Brood Parasites as Predators: Farming and Mafia Strategies. In: Soler M, editor. Avian Brood Parasitism. Cham: Springer International Publishing; 2017. pp. 271-86. [DOI: 10.1007/978-3-319-73138-4_15]

Three sympatric host nestlings eavesdrop on cuckoo nestling distress calls

In predator-prey interactions, the prey faces extreme challenges from predation, which drives the evolution of defense or anti-predator mechanisms. Compared with adult birds, nestlings are more vulnerable but not helpless. However, data on whether nestlings eavesdrop on the danger signals transmitted by other prey nestlings and the mechanisms of eavesdropping remain limited. In brood parasitism, common cuckoo (Cuculus canorus) nestlings, raised by host adults who are not closely related, offer an instructive system for studying the transmission and recognition of danger signals among nestlings of different species that share special relationships. We played back the distress calls of common cuckoo nestlings to nestlings of three sympatric host species (the oriental reed warbler Acrocephalus orientalis, which is a primary host of the common cuckoo, the reed parrotbill Paradoxornis heudei, an occasional host, and the vinous-throated parrotbill Sinosuthora webbiana, which is not parasitized in the study area) to investigate whether the host nestlings reduced their begging behavior. We also quantified the degree of inhibition toward begging behavior for these nestlings. The results revealed that, in response to the distress calls, the three sympatric host species markedly suppressed their begging behavior. This response can likely be attributed to the innate response of host nestlings caused by the general characteristics of distress calls, rather than the acoustic similarity and phylogenetic relationship between host nestlings and cuckoo nestlings. Furthermore, we observed that upon hearing the distress calls of cuckoo nestlings, the oriental reed warbler nestlings exhibited the greatest reduction in the total number of calls compared to the other two host species, potentially owing to stronger predation and parasitic pressures. This study suggests that host nestlings can detect danger signals emitted by parasitic nestlings; however, further investigation is needed to determine whether they can respond to distress calls from unfamiliar nestlings in different regions.

Video evidence that cuckoos farm their hosts by ejecting nestlings

When host nests are scarce, avian brood parasites would benefit from behaviours that increase the availability of suitable nests. Several studies reported ejection of host nestlings from nests by brood parasites; however, whether brood parasites do so to induce the host to re-nest and thus increase opportunities for future parasitism (i.e. 'farming' behaviour) remains unclear. Here, we report observational evidence of farming behaviour by a common cuckoo Cuculus canorus female in a Daurian redstart Phoenicurus auroreus population: (1) the cuckoo destroyed a host nest by ejecting all nestlings, (2) the host then produced a new nest and (3) the cuckoo successfully parasitized the replacement nest. We suggest that farming behaviour may be more common, but often goes undetected because it requires intense nest monitoring.

Behavioral and Acoustic Responses of the Oriental Reed Warbler (Acrocephalus orientalis), at Egg and Nestling Stages, to the Common Cuckoo (Cuculus canorus)

Cuckoo nest parasites lay eggs in host nests and thereby transfer all reproduction costs to the hosts. This greatly reduces host fitness. Parasitism has selected for the evolution of anti-parasitic strategies in hosts, including nest defense. The dynamic risk assessment hypothesis holds that nest parasitism only threatens the nests during the egg stage, so hosts should reduce the level of defense against nest parasites after the egg stage. We studied the behavioral and acoustic responses of oriental reed warblers (Acrocephalus orientalis), during both the egg and nestling stages, toward the common cuckoo (Cuculus canorus), sparrowhawks (Accipiter nisus) and oriental turtle doves (Streptopelia orientalis). A. orientalis can visually distinguish cuckoos from sparrowhawks and doves, indicating that hawk mimicry did not work for the cuckoos. The behavioral response of hosts in the nestling stage was stronger than in the egg stage, which supports the offspring value hypothesis and suggests that cuckoos may also act as nest predators. However, there was no difference in the alarm calls A. orientalis produce in response to different invaders, indicating that different types of alarm calls may not contain specific information.

Host personality predicts cuckoo egg rejection in Daurian redstarts Phoenicurus auroreus

In species that are subject to brood parasitism, individuals often vary in their responses to parasitic eggs, with some rejecting the eggs while others do not. While some factors, such as host age (breeding experience), the degree of egg matching and the level of perceived risk of brood parasitism have been shown to influence host decisions, much of the variation remains unexplained. The host personality hypothesis suggests that personality traits of the host influence its response to parasitic eggs, but few studies have tested this. We investigated the relationship between two personality traits (exploration and neophobia) and a physiological trait (breathing rate) of the host, and egg-rejection behaviour in a population of Daurian redstarts Phoenicurus auroreus in northeast China. We first show that exploratory behaviour and the response to a novel object are repeatable for individual females and strongly covary, indicating distinct personality types. We then show that fast-exploring and less neophobic hosts were more likely to reject parasitic eggs than slow-exploring and more neophobic hosts. Variation in breathing rate-a measure of the stress-response-did not affect rejection behaviour. Our results demonstrate that host personality, along the bold-shy continuum, predicts the responses to parasitic eggs in Daurian redstarts, with bold hosts being more likely to reject parasitic eggs.

Threat recognition and response in an avian brood-parasite host from New Caledonia

Nest predation and avian brood parasitism are the main sources of nest failure in many passerine birds. Large predators threaten both brood and parents, whereas brood parasites pose only a danger to eggs or nestlings. The fan-tailed gerygone Gerygone flavolateralis from New Caledonia is subjected to high rates of nest predation by the New Caledonian crow Corvus moneduloides (responsible for about 20–40% of predation) and moderate rates of brood parasitism by the shining bronze-cuckoo Chalcites lucidus (parasitizing about 18% of nests), which also depredates nests that are too advanced for parasitism (13% of nests). To test if fan-tailed gerygones are able to discriminate predators from brood parasites, we presented 3 bird models at active gerygone nests: a brood parasite/small nest predator (shining bronze-cuckoo), a large nest predator (crow), and a small non-native bird (common chaffinch Fringilla coelebs), which is unknown to the gerygone, as a control. We assessed the response of adult gerygones to the presentation of each model by measuring the minimum approach distance, number of alarm calls, number of attacks, and time to first nest visit after the presentation (latency). Adult gerygones often attacked the cuckoo, approached but never attacked the chaffinch and always avoided the crow. Latency was shorter after an attack response and during brooding, but similar among models. We did not find any link between the cuckoo model presentation and later ejection of cuckoo nestlings. We conclude that adult fan-tailed gerygones discriminate between different models and respond accordingly to the level of threat, but do not show awareness of parasitism risk and increase of nestling ejection rates following exposure to the cuckoo model.

Signal detection and optimal acceptance thresholds in avian brood parasite-host systems: implications for egg rejection

Signal detection theory addresses the challenge of successfully identifying informative signals in noisy information contexts, allowing optimal behavioural decisions in diverse ecological contexts. The optimal acceptance threshold hypothesis proposed by Reeve (Reeve 1989 Am. Nat. 133, 407-435. (doi:10.1086/284926)) is an elegant theoretical model to predict the flexibility of acceptance thresholds for conspecific discrimination. This model has provided a robust framework used to explore recognition systems in a broad range of contexts such as animal communication, nest-mate discrimination or anti-parasitic host responses. In this review, we discuss key concepts related to the optimal acceptance threshold hypothesis applied to egg rejection decisions in avian brood parasite-host interactions. We explore those factors determining signal detectability in parasitized nests and how hosts adjust their rejection decisions to both the risk of parasitism and the potential costs associated with egg rejection. In addition, we discuss recent results that challenge some traditional assumptions of the optimal acceptance threshold hypothesis and provide a novel perspective to explore rejection decisions, such as the existence of single-threshold decision rules or acceptance decisions. An integrative view combining current evidence with traditional theory is needed to further advance the comprehension of optimal acceptance thresholds. This article is part of the theme issue 'Signal detection theory in recognition systems: from evolving models to experimental tests'.

Flexible responses to stage-specific offspring threats

When caring for their young, parents must compensate for threats to offspring survival in a manner that maximizes their lifetime reproductive success. In birds, parents respond to offspring threats by altering reproductive strategies throughout the breeding attempt. Because altered reproductive strategies are costly, when threats to offspring are limited, parents should exhibit a limited response. However, it is unclear if response to offspring threat is the result of an integrated set of correlated changes throughout the breeding attempt or if responses are a flexible set of dissociable changes that are stage-specific. We test these hypotheses in a system where house wrens (Troglodytes aedon) compete for nesting cavities with Carolina chickadees (Poecile carolinensis) by usurping and destroying their nests during the early stage of the breeding attempt (the egg stage). Due to the specificity of the house wren threat, we can test whether parental responses to an offspring threat show flexibility and stage specificity or if parental strategies are an integrated and persistent response. We monitored nests in a natural population to compare life history traits of chickadees nesting in boxes that were in the presence of house wrens to chickadees nesting in boxes that did not overlap with house wrens. Carolina chickadees that nested near house wrens laid significantly smaller clutch sizes (early change in reproductive strategy) but did not alter nestling provisioning or nestling stage length (late change in reproductive strategy), suggesting that chickadees respond in a flexible and stage-specific manner to the threat of house wrens. By responding only when a threat is highest, parents minimize the cost of antithreat responses. Our study suggests that parents can respond in subtle and nuanced ways to offspring threats in the environment and specifically alter reproductive behaviors at the appropriate stage.