Abstract
Animals invest in costly vigilance to detect threats. Joining groups reduces these costs, which can be further reduced in mixed-species assemblages. In colour-polymorphic species, morphs often experience different predation pressure and vary in a variety of traits. However, little is known about differences in vigilance or how group composition affects vigilance. The aim was to investigate whether higher conspicuousness increased vigilance and whether vigilance was reduced in mixed-morph groups like in mixed-species assemblages. I tested vigilance in the colour-polymorphic Gouldian Finch (Chloebia gouldiae). Same sex pairs of different age and of either pure (red-red or black-black) or mixed head colour were exposed to three contexts (familiar, changed and novel environment) and head movements were recorded. All birds reduced the frequency of head movements with increasing novelty, indicating different vigilance strategies (switching from a searching to a tracking strategy) depending on the situation. While vigilance did not differ between morphs, morph composition mattered. Black-headed pairs made fewer head movements than mixed-head colour pairs. Results indicated that conspicuousness did not affect vigilance, possibly due to existing adaptations to reduce predation risk. However, whenever red-headed birds were involved, vigilance increased either because of higher group conspicuousness or prevalence of aggression.
Highlights
We investigated the effects of head colour, head colour composition and effects of age on vigilance
When black-headed birds were partnered with another black-headed bird, the focal bird made significantly fewer head movements than when partnered with a red-headed bird (t = −2.327, p = 0.023)
Partner head colour had no significant effect on the frequency of head movements in red-headed birds (t = 1.200, p = 0.235)
Summary
Vigilance—and, costs—are often reduced even further in mixed-species assemblages [5,6,7,8]. Reasons for this can include less foraging competition, requiring less tracking of nearby companions [9,10], but more importantly, different perception abilities can be combined resulting in earlier detection of threats [11]. Vigilance is generally assessed in a foraging context by measuring the frequency and duration of head-up [1,12,13,14] or the interscan interval [14,15]. Studies have found that a higher frequency of head-up positively correlates with predator detection [16]. Scan frequency is higher in novel environments [17,18], when further away from cover [19] or at the periphery of groups [20], when in smaller groups [12,21] and once a predator has been detected [13,17]
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