Black-tailed Godwits Limosa limosa islandica and Redshanks Tringa totanus respond differently to macroalgal mats in their foraging areas

In Clonakilty Bay, SW Ireland, we examined the effects of macroalgal mats on two shorebird species, Black-tailed Godwit Limosa limosa islandica and Redshank Tringa totanus, in the winters of 2000–2001 and 2001–2002. In both winters, coverage with macro algae was variable and declined to zero by January. Godwit densities decreased over the course of each winter, while Redshank densities showed an increase. Godwits were more numerous in the second winter, while Redshanks were more numerous in the first winter. These opposite intra-seasonal and annual trends are not consistent with a general negative effect of algal mats on shorebirds. The potential effects of macroalgae were assessed through (1) association between site use (expressed in bird-days) and environmental parameters (macroalgal cover and biomass, prey densities, sediment silt content and organic matter), (2) individual habitat selection, and (3) foraging behaviour. Strong positive correlations between site use and prey densities were found for both shorebirds. Site use by Redshanks was negatively correlated with algal cover, but this was not the case for godwits. The foraging behaviour of both species was affected by algae, but in a different manner. Godwits made ca. 30% fewer foraging attempts in algal patches than in clear patches, but foraging success was similar in both. Redshanks attempted prey captures at similar rates in clear and algal patches but success was 50% lower in the latter. This suggests that Redshanks may be constrained in obtaining their required daily energy intake on algal-covered mudflats in contrast to godwits. How much the observed increase in Redshank densities over the winter (when godwit densities decrease) is related to inter-specific interference or to the decline in algal cover remains an interesting research question.