Eavesdropping magpies respond to the number of heterospecifics giving alarm calls but not the number of species calling

Abstract

Social information varies in its reliability and relevance, requiring individuals to use rules to avoid inappropriate responses to false information. A simple rule is only to respond when a certain number of individuals provides similar information. Although individuals within social groups can use such numerical rules to assess conspecific information and make consensus decisions, it is unknown whether individuals apply similar rules when assessing the value of heterospecific information. We consider the case of individuals eavesdropping on heterospecific alarm calls. Eavesdroppers may be particularly vulnerable to false alarms because of the large pool of potential callers and variability in the specific threats to which they call. Individuals might therefore value alarm calls more if they come from multiple callers or multiple species than from a single caller or a single species. We tested these predictions using field playback experiments on wild Australian magpies (Gymnorhina tibicen). Magpies responded more strongly to alarm calls coming from two callers versus one caller of the same heterospecific species. However, in contrast to our prediction, magpies responded similarly to alarm calls from two individuals of different species as they did to alarm calls from two individuals of the same species. We conclude that the number of calling individuals does affect response, probably because information from multiple callers is more reliable, but that the value or reliability of information from multiple species may depend on the types of alarm calls and combination of species involved.

Mobbing responses of great tits (Parus major) do not depend on the number of heterospecific callers

“…Previous study compared one versus three and five conspecific callers (Coomes et al., 2019; Dutour et al., 2021). Moreover, Australian magpies ( Gymnorhina tibicen ) responded stronger toward the calls of a heterospecific when two callers were simulated rather than one (Igic et al., 2019), which is based on the discrimination between two heterospecific individuals.…”

Eavesdropping of an African ground squirrel on the heterospecific alarm calls of a noisy ground‐nesting bird

“…Perhaps it is the differences that matter more than the similarities. Heterospecific calls may provide information that is not otherwise available to the eavesdropper (Igic et al., 2019), which would support the community informant hypothesis (Carlson et al., 2020). Lapwings may give different information to the squirrels than can either conspecifics or mongooses scanning from ground level (Makenbach et al., 2013; Waterman & Roth, 2007).…”

“…In the yellow mongoose study, a long latency in squirrel vigilance occurred only after conspecific alarm calls (Makenbach et al., 2013). Individuals are expected to direct energy to eavesdrop on more reliable and relevant information from species sharing habitat and predators to reduce time spent vigilant (Igic et al., 2019; Palmer & Gross, 2018). Yellow mongoose and Cape ground squirrels share burrows, are of similar body size and share predators, suggesting that alarm calls from mongooses would be as relevant as alarms from lapwings (Makenbach et al., 2013; Waterman & Roth, 2007).…”

“…In birds, mixed-species flocks are common and species benefit greatly from eavesdropping by decreasing time spent vigilant (Igic et al, 2019;Ridley et al, 2014). Heterospecific alarm call recognition has also been demonstrated in many mammals (e.g.…”

“…The ability to recognize heterospecific alarm calls has been demonstrated in a number of vertebrates, including many species of birds and mammals (see Magrath et al (2020) for a review). In birds, mixed‐species flocks are common and species benefit greatly from eavesdropping by decreasing time spent vigilant (Igic et al., 2019; Ridley et al., 2014). Heterospecific alarm call recognition has also been demonstrated in many mammals (e.g., Marmota monax eavesdrop on Tamias striatus , Aschemeier & Maher, 2011; Odocoileus hemionus eavesdrop on Marmota flaviventris , Carrasco & Blumstein, 2012; Helogale parvula eavesdrop on Paraxerus cepapi , Morris‐Drake et al., 2017; Xerus inauris and Cynictis pencillata mutually eavesdrop, Makenbach et al., 2013, and likewise Marmota flaviventris and Callospermophilus lateralis mutually eavesdrop, Shriner, 1998).…”