Discrimination and ejection of eggs and nestlings by the fan-tailed gerygone from New Caledonia

Abstract

Nestling rejection is a rare type of host defense against brood parasitism compared with egg rejection. Theoretically, host defenses at both egg and nestling stages could be based on similar underlying discrimination mechanisms but, due to the rarity of nestling rejector hosts, few studies have actually tested this hypothesis. We investigated egg and nestling discrimination by the fan-tailed gerygone Gerygone flavolateralis, a host that seemingly accepts nonmimetic eggs of its parasite, the shining bronze-cuckoo Chalcites lucidus, but ejects mimetic parasite nestlings. We introduced artificial eggs or nestlings and foreign gerygone nestlings in gerygone nests and compared begging calls of parasite and host nestlings. We found that the gerygone ejected artificial eggs only if their size was smaller than the parasite or host eggs. Ejection of artificial nestlings did not depend on whether their color matched that of the brood. The frequency of ejection increased during the course of the breeding season mirroring the increase in ejection frequency of parasite nestlings by the host. Cross-fostered gerygone nestlings were frequently ejected when lacking natal down and when introduced in the nest before hatching of the foster brood, but only occasionally when they did not match the color of the foster brood. Begging calls differed significantly between parasite and host nestlings throughout the nestling period. Our results suggest that the fan-tailed gerygone accepts eggs within the size range of gerygone and cuckoo eggs and that nestling discrimination is based on auditory and visual cues other than skin color. This highlights the importance of using a combined approach to study discrimination mechanisms of hosts.

Keywords: begging calls; brood parasitism; co-evolutionary arms race; egg discrimination; nestling discrimination; nestling polymorphism.

Figure 1.

(A) Eggs of shining bronze-cuckoo (left) and fan-tailed gerygone (right). (B) Artificial eggs for Experiment 1: parasite-like brown (left), blue (center), and host-like whitish-gray with brown speckles (right). (C) Artificial eggs for Experiment 2: low reflectance (left, only large size shown) and high reflectance (right, only small size shown). (D) Bright artificial nestling (left) with bright fan-tailed gerygone nestling (right). (E) Dark artificial nestling (left) with dark fan-tailed gerygone nestling (right). (F) Bright shining bronze-cuckoo nestling. (G) Dark shining bronze-cuckoo nestling. All nestlings in the photos are newly hatched (Day 0). Gerygone and cuckoo nestlings are not at the same scale.

Figure 2.

Proportion (with 95% Bonferroni CIs) of ejected artificial eggs in Experiment 1 (A, n = 15 blue, n = 15 parasite-like, n = 14 host-like) and ejected cross-fostered nestlings according to presence/absence of natal down (B, n = 8 natal down present, n = 6 natal down absent) and timing of introduction to the foster brood (C, n = 7 before hatching of the foster brood, n = 7 after hatching).

Figure 3.

Logistic regression (estimate = 0.053, se = 0.028) for the probability of fan-tailed gerygones to accept (0) or eject (1) the artificial nestling in relation to day of the season (1 = 9th September). Black points are artificial nestling ejection trials and the gray area around the regression line is the 95% confidence band. Vertical bars are the proportions of cuckoo nestlings ejected by host parents observed within 20-day periods during the 8 breeding seasons.

Figure 4.

Canonical plot of the first and second linear discriminant functions for the 8 measured parameters of the begging calls in 0–3 days old nestlings. Orange dots are fan-tailed gerygone nestlings, red dots are shining bronze-cuckoo nestlings accepted by host parents and blue dots are shining bronze-cuckoo nestlings ejected by their host parents within 1–2 days from hatching. Ellipses are 95% CIs around the centroid of the distribution for each nestling type.

Figure 5.

Variation of begging call parameters in relation to age of fan-tailed gerygone (orange) and shining bronze-cuckoo (blue) nestlings. The grey shaded areas around the regression lines are the 95% confidence bands.