Wetlands and predation in boreal ecosystems play essential roles throughout the breeding season for bird assemblages. We found a positive association of beaver activity and a negative influence of American red squirrels (Tamiasciurus hudsonicus) on bird assemblages. We used a multispecies hierarchical model to investigate whether bird communities differ between two major wetland habitats in boreal Canada: beaver ponds and peatland ponds. In addition to including variables such as forest cover and latitude, we adopted a structural equation model approach to estimate the occupancy of American red squirrels and its potential influence on bird communities. Using automated recording stations deployed at 50 ponds, we detected 96 bird species in 2018 and 2019. Bird species were grouped into four taxonomic guilds according to their habitat successional requirements: early successional species, late successional species, generalists, and wetland species. Beaver ponds harbored higher species richness, a pattern driven primarily by early successional species. The occupancy of almost a quarter of the species was lower in the presence of red squirrels. Late successional species responded positively to the cover of forest surrounding the pond. Our results highlight the value of considering acoustic data of red squirrels to quantify habitat quality in boreal forests. We conclude that beaver activity shapes bird assemblages through modification of their habitat, and that some bird guilds are associated negatively with the presence of American red squirrels.

Acoustic sampling and covariates.

We used a SM4 Song meter (Wildlife Acoustics Incorporation, Concord, Massachusetts, USA) at each pond to record bird and squirrel vocalizations. During a given sampling period, ARUs were set with both microphones facing upward for four consecutive days to record a 5-min sample at 00600 h. Each file was encoded at a sampling rate of 44.1 kHz and a 16-bit resolution. Each pond was sampled for four consecutive days during two periods in both years, yielding an average of 80 minutes per pond. All recording data were analyzed by an ornithologist with extensive field experience in the study region to identify species based on their vocalizations or other aural cue. Species were categorized into four guilds based on their requirements for various successional stages of black spruce: (1) early successional species, associated with open areas of low-vegetation such as young forests or regenerating shrublands; (2) late-successional species preferring older forest stands; (3) generalists, consisting of species that use a wide range of successional stages; and (4) wetland and aquatic species. In addition to birds, we identified the vocalizations of the American red squirrel from recordings for use in occupancy analysis. We included four candidate variables as potential predictors of boreal bird occupancy: pond type, red squirrel latent occupancy, proportion of all forest types over 2 m (deciduous, mixed, and coniferous forest) within circular buffers of 1000 m centered on acoustic recorder locations, and we included latitude to reflect habitat changes in the study area. We considered two parameters that potentially influenced detection probability during acoustic surveys: the quality of the recording and the number of days after snowmelt.

Data processing and analysis.

We investigated occupancy patterns of the American red squirrel, and the entire bird community detected at least once in our recordings. We prepared detection histories for each species at each pond, where we indicated detection (1) or non-detection (0) in each recording of the two periods of four consecutive days in each year. Each pond in each year was considered as an independent site and we included a fixed year effect as well as a site random effect to account for potential differences in occupancy between years. Thus, detection histories for a given species were arranged in a matrix of 100 rows (50 sites x 2 years) and 8 columns (4 recordings x 2 periods). We estimated bird community structure with a multispecies site-occupancy model. We adapted a structural equation strategy to our multispecies occupancy model. Specifically, one component of our model estimated the occupancy of American red squirrels as a function of different explanatory variables, and then used the latent squirrel occupancy as an explanatory variable in the multispecies occupancy model for the bird communities. We used the multispecies model to derive the species richness from the posterior distribution of the true occurrence of each species at each site, expressed as the mean of the posterior distribution. We investigated the relationship between species richness against the explanatory variables pond type, forest cover, latitude, and the posterior mean of the squirrel occupancy state in a linear mixed-effect model.

The description of each column header for each spreadsheet is provided in an associated ReadMe file. The steps to follow for each analysis are provided in Supplementary material the R code files. The final dataset included detection/non-detection data for 96 species from 50 ponds from May to August 2018 and 2019 ("Bird_multisp_v2.Rdata"). This dataset is shared in one file in the form of a 3-dimensional array table, which contained rows for pond units (n=100), columns for visits (n=8), and rectangular matrices for species (n=96). Please contact the author for any additional inquiries of how data was collected and analyze.