Michael E. Akresh

Recent sightings of Kirtland's Warblers on San Salvador Island, The Bahamas

Abstract Here we present recent accounts of Kirtlands Warblers (Setophaga kirtlandii) captured and observed on San Salvador Island, The Bahamas, in 2012 and 2013. We conducted mist-netting and passive surveys in a variety of habitats on the island from January–March 2012 and December 2012–March 2013. We captured four warblers and sighted at least six other individuals, including two color-banded birds originally banded near Mack Lake, Michigan, USA. Warblers were primarily captured or observed in coastal scrub and inland low coppice habitats where vegetation height is <4 m, has a scrub/shrub appearance, and openings because of disturbance. These habitats are similar to habitats preferred by Kirtlands Warblers on other Bahamian islands, and also have fruiting plants the warblers prefer such as Erithalis fruticosa, and Lantana involucrata. To our knowledge, these are the first confirmed cases of observed Kirtlands Warblers on the island in over 46 years.

A proxy of social mate choice in prairie warblers is correlated with consistent, rapid, low-pitched singing

In songbirds, female mate choice may be influenced by how well a male performs his songs. Performing songs well may be especially difficult if it requires maximizing multiple aspects of performance simultaneously. We therefore hypothesized that, in a population of prairie warblers, the males most attractive to females would be those with superior performance in more than one facet of singing. We tested this prediction by comparing different aspects of song performance, as well as different combinations of these aspects, to determine which were the best predictors of first-egg date, which we took to be a proxy for social mate choice. We found that first-egg date was best predicted by a combination of song performance traits that included consistent performance, rapid rate, and low pitch. Female preference for males capable of physically challenging song performance may have contributed to the evolution of acoustically complex vocalizations in oscine songbirds, because if complex sounds are more difficult to perform, they may be favored by selection for signal reliability.

Effect of nest characteristics on thermal properties, clutch size, and reproductive performance for an open-cup nesting songbird

Maintaining avian eggs and young at optimum temperatures for development can increase hatching success and nestling condition, but this maintenance requires parental energetic demands. Bird nests, which often provide a structure to safely hold the eggs and nestlings and protect them from predators, can additionally be designed to help maintain eggs’ optimum temperatures by minimising heat loss, especially in climates where eggs cool rapidly when unattended. We collected and measured Prairie Warbler (Setophaga discolor) nests in western Massachusetts, U.S. in 2009 and conducted a climate-controlled, nest-cooling experiment to determine how nest characteristics affect thermal properties for small, open-cup nesting birds. We then assessed if nests with better insulation properties resulted in any fitness benefits, and also tested if nest structural characteristics affected birds’ fitness. We found that nest characteristics influenced their thermal properties, with thicker, heavier, and larger nests having slower cooling rates and higher predicted equilibrium egg temperatures. Both nest cup depth and clutch size significantly declined over the breeding season, and we observed a trend, although non-significant, that nests with shallower cups had smaller clutches. Contrary to studies on cavity-nesting birds, we found no significant effects of nest thermal properties or nest structure on hatching and fledging success, nestling condition, brood parasitism, or nest survival. Prairie Warblers in our study site may already be adapted to build nests within a range that maximises their fitness. Furthermore, studies have shown that open-cup nests of other species are relatively thicker and more insulated in colder environments at higher latitudes than our study. Instead of building nests to solely minimise heat loss, open-cup nesting birds in temperate climates may also be driven by opposing selection pressures when building their nests, such as to prevent nestlings from overheating during hot days.