Logging Affects Fledgling Sex Ratios and Baseline Corticosterone in a Forest Songbird

Silviculture (logging) creates a disturbance to forested environments. The degree to which forests are modified depends on the logging prescription and forest stand characteristics. In this study we compared the effects of two methods of group-selection (“moderate” and “heavy”) silviculture (GSS) and undisturbed reference stands on stress and offspring sex ratios of a forest interior species, the Ovenbird (Seiurus aurocapilla), in Algonquin Provincial Park, Canada. Blood samples were taken from nestlings for corticosterone and molecular sexing. We found that logging creates a disturbance that is stressful for nestling Ovenbirds, as illustrated by elevated baseline corticosterone in cut sites. Ovenbirds nesting in undisturbed reference forest produce fewer male offspring per brood (proportion male = 30%) while logging with progressively greater forest disturbance, shifted the offspring sex ratio towards males (proportion male: moderate = 50%, heavy = 70%). If Ovenbirds in undisturbed forests usually produce female-biased broods, then the production of males as a result of logging may disrupt population viability. We recommend a broad examination of nestling sex ratios in response to anthropogenic disturbance to determine the generality of our findings

Results of a linear mixed model for nestling baseline corticosterone.

<p>Both the full model containing, treatment, body condition, year, sex and a treatment×sex interaction and a minimal model containing just treatment are presented. Pairwise significances were determined using Tukey's post-hoc analysis, estimate, standard error and p-values are presented.</p

The top logistic exposure models of daily survival rate of Ovenbird nests in Algonquin Provincial Park in 2008 and 2009 as chosen using AIC model selection corrected for small sample size (AICc).

<p>Models with ΔAICc values <2 were considered the most supported. Treatment was not a well supported model as it failed to appear in any of the top models. The chosen model is displayed in bold.</p

The proportion of males within a brood within a given treatment.

<p>Means are presented with standard error, and the number of nestlings and the number of broods within each treatment respectively are in parentheses. Different letters indicate significant differences.</p

Aerial image of two forest stands cut with group-selection silviculture (GSS) in Algonquin Provincial Park.

<p>Heavy group-selection is outlined with a solid line in the upper left and Moderate group-selection is outlined with a dotted line in the bottom right. Undisturbed sites are similar to the unharvested forest in the upper right of the image (Source: Algonquin Forestry Authority).</p

Results of a linear mixed model for nestling male sex ratio, represented as the proportion of male nestlings within a given brood.

<p>Both the full model containing, treatment, complete (whether the brood size equalled the clutch size), and year, and a minimal model containing just treatment are presented. Pairwise significances were determined using Tukey's post-hoc analysis, estimate, standard error and p-values are presented.</p

Effect of harvesting treatment on the mean corticosterone levels in broods of Ovenbird nestlings.

<p>Brood corticosterone levels were significantly higher in the Heavy logging treatment than the Undisturbed logging treatment (<i>P</i> = 0.003). The number of nestlings and the number of broods within each treatment respectively are in parentheses. Means and SE are presented. Different letters indicate significant differences using Tukey's unequal N HSD post hoc test.</p

Results of a linear mixed model for nestling body condition.

<p>Both the full model containing, treatment, sex, year and a treatment×sex interaction and a minimal model containing just treatment are presented. Pairwise significances were determined using Tukey's post-hoc analysis, estimate, standard error and p-values are presented.</p