Energetic condition and mass gain in six species of Parulid Warbler during stopover along the southern shore of Lake Ontario

Passerines use stopover sites to refuel during migration. However, some studies have shown that refueling rates during stopover can vary with sex, time of arrival, density of conspecifics, or season. Using AIC model sets, we addressed two main questions: (1) Which factors – season, sex, arrival date, or time of day – best explain variability in energetic condition and (2) do season, sex, and arrival date influence mass-gain rate? Data were collected for six species of migratory warbler between 1999-2016 at the Braddock Bay Bird Observatory, a stopover site near Rochester, New York. While there was significant species variation, arrival date, sex, and season explained more variation in condition than time of day. For all species, birds arriving later in the season were in better condition than earlier migrants, females were in better condition than males, and spring birds were in better condition than fall. Likewise, all birds gained mass over the course of the day. There was no difference in mass gain between the sexes in any species. Mass gain was consistent across season and arrival date for Geothlypis trichas and Setophaga caerulescens, fall birds gained mass at a greater rate than spring birds in S. ruticilla and S. magnolia, and arrival date influenced mass gain of Cardellina pusilla and S. coronata. This suggests that the factors that most influence energetic condition and mass gain may be species specific. Further studies need to be conducted in order to determine the causes of these variations.