Social information modifies the associations between forest fragmentation and the abundance of a passerine bird

Habitat loss and fragmentation are the main factors driving the occurrence and abundance of species in the landscape. However, the local occurrence and abundance of species may also depend on conspecific and heterospecific social information e.g. clues of animals' presence or their voices. We investigated the impact of the interaction between different types of social information and forest fragmentation on the abundance of the song thrush, Turdus philomelos, in Central Europe. Three types of social information (attractive, repulsive, and mixed) and procedural control were broadcasted via loudspeakers in 150 forest patches that varied in size and isolation metrics. Repulsive social information (cues of presence of predator) decreased abundance of song thrush. Also, the repulsive social information changed the association between forest patch isolation, size and the abundance. Attractive social information (songs of the studied thrush) had no effect on song thrush abundance. However, the attractive social information reversed the positive correlation between habitat patch size and the abundance. Mixed social information (both repulsive and attractive) had no impact on the abundance nor interacted with habitat fragmentation. The observed effects mostly did not last to the next breeding season. Overall, our findings indicate that lands of fear and social attraction could modify the effect of habitat fragmentation on the species abundance but these effects probably are not long-lasting.

A review of conspecific attraction for habitat selection across taxa

Many species across taxa select habitat based on conspecific presence, known as conspecific attraction. Studies that document conspecific attraction typically provide social information (i.e., cues that indicate the presence of a given species) and then determine if a given species is more likely to settle at locations where the social information is provided compared to those locations that do not. Although the number of studies examining conspecific attraction has grown in recent years, a comprehensive review has not yet been undertaken. Here, we conducted a review of the literature and found 151 studies investigating conspecific attraction across eight taxa. We found that conspecific attraction is widespread with between 80% and 100% of studies, depending on taxa, documenting positive associations between habitat selection and the presence of conspecific cues. Conspecific attraction has been documented more frequently in bird and fish species with less attention given to invertebrate and mammal species. We use the patterns we found to (a) provide an overview of the current state of research on conspecific attraction and (b) discuss how important factors, such as cue characteristics and life history traits, may play a role in shaping conspecific attraction patterns within and across taxa.

Heterospecific eavesdropping on an anti-parasitic referential alarm call

Referential alarm calls occur across taxa to warn of specific predator types. However, referential calls may also denote other types of dangers. Yellow warblers (Setophaga petechia) produce “seet” calls specifically to warn conspecifics of obligate brood parasitic brown-headed cowbirds (Molothrus ater), which lay their eggs in the warblers’ and other species’ nests. Sympatric hosts of cowbirds that do not have referential alarm calls may eavesdrop on the yellow warbler’s seet call as a warning system for brood parasites. Using playback presentations, we found that red-winged blackbirds (Agelaius phoeniceus) eavesdrop on seet calls of yellow warblers, and respond as much to seet calls as to cowbird chatters and predator calls. Red-winged blackbirds appear to eavesdrop on seets as warning system to boost frontline defenses on their territories, although they do not seem to perceive the warblers’ seets as a cue for parasitism per se, but rather for general danger to the nest.

Lawson et al. study the response of the redwinged blackbirds to referential alarm calls in other bird species. They show that the blackbirds eavesdrop on heterospecific warning calls of the yellow warbler related to the nest parasites and respond by boosting their defenses to general danger on their territories.

Heterospecific social attraction in migrant birds: habitat niche overlap between two threatened shrikes

Context Effective biodiversity conservation requires information about a species’ ecology and its relationship with the geographical context. To achieve this efficiently, species distribution models can be developed, also taking into account species associations. Aims I aimed to illustrate the habitat requirements of two threatened passerines of semi-open landscapes, i.e. the lesser grey shrike, Lanius minor, and the woodchat shrike, Lanius senator, in southern Italy. Further, I assessed whether there is heterospecific social attraction between them, by assessing how their habitat niches overlap. Methods Data were collected by territory mapping during the 2009–13 breeding seasons. The habitat requirements of the species were investigated with the help of ecological niche factor analysis (ENFA). In addition, the areas of sympatry between the two species were defined by indicator kriging and the habitat niche overlap was analysed using the generalised linear model (GLM). Key results The results showed that these two species inhabit open areas, with scattered vegetation such as shrubs and trees, while avoiding intensively cultivated permanent crops, such as vineyards and olive groves. Moreover, a little difference was observed in the percentage cover of shrublands, which were higher in the woodchat shrike territories than in the lesser grey ones. The strong overlap in their habitat niches suggests heterospecific social attraction between them, especially because of the species rarity. Conclusions The lesser grey and the woodchat shrike were found to occur in very similar environments, by evidencing the overlap of their macrohabitat niches. This suggests the presence of heterospecific social attraction, corroborating the understudied hypothesis that this phenomenon is valid for migrant birds as well. The lesser grey shrike, in particular, may use occurrences of the woodchat shrike as a cue for assessing habitat quality and, thus, to establish its territory. Implications Considering the high habitat niche overlap, habitat changes occurring in the landscape might affect them equally. Furthermore, the use of heterospecific social information in habitat selection potentially has many important conservation implications. Indeed, by reproducing the appropriate information, birds could be attracted to specific sites known to be high-quality habitat.