Nest-site selection of a subarctic-breeding shorebird: evidence for tree avoidance without fitness consequences

Vegetation communities in the subarctic are at risk of change due to climate-driven tree and shrub encroachment. Vegetation change may lead to unsuitable habitat for arctic-breeding birds, many of whom are declining. Although many possible factors are contributing to their decline, loss of breeding habitat could be a major contributor. We examined nest-site selection in Dunlin (Calidris alpina hudsonia (Todd, 1953)), a shorebird that nests in open fen habitats in the Churchill, Manitoba, Canada, region. Our objective was to determine whether this species avoids treed habitats and the possible fitness consequences for this. We examined the role of vegetative horizontal and vertical concealments on nest-site selection and nest fate. Dunlin selected nest sites with lower densities of trees than present at unused sites (40 m radius). Both horizontal and vertical concealments were significantly greater at nests than at unused sites, and horizontal concealment was greatest in the north. No measure of tree density or height, or concealment, significantly predicted nest fate. Although Dunlin appear to select nest sites that may minimize exposure to northerly winds and that may provide cover against potential predators, the current nest-site characteristics are not reinforced by contemporary selection.

Choice consequences: salinity preferences and hatchling survival in the mangrove rivulus (Kryptolebias marmoratus)

In heterogeneous environments, mobile species should occupy habitats in which their fitness is maximized. Mangrove rivulus fish inhabit mangrove ecosystems where salinities range from 0 to 65 ppt, but are most often collected from areas with salinities of ∼25 ppt. We examined the salinity preference of mangrove rivulus in a lateral salinity gradient, in the absence of predators and competitors. Fish could swim freely for 8 h throughout the gradient with chambers containing salinities ranging from 5 to 45 ppt (or 25 ppt throughout in the control). We defined preference as the salinity in which the fish spent most of their time, and also measured preference strength, latency to begin exploring the arena, and number of transitions between chambers. To determine whether these traits were repeatable, each fish experienced three trials. Mangrove rivulus spent a greater proportion of time in salinities lower (5-15 ppt) than they occupy in the wild. Significant among-individual variation in the (multivariate) behavioral phenotype emerged when animals experienced the gradient, indicating strong potential for selection to drive behavioral evolution in areas with diverse salinity microhabitats. We also showed that mangrove rivulus had a significantly greater probability of laying eggs in low salinities compared with control or high salinities. Eggs laid in lower salinities also had higher hatching success compared with those laid in higher salinities. Thus, although mangrove rivulus can tolerate a wide range of salinities, they prefer low salinities. These results raise questions about factors that prevent mangrove rivulus from occupying lower salinities in the wild, whether higher salinities impose energetic costs, and whether fitness changes as a function of salinity.