Visual discrimination of polymorphic nestlings in a cuckoo-host system

Abstract

Mimicry by avian brood parasites favours uniformity over variation within a breeding attempt as host defence against parasitism. In a cuckoo-host system from New Caledonia, the arms race resulted in both host (Gerygone flavolateralis) and parasite (Chalcites lucidus) having nestlings of two discrete skin colour phenotypes, bright and dark. In our study sites, host nestlings occurred in monomorphic and polymorphic broods, whereas cuckoo nestlings only occurred in the bright morph. Irrespective of their brood colour, host parents recognised and ejected parasite nestlings but never ejected their own. We investigated whether host parents visually recognised their own nestlings by using colour, luminance and pattern of multiple body regions. We found that the parasite mimicked multiple visual features of both host morphs and that the visual difference between host morphs was larger than the difference between the parasite and the mimicked host morph. Visual discrimination alone may result in higher chances of recognition errors in polymorphic than in monomorphic host broods. Host parents may rely on additional sensorial cues, not only visual, to assess nestling identity. Nestling polymorphism may be a trace of evolutionary past and may only have a marginal role in true-recognition of nestlings in the arms race in New Caledonia.

Figure 1

Colour (a) and luminance (b) just noticeable differences (JND, mean ± 95% CI). Horizontal bars with P values indicate pairwise comparisons between groups or regions. Higher JND values indicates higher colour differences in the eyes of host parents between nestling types in each group. Sample size (number of comparisons) for dorsum and skin regions are Bright-Dark = 60, Cuckoo-Bright = 45, Cuckoo-Dark = 12 and for the flanges region are Bright-Dark = 27, Cuckoo-Bright = 18, Cuckoo-Dark = 6.

Figure 2

Regressions lines (with 95% CI as shaded area) of skin luminance in relation to age for the bright (solid line) and dark (dashed line) morphs of Fan-tailed Gerygone nestlings.

Figure 3

Mean pattern differences (pattern size, spacing and contrast) with 95% CI between nestling types in each group for each region of interest. Horizontal bars with P values indicate pairwise comparisons between groups or regions.