Tidal Current Speed and Bird Abundance: an investigation into bird behavior patterns in the San Juan Channel

Abstract

Seabird abundance in the San Juan Channel is affected by variations in tidal current speed and direction. In this paper we investigate the abundance and community structure of seabirds in a tidally active transect of San Juan Channel. Using a variation of the strip-transect vessel survey method, we observed the abundance of birds in a 200 m wide strip between Turn Rock and Shaw Island. We divided tidal current into five different categories, and conducted 36 transects, recording the number of birds observed during each tidal current category. Our results suggest that the presence of birds in San Juan Channel correlates with changes in tidal current speed and direction. We observed a greater number of birds during periods of fast current, and fewer birds during slack and slow ebbs. When focusing our analysis on gulls and alcids, we discovered that while alcids prefer fast floods, gulls prefer fast ebbs. Finally, we observed that Rhinoceros Auklets and Common Murres have similar tidal current speed and direction preference. These surveys demonstrate the importance of the variation in current speed and direction on marine bird behavior in the San Juan Channel.