamics.  Previous research has shown that avian nest-predation by red squirrels strongly affects local breeding productivity of birds, and that red squirrel populations are regionally synchronous.  Four lines of evidence were consistent with the premise that squirrel population fluctuations can affect bird populations over large areas.  Squirrel populations in the PEF peaked in 1995 and crashed in 1996, while bird densities decreased from 1995 to 1996, then increased sharply from 1996 to 1997.  Breeding Bird Survey data showed a similar pattern of avian population change (especially for coniferous-forest birds) from 1995-1997 at much larger scales.