Specific responses of cuckoo hosts to different alarm signals according to breeding stage: a test of the offspring value hypothesis

Abstract

The selective pressure exerted by avian brood parasites forces their hosts to evolve specific defense strategies. When subject to brood parasite attack, avian hosts will often emit alarm calls. To date, few studies have examined whether and how host responses to different alarm calls indicative of different enemies vary with the host’s breeding stage. We carried out alarm call playback experiments during both the egg and nestling stages of the oriental reed warbler (Acrocephalus orientalis), a host of the common cuckoo (Cuculus canorus). The playback exemplars were selected from recorded alarm calls of the warbler to the presence of common cuckoos, sparrowhawks (Accipiter nisus), and oriental turtle doves (Streptopelia orientalis), which represented brood parasite, predator, and harmless control, respectively. The results showed that the oriental reed warblers did not discriminate alarm calls issued to different intruder types, but the intensity of the response was significantly higher in the nestling stage than in the egg stage. Attack behavior related to sparrowhawk alarm calls was absent in the egg stage, but aggressive behavior increased dramatically and exceeded the attack frequency in response to the cuckoo alarm call in the nestling stage, implying a shift in the tradeoff between the parents’ own survival and the loss of offspring. Alarm calls attracted a larger number of conspecifics than members of other species. In general, the oriental reed warbler had consistently stronger responses to different alarm calls in the nestling stage than in the egg stage, supporting the offspring value hypothesis.

Parent–offspring and inter-offspring responses to conspecific versus heterospecific distress calls in 2 sympatric birds

“…The average rainfall is 527.8 mm, and the average annual temperature is 12.9 °C. The main vegetation in this area includes reeds Phragmites australis , cattails Typha latifolia , and other herbaceous species ( Wang and Yang 2020 ). There are many species of predators for VP and ORW in this wetland, including the Siberian weasel Mustela sibirica , the Erythema snake Dinodon rufozonatum , and the Brown rat Rattus norvegicus .…”

“…The chicks that were used to record the distress calls were excluded from the playback experiment. To obtain high-quality distress calls, we used Raven Pro version 1.4 software (Cornell lab of ornithology, Cornell University) to chip the sound that we recorded, removed noise below 0.2 kHz, and randomly combined the recordings to form 30 and 60 s distress calls ( Wang and Yang 2020 ), but we did not change the syllable type or call rate in the segment of distress calls. We recorded the background noise from the wetland and randomly chose and chipped the background noise to make 3 0.3-s sound fragments, and we randomly combined these fragments to form the 30 and 60 s background noise ( Supplementary Figure S1 ).…”

Aggressive hosts are undeterred by a cuckoo's hawk mimicry, but probably make good foster parents

“…Either drongos adaptively mob each species independently to the same degree, or they do not discriminate between the two. If the latter, this would contrast with multiple other brood-parasitic systems where hosts distinguish other cuckoo species from the local raptor species they resemble [17,21,23,[54][55][56][57]. It is likely that for those hosts, interacting with a hawk represents a greater risk than it does for a drongo, a species which regularly responds aggressively to raptors [25].…”

Three sympatric host nestlings eavesdrop on cuckoo nestling distress calls

“…The average annual rainfall is 527.8 mm, which is primarily concentrated in the summer, and the annual average temperature is 12.9°C. The primary vegetation of this wetland includes reeds Phragmites australis , cattails Typha latifolia , and other herbaceous plants (Wang & Yang, 2020 ).…”

“…One such example is the oriental reed warbler ( Acrocephalus orientalis ); upon detecting a predator, this species exhibits aggressive behavior and emits alarm calls to attract help from conspecifics and heterospecifics, thereby increasing the chance of expelling the predator from its territory (Wang et al., 2020 , 2021 ). Additionally, they adjust their defense strategies according to their breeding status (Montgomerie & Weatherhead, 1988 ; Shew et al., 2016 ; Wang & Yang, 2020 ). Conversely, nestlings cannot actively flee and are therefore vulnerable to predators.…”