Social learning spreads knowledge about dangerous humans among American crows - PubMed (original) (raw)

Social learning spreads knowledge about dangerous humans among American crows

Heather N Cornell et al. Proc Biol Sci. 2012.

Abstract

Individuals face evolutionary trade-offs between the acquisition of costly but accurate information gained firsthand and the use of inexpensive but possibly less reliable social information. American crows (Corvus brachyrhynchos) use both sources of information to learn the facial features of a dangerous person. We exposed wild crows to a novel 'dangerous face' by wearing a unique mask as we trapped, banded and released 7-15 birds at five study sites near Seattle, WA, USA. An immediate scolding response to the dangerous mask after trapping by previously captured crows demonstrates individual learning, while an immediate response by crows that were not captured probably represents conditioning to the trapping scene by the mob of birds that assembled during the capture. Later recognition of dangerous masks by lone crows that were never captured is consistent with horizontal social learning. Independent scolding by young crows, whose parents had conditioned them to scold the dangerous mask, demonstrates vertical social learning. Crows that directly experienced trapping later discriminated among dangerous and neutral masks more precisely than did crows that learned through social means. Learning enabled scolding to double in frequency and spread at least 1.2 km from the place of origin over a 5 year period at one site.

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Figures

Figure 1.

Figure 1.

Responses of lone, unbanded crows (solid symbols, scolding; open symbols, silent) at site B (urban and dense suburban Bellevue, WA; for other sites see the electronic supplementary material). Each of the 11 repeated trials in which the dangerous mask was worn is represented as a straight line with segments proportional to the actual route (lines 1–11 under the study route, letters orient lines to route; dashed lines delineate the major segments). The locations of all mobs given in response to the dangerous person's presence and the responses of all lone unbanded (never captured) crows are plotted in a spatially explicit manner for each trial. Scolding crows exposed to a mob (observed within 100 m of the location where mobbing occurred on a previous trial; these crows have a plus symbol) are evidence of peer-to-peer social learning. Scolding crows that were not exposed to a mob are evidence of social learning by observing the trapping event (trial 1), evidence of inherent scolding of a masked person (trials 2–11) or the conservative nature of our assessment (trials 2–11).

Figure 2.

Figure 2.

Scolding of the (a,b) dangerous and (c) neutral mask at our long-term research site at the University of Washington (UW) campus. The same responses to the dangerous face are plotted as a function of real time (a) and the number of trials during which birds could witness others scolding (exposures to dangerous mask; b). Responses were obtained during the breeding and non-breeding seasons by the authors and naive observers blind to the study design. A linear model was fit to all the data (solid lines with confidence intervals; a,c). An exponential rise to maximum model was also fit to the responses to the dangerous mask (dashed line; b).

Figure 3.

Figure 3.

Spatial increase in intensity and extent of scolding at the UW site. Locations of consistent (cross hatched circles) and inconsistent (filled circles) scolding to the dangerous mask from (a) two weeks, (b) 1.25 years and (c) 2.8 years post-trapping. Circle diameter is proportional to the number of scolding crows: smallest circles represent single crows; largest represent groups of seven crows. Solid line marks survey route. (d) Responses with scolding (filled circles) and no scolding (open circles) during the expanded search (large dashed circle superimposed on street network) carried out 2.7 years after trapping.

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