Solo and duet calling in the pheasant coucal: sex and individual call differences in a nesting cuckoo with reversed size dimorphism

Duetting and female vocalisations have rarely been studied in tropical non-passerines. In coucals, a subfamily of nesting cuckoos, these behaviours have evolved under unusual conditions of male-biased parental care and reversed sexual size dimorphism. Here we provide the first detailed description of the structure and occurrence of sex-specific calls and duets in a monogamous coucal species, the pheasant coucal, Centropus phasianinus. Pheasant coucals of either sex use two types of far-reaching (>1 km) calls in their solo and duet displays: the ‘descending whoops’ call and, less frequently (25%), ‘scale’ calls. Both calls are series of very deep hooting notes that the larger females produce at lower frequency than the males (~326 versus 480 Hz). Descending whoops calls also vary among individuals but this difference is not consistent enough for individual identification. Most duets (63%) comprise a single scale call by each partner and the sexes start duets with equal frequency. Duetting triggers neighbouring pairs to duet too, suggesting a role in territory defence. Calling is most intense in the morning and early in the breeding season. The cryptic behaviour of coucals makes their calls an important tool for studies on their mating system, survey work and taxonomic research.

Mating Patterns and Reversed Size Dimorphism in Southern Skuas (Stercorarius skua lonnbergi)

Numerous explanations exist for the evolution of reversed size dimorphism in raptorial species. A recent study concluded that reversed size dimorphism in skuas and jaegers was probably not attributable to breeding-role specialization, but that there was evidence for sexual selection, and in particular intrasexual competition by females for males. Our study tested the applicability of those conclusions for Southern (or Brown) Skuas (Stercorarius skua lonnbergi) breeding in South Georgia. Clutch volume was related positively to size and condition of females and negatively to condition of males, but there was no evidence of assortative mating for size or condition within pairs. Potential explanations for the discrepancy between this and previous studies are that size is less closely correlated with individual quality because of highly diverse foraging strategies, territory quality is a confounding factor, or because lower aggression in Southern Skuas reduces the necessity for small females to avoid large males.