Scanning behaviour of foraging Ruffs Philomachus pugnax during spring migration: is flock size all that matters?

Abstract

This study tested for effects of flock size and other potentially important variables (location, vegetation cover, wind force, cloud cover, date, time of day, feeding habitat, number of other waders and peck rate) on the vigilance level of foraging Ruffs Philomachus pugnax during spring migration at Seewinkel, an important stopover site for waders in Eastern Austria. Therefore, foraging Ruffs were filmed at four different salt ponds, with a final total of 681 film sequences being available for analysis. To test for effects of predictor variables on scan rate (number of scans per 30 s) of foraging Ruffs, generalised linear models (GLMs) were calculated including all variables and all possible subsets. Three variables remained in all 30 best GLMs (selected according to Akaike’s information criterion) testing for effects on scan rate of foraging Ruffs: feeding location, feeding habitat and flock size. These variables also significantly affected Ruffs’ scan rates according to Wald statistics. Besides differences of scan rates between feeding locations, the vigilance level was significantly higher in terrestrially foraging Ruffs than in birds feeding at semi-aquatic habitat patches. Furthermore, scan rate decreased with increasing flock size. Our study emphasized that, even when controlled for other variables affecting scan rates, flock size still remains important for explaining variance in vigilance levels of foraging Ruffs.