Macroalgae composition alters occupancy of multiple bird guilds in rocky intertidal communities

Abstract

In the north Atlantic, rocky intertidal shores with diverse macroalgal assemblages are common. Despite this, the effect of macroalgae on bird use of these habitats is not well understood. We used a single-season, single-species occupancy framework to investigate the influence of the macroalgal assemblage on 3 bird guilds and 2 single-species groups: shorebirds (waders), scavengers, divers, common eider Somateria mollissima non-parenting adults, and common eider broods. In particular, we considered whether bird occupancy is more strongly determined by the taxonomic composition of the macroalgal assemblage or by structural characteristics of the collective assemblage. We conducted avian point counts and intertidal habitat surveys at 75 sites along the Maine (USA) coast in 2016 and 2017. Macroalgae increased the probability of site occupancy for 1 guild and 2 single-species groups. Most notably, we found that shorebird occupancy had a positive relationship with the spatial extent and mat depth of Ascophyllum nodosum, indicating a preference for more homogeneous and deeper macroalgal assemblages dominated by this common, tall-canopied taxon. In contrast, common eider presence had a positive relationship with the spatial extent of 2 categories of shorter-canopied macroalgae that are less dominant members of the macroalgal assemblage. Both shorebirds and eiders were less likely to occur in areas with steeper intertidal slopes, which provide less potential habitat during low tides. Overall, our results suggest that macroalgae composition and availability influences bird occupancy in rocky intertidal habitats, but more mechanistic studies (e.g. behavioral) are needed to understand the exact role macroalgae play in coastal bird ecology.