Imperfect mimicry of host begging calls by a brood parasitic cuckoo: a cue for nestling rejection by hosts?

Abstract

Coevolutionary interactions between avian brood parasites and their hosts often lead to the evolution of discrimination and rejection of parasite eggs or chicks by hosts based on visual cues, and the evolution of visual mimicry of host eggs or chicks by brood parasites. Hosts may also base rejection of brood parasite nestlings on vocal cues, which would in turn select for mimicry of host begging calls in brood parasite chicks. In cuckoos that exploit multiple hosts with different begging calls, call structure may be plastic, allowing nestlings to modify their calls to match those of their various hosts, or fixed, in which case we would predict either imperfect mimicry or divergence of the species into host-specific lineages. In our study of the little bronze-cuckoo (LBC) Chalcites minutillus and its primary host, the large-billed gerygone Gerygone magnirostris, we tested whether: (1) hosts use nestling vocalizations as a cue to discriminate cuckoo chicks; (2) cuckoo nestlings mimic the host begging calls throughout the nestling period; and (3) the cuckoo begging calls are plastic, thereby facilitating mimicry of the calls of different hosts. We found that the begging calls of LBCs are most similar to their gerygone hosts shortly after hatching (when rejection by hosts typically occurs) but become less similar as cuckoo chicks get older. Begging call structure may be used as a cue for rejection by hosts, and these results are consistent with gerygone defenses selecting for age-specific vocal mimicry in cuckoo chicks. We found no evidence that LBC begging calls were plastic.

Keywords: begging call; bronze-cuckoo; brood parasitism; coevolution; gerygone.

Figure 1.

(A) Mean ± SE duration (seconds) of begging calls produced by LBG chicks (n = 7), accepted cuckoo chicks (n = 4), and rejected cuckoo chicks (n = 3) on hatching day. (B) Sonograms of nestling begging calls for LBGs chicks, accepted cuckoo chicks, and rejected cuckoo chicks.

Figure 2.

Duration (seconds) of begging calls in relation to age of LBGs (green, n = 36 chicks), LBCs (blue, n = 38), and fairy gerygones (red, n = 2).

Figure 3.

Sonograms of nestling begging calls for LBCs, LBGs (primary host), fairy gerygones (secondary host), and LFWs (nonhost) at 3 different ages (Days 3, 7, and 13) during the nestling period.

Figure 4.

Canonical plots from discriminant function analysis based on the 6 begging call measurements (call duration [sec], high frequency [kHz], low frequency [kHz], peak frequency [kHz], frequency bandwidth [kHz], and the difference in frequency between the beginning and the end of the call) from LBG chicks (blue), and LBC chicks reared by LBGs (red) at 3 different nestling stages, Day 0 (double solid line) and Day 7 (single solid line), and Day 13 (dotted line). Discriminant function analysis labels each multivariate mean with a circle. The size of the circle corresponds to a 95% confidence limit for the mean. Groups that are significantly different have nonintersecting circles.

Figure 5.

Canonical plots from discriminant function analysis based on the 6 begging call measurements from LBG (blue), LFW chicks (yellow), and LBC chicks reared by LBG LBG_cuckoo, red) and LFWs (LFW_cuckoo, green) at 3 different ages (Days 3, 7, and 13) during the nestling period. Discriminant function analysis labels each multivariate mean with a circle. The size of the circle corresponds to a 95% confidence limit for the mean. Groups that are significantly different have nonintersecting circles.

Figure 6.

(A) Growth of LBC chicks in LBG nests (primary host, n = 28, red points and line) and LFW nests (naïve host, n = 14, blue points and line). The average growth curve from all data points is shown in black. (B) Boxplot of residuals of each data point from the average growth curve.

Figure 7.

Regression plots for LBC and LBG showing coefficients of variation by nestling age for (A) call duration, (B) frequency bandwidth, (C) maximum frequency, (D) minimum frequency, (E) peak frequency, and (F) difference in frequency.