Differential reproductive success favours strong host preference in a highly specialized brood parasite

et al. 2018

Ibis

Young birds communicate their need to parents through complex begging displays that include visual and acoustic cues. Nestlings of interspecific brood parasites must ‘tune’ into these communication channels to secure parental care from their hosts. Various studies show that parasitic nestlings can effectively manipulate host parental behaviour through their begging calls, but how these manipulative acoustic signals develop in growing parasites remains poorly understood. We investigated the influence of social experience on begging call development in a host‐specialist brood parasite, the Screaming Cowbird Molothrus rufoaxillaris. Screaming Cowbird nestlings look and sound similar to those of the primary host, the Greyish Baywing Agelaioides badius. This resemblance is likely to be adaptive because Baywings discriminate against fledglings unlike their own and provision nests at higher rates in response to Baywing‐like begging calls than to non‐mimetic begging calls. By means of cross‐fostering and playback experiments, we tested whether the acoustic cues that elicit recognition by Baywings develop innately in Screaming Cowbird nestlings or are acquired through social experience with host parents or nest mates. Our results suggest that begging call structure was partially modulated by experience because Baywing‐reared Screaming Cowbird and host nestlings were acoustically more similar as age increased, whereas acoustic similarity between cross‐fostered and Baywing‐reared Screaming Cowbird nestlings decreased from 4–5 to 8–10 days of age. Cross‐fostered Screaming Cowbirds developed begging calls of lower minimum frequency and broader bandwidth than those of Baywing‐reared Screaming Cowbirds by the age of 8–10 days. Despite the observed differences in begging call structure, however, adult Baywings responded similarly to begging calls of 8‐ to 10‐day‐old cross‐fostered and Baywing‐reared Screaming Cowbirds, suggesting that these were functionally equivalent from the host's perspective. These findings support the idea that, although rearing environment can influence certain begging call parameters, the acoustic cues that serve for offspring recognition by Baywings develop in young Screaming Cowbirds independently of social experience.

Section: Discussionsupporting

confidence: 77%

Section: Discussionmentioning

confidence: 99%

Section: Data Collection and Analysismentioning

confidence: 99%

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Innate development of acoustic signals for host parent–offspring recognition in the brood‐parasitic Screaming CowbirdMolothrus rufoaxillaris

Ripari

Ursino

et al. 2018

Ibis

“…When translocated to nests of Chalk-browed Mockingbirds (Mimus saturninus), a species not used as a host by this parasite, Screaming Cowbird chicks are often outcompeted for food by host chicks (De Mársico and Reboreda 2008). By contrast, Screaming Cowbird chicks are rarely outcompeted for food in nests of Brown-and-yellow Marshbirds (Mermoz and Fernández 2003) or Chopi Blackbirds (present study).…”

Section: Discussionmentioning

confidence: 50%

Reproductive success of the specialist brood parasite Screaming Cowbird in an alternative host, the Chopi Blackbird

Giácomo¹,

2015

The Auk

The Screaming Cowbird (Molothrus rufoaxillaris) is the most specialized brood-parasitic cowbird, relying almost entirely on the Bay-winged Cowbird (Agelaioides badius) as host. Recently, Screaming Cowbirds have expanded their range to areas where Bay-winged Cowbirds are absent, and they are exploiting the Chopi Blackbird (Gnorimopsar chopi). Interactions between Screaming Cowbirds and Chopi Blackbirds are largely unexplored, as is the reproductive success of the parasite in this host. Screaming Cowbirds, Chopi Blackbirds, and Bay-winged Cowbirds coexist in northeastern Argentina, providing an ideal system to explore interactions between a specialist brood parasite and an alternative host and to compare the reproductive success of the parasite in its main host and in an alternative host. Screaming Cowbirds parasitized both hosts throughout their breeding seasons (Chopi Blackbirds, mid-October to mid-January; Bay-winged Cowbirds, mid-November to mid-March). Frequency of parasitism was lower in Chopi Blackbirds than in Bay-winged Cowbirds (46% vs. 74%). Nest survival was higher in Chopi Blackbirds than in Bay-winged Cowbirds (37% vs. 15%). In successful nests, survival of Screaming Cowbird eggs and chicks was high and relatively similar in both hosts (Chopi Blackbirds: eggs, 99%; chicks, 90%; Bay-winged Cowbirds: eggs, 93%; chicks, 93%), but hatchability was lower in Chopi Blackbirds than in Bay-winged Cowbirds (52% vs. 92%). Considering (1) nest survival and (2) egg survival, hatchability, and chick survival in successful nests, the reproductive success of Screaming Cowbirds (i.e. proportion of eggs that resulted in fledglings) was 0.17 in Chopi Blackbirds and 0.12 in Bay-winged Cowbirds. Our results indicate that the Chopi Blackbird is a frequent host of the Screaming Cowbird, and parasitism of this alternative host may help explain the range expansion of this parasite in areas of Brazil where the Bay-winged Cowbird is absent.Keywords: brood parasitism, Gnorimopsar chopi, host use, Molothrus rufoaxillariś Exito reproductivo del parasito de cría especialista Molothrus rufoaxillaris en un hospedador alternativo, Gnorimopsar chopi RESUMENMolothrus rufoaxillaris es el tordo parásito de cría con mayor grado de especialización y depende casi exclusivamente de Agelaioides badius como hospedador. Recientemente M. rufoaxillaris ha expandido su rango hacia áreas en las que A. badius está ausente y la evidencia indica que estarían utilizando a Gnorimopsar chopi como hospedador. Las interacciones entre M. rufoaxillaris y G. chopi son mayormente desconocidas así como eléxito reproductivo del parásito en este hospedador. Las tres especies (M. rufoaxillaris, G. chopi y A. badius) coexisten en el NE de Argentina, ofreciendo un sistema ideal para explorar las interacciones entre un parásito de cría especialista y un hospedador alternativo y para comparar eléxito reproductivo del parásito en sus hospedadores principal y alternativo. M. rufoaxillaris parasitó a ambos hospedadores a lo largo de sus temporadas reproductivas (...

“…Females in other cowbird species do not remove eggs, but puncture one or more host eggs. Such behavior has been observed in Shiny ( M. bonariensis ; Hoy and Ottow 1964, Post and Wiley 1977, Nakamura and Cruz 2000), Screaming ( M. rufoaxillaris ; Fraga 1998, De Mársico and Reboreda 2008), and Bronzed ( M. aeneus ; Carter 1986, Peer and Sealy 1999) cowbirds. Egg‐puncturing behavior appears to have evolved at least twice in other brood parasitic lineages, including Great Spotted Cuckoos ( Clamator glandarius ; Soler et al 1997) and Greater Honeyguides ( Indicator indicator ; Spottiswoode and Colebrook‐Robjent 2007).…”

mentioning

confidence: 81%

Function of egg punctures by Shiny Cowbirds in parasitized and nonparasitized Creamy-bellied Thrush nests

Astié

Journal of Field Ornithology

2009

Avian brood parasites usually remove or puncture host eggs. Several hypotheses have been proposed to explain the function of these behaviors. Removing or puncturing host eggs may enhance the efficiency of incubation of cowbird eggs (incubation‐efficiency hypothesis) or reduce competition for food between cowbird and host chicks in parasitized nests (competition‐reduction hypothesis) and, in nonparasitized nests, may force hosts to renest and provide cowbirds with new opportunities for parasitism when nests are too advanced to be parasitized (nest‐predation hypothesis). Puncturing eggs may also allow cowbirds to assess the development of host eggs and use this information to decide whether to parasitize a nest (test‐incubation hypothesis). From 1999 to 2002, we tested these hypotheses using a population of Creamy‐bellied Thrushes (Turdus amaurochalinus) in Argentina that was heavily parasitized by Shiny Cowbirds (Molothrus bonariensis). We found that 56 of 94 Creamy‐bellied Thrush nests (60%) found during nest building or egg laying were parasitized by Shiny Cowbirds, and the mean number of cowbird eggs per parasitized nest was 1.6 ± 0.1 (N= 54 nests). At least one thrush egg was punctured in 71% (40/56) of parasitized nests, and 42% (16/38) of nonparasitized nests. We found that cowbird hatching success did not differ among nests where zero, one, or two thrush eggs were punctured and that the proportion of egg punctures associated with parasitism decreased as incubation progressed. Thus, our results do not support the incubation‐efficiency, nest‐predation, or test‐incubation hypotheses. However, the survival of cowbird chicks in our study was negatively associated with the number of thrush chicks. Thus, our results support the competition‐reduction hypothesis, with Shiny Cowbirds reducing competition between their young and host chicks by puncturing host eggs in parasitized nests.