Foraging behavior of three species of songbirds during stopover in southeastern Morocco during spring migration

Investigators studying the stopover ecology of migrating birds typically use the capture-recapture method to examine important parameters such as fuel deposition rates (FDR) and stopover duration. However, such studies can be constrained by the number of recaptures. An alternative method is to calculate a regression of mass over time of day, but this method may not be reliable because patterns of mass change of individual birds through the day may not reflect that of the whole population. Given the potential constraints of these methods, using them in combination with other methods, such as behavioral observations of foraging birds, may improve our understanding of the patterns of fuelling in birds at stopover sites. We observed the foraging behavior of three songbird species, including Western Bonelli's (Phylloscopus bonelli), Subalpine (Sylvia cantillans), and Willow (Phylloscopus trochilus) warblers, from 15 March to 30 April 2011 at a small oasis at the northern border of the Sahara desert in southeast Morocco. Given the location of our study site at the northern edge of the Sahara desert, birds migrating north likely needed to replenish their energy reserves at this stage of their journey. We assessed foraging effort by determining the rate (number per unit time) at which birds pecked at substrates or made aerial forays after flying insects. Peck rates were higher for Western Bonelli's Warblers than for Subalpine and Willow warblers, suggesting either species-specific adaptations to feeding in arid environments or differences in the motivation to feed. In addition, Western Bonelli's Warblers had FDRs that were negative or close to zero and, therefore, were apparently unable to refuel successfully (i.e., increase their fuel stores) despite greater effort, possibly indicating less efficiency in obtaining food (i.e., more unsuccessful pecks). The lower peck rates of Subalpine and Willow warblers suggest either that they were less efficient at finding prey or were simply foraging at lower rates. For all three species, peck rates were lower at higher wind speeds, suggesting that wind may alter prey availability and detectability, especially of flying insects. Interactions among species-specific migration strategies, environmental conditions, and habitat quality ultimately define the success of migration. Our results suggest that using observational data in combination with capture data may improve our understanding of these interactions at migration stopover sites.