Experimental evidence of specialised phenotypic roles in a mobbing raptor |
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Authors: | Audrey Sternalski Vincent Bretagnolle |
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Institution: | (1) CEBC–CNRS, 79360 Beauvoir-sur-Niort, France |
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Abstract: | Group living is associated with costs but also with potential benefits, such as a decrease in predation risk through, for
example, higher defence efficiency. Mobbing is among the most specialised forms of anti-predator strategies involving group
defence and has mainly been investigated in passerine birds and some mammals. Variation in the mobbing response has been found
in several species according to phenotypic variation such as sex or age. This suggests that there are differential benefits
between mobbers, which may have promoted individual specialisation in mobbing behaviour. We studied mobbing behaviour in a
communal roosting raptor, the Marsh harrier (Circus aeruginosus), which shows active group defence. Our study population exhibits extreme colour polymorphism, with two colour morphs in
males, as well as sexual dichromatism and colour variation with age. We used different decoys, placed at different distances
from the roost, to manipulate experimentally the perceived predation risk and to elicit mobbing behaviour. Using the experimental
design that maximised mobbing response in harriers, we then focused on the sequence and the specific behaviours involved in
recruitment of mobbers, and whether individual investment in terms of defence was associated with phenotypic characteristics
of individuals (i.e. sex, age and colour morph). We found that the main behaviour involved in successfully attracting mobbers
was alarm calling. We also detected differential individual investment in relation to sex and age, but more importantly, we
provide the first evidence for specialised male phenotypic roles during mobbing events, signalled by colour polymorphism:
grey males tended not to be involved in mobbing and almost never behaved as recruiters or mobbers, while brown males behaved
mainly as recruiter birds. These findings suggest that colour morph may signal the individual’s anti-predatory abilities through
different behavioural strategies between males. |
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Keywords: | |
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