Host alarm calls attract the unwanted attention of the brood parasitic common cuckoo

Abstract

It is well known that avian brood parasites lay their eggs in the nests of other bird species, called hosts. It remains less clear, however, just how parasites are able to recognize their hosts and identify the exact location of the appropriate nests to lay their eggs in. While previous studies attributed high importance to visual signals in finding the hosts' nests (e.g. nest building activity or the distance and direct sight of the nest from vantage points used by the brood parasites), the role of host acoustic signals during the nest searching stage has been largely neglected. We present experimental evidence that both female and male common cuckoos Cuculus canorus pay attention to their host's, the great reed warbler's Acrocephalus arundinaceus alarm calls, relative to the calls of an unparasitized species used as controls. Parallel to this, we found no difference between the visibility of parasitized and unparasitized nests during drone flights, but great reed warblers that alarmed more frequently experienced higher rates of parasitism. We conclude that alarm calls might be advantageous for the hosts when used against enemies or for alerting conspecifics, but can act in a detrimental manner by providing important nest location cues for eavesdropping brood parasites. Our results suggest that host alarm calls may constitute a suitable trait on which cuckoo nestlings can imprint on to recognize their primary host species later in life. Our study contributes to the growing body of knowledge regarding the context-dependency of animal signals, by providing a novel example of a beneficial acoustic trait intercepted by a heterospecific and used against the emitter.

Figure 1

Mosaic plot showing that both female (n = 14) and male (n = 30) common cuckoos exhibited intensive responses during the 2 minutes when their host, the great reed warbler, engaged in nest defense activities, compared to the 2 minutes prior to the reed warblers alarming. Both female and male cuckoo responses were scored on the following scale: 0 – no response; 1 – typical female or male call within 100 m; 2 – flying towards the alarming great reed warblers; 3 – flying towards the alarming great reed warblers and vocalizing.

Figure 2

Mosaic plot showing that both female (n = 32) and male (n = 32) common cuckoos responded positively (i.e. flew closer to the playback device compared to the position where they were initially observed) to the alarm call of great reed warblers by approaching the loudspeaker, compared to the trials when the calls of collared doves were played as a control treatment.

Figure 3

Barplot showing that alarming great reed warblers experience higher probability of brood parasitism by common cuckoos than non-alarming conspecifics. Values indicating the probability of brood parasitism are predicted based on the minimal adequate model (see Table 1) from the correlative study (see main text). Mean predicted values ± standard error (SE) are shown.

Figure 4

The 3D printed common cuckoo decoy used in our study. The decoy was painted with acrylic paint and mounted on a pole supplied with a noiseless electric motor to stimulate horizontal movement.