Learning in advance? Interspecific recognition ability in male Eurasian blackcaps

Interspecific aggression is common in bird communities. It is thought to be a consequence of learned behavior or misdirected intraspecific aggression. In the former case birds acquire the ability to identify individuals of other species by social learning of species-specific traits during interactions with heterospecifics in a shared habitat. I conducted a two-choice song playback experiment to investigate the role of associating signals from two sensory modalities, auditory and visual, in shaping the agonistic response of male Eurasian Blackcaps (Sylvia atricapilla) to simulated male Garden Warbler (S. borin) intruders. I measured focal males’ response to playbacks to the song of a Blackcap or a Garden Warbler while giving them a choice between stuffed male models of both species presented in their territories. The experiments were carried out in early spring, before the arrival of Garden Warblers from Africa. I found that male Blackcaps were able to associate species-specific songs with species-specific plumage types. The ability to associate signals characterized not only experienced after-second-year old males, but also second-year old males that had not had the opportunity to defend breeding territories against heterospecifics. This suggests that second-year old male Blackcaps acquire the ability to associate Garden Warbler song with plumage before they use this skill when defending territory during their first breeding episode. Male Blackcaps do not mistake male Garden Warblers for males of their species, but rather learn these associations during the first months of life. However, the possibility that these associations are innate cannot be excluded.

Energy Stores, Oxidative Balance, and Sleep in Migratory Garden Warblers (Sylvia borin) and Whitethroats (Sylvia communis) at a Spring Stopover Site

Synopsis Little is known about how songbirds modulate sleep during migratory periods. Due to the alternation of nocturnal endurance flights and diurnal refueling stopovers, sleep is likely to be a major constraint for many migratory passerine species. Sleep may help to increase the endogenous antioxidant capacity that counteracts free radicals produced during endurance flight and reduces energy expenditure. Here, we investigated the relationship between sleep behavior, food intake, and two markers of physiological condition—the amount of energy reserves and oxidative status—in two migratory songbird species, the garden warbler (Sylvia borin) and the whitethroat (Sylvia communis). In garden warblers, birds with high energy stores were more prone to sleep during the day, while this condition-dependent sleep pattern was not present in whitethroats. In both species, birds with low energy stores were more likely to sleep with their head tucked in the feathers during nocturnal sleep. Moreover, we found a positive correlation between food intake and the extent of energy reserves in garden warblers, but not in whitethroats. Finally, we did not find significant correlations between oxidative status and sleep, or oxidative status and energy stores. Despite our study was not comparative, it suggests that different species might use different strategies to manage their energy during stopover and, additionally, it raises the possibility that migrants have evolved physiological adaptations to deal with oxidative damage produced during migration.

Orientation of Trans-Saharan Passerine Migrants in Southwestern Spain – Revisited

At the end of the 1980s, a relatively large data set was collected from orientation cage tests of nocturnal migrants caught in one locality in south-western Spain. It was published in The Auk in 1989. At the time of publication, the use of orientation cages was a major advance in detailed studies on passerine migration patterns. The basic evaluation procedure was to calculate the average vector of all numbers representing bird activity in a number of sectors of the full wind-rose and to use the Rayleigh test to determine the statistical significance of the distribution. However, due to hidden assumptions in the procedure used at that time, the resulting heading patterns could be called into question in light of further methodological development. The paper revisits the original set of orientation data collected in the last century in the reported study. The new results from the multimodal distribution accepting procedure are compared with the older pictures and are found to be much better at explaining the migration pattern there. In particular, the Garden Warbler and Reed Warbler, which were claimed to be ‘disoriented’, now show acceptable directionality patterns which do not differ in shape from those of other species.