Host–parasite coevolution beyond the nestling stage? Mimicry of host fledglings by the specialist screaming cowbird

Abstract

Egg mimicry by obligate avian brood parasites and host rejection of non-mimetic eggs are well-known textbook examples of host-parasite coevolution. By contrast, reciprocal adaptations and counteradaptations beyond the egg stage in brood parasites and their hosts have received less attention. The screaming cowbird (Molothrus rufoaxillaris) is a specialist obligate brood parasite whose fledglings look identical to those of its primary host, the baywing (Agelaioides badius). Such a resemblance has been proposed as an adaptation in response to host discrimination against odd-looking young, but evidence supporting this idea is scarce. Here, we examined this hypothesis by comparing the survival rates of young screaming cowbirds and non-mimetic shiny cowbirds (Molothrus bonariensis) cross-fostered to baywing nests and quantifying the similarity in plumage colour and begging calls between host and cowbird fledglings. Shiny cowbirds suffered higher post-fledging mortality rates (83%) than screaming cowbirds (0%) owing to host rejection. Visual modelling revealed that screaming cowbirds, but not shiny cowbirds, were indistinguishable from host young in plumage colour. Similarly, screaming cowbirds matched baywings' begging calls more closely than shiny cowbirds. Our results strongly support the occurrence of host fledgling mimicry in screaming cowbirds and suggest a role of visual and vocal cues in fledgling discrimination by baywings.

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

“…The variables chosen have previously been used to analyse begging call structure of brood-parasitic nestlings (Madden & Davies 2006, Langmore et al 2008, De M arsico et al 2012, Gloag & Kacelnik 2013. We converted digital audio recordings to spectrograms using RAVEN PRO 1.4 (Cornell Bioacoustics Research Program, Ithaca, NY, USA) using default settings (Hann window of 256 samples and 248-Hz filter bandwidth, time grid with hop size of 128 samples and 50% overlap, and frequency grid with discrete Fourier transform size of 256 samples and grid spacing of 172 Hz).…”

“…Screaming Cowbird nestlings and fledglings closely resemble host young in visual appearance and begging calls (Fraga 1979, De M arsico et al 2012. Screaming Cowbird nestlings and fledglings closely resemble host young in visual appearance and begging calls (Fraga 1979, De M arsico et al 2012.…”

Avian vocal mimicry: a unified conceptual framework

“…However, under our functional definition, the begging calls of the common cuckoo chick are almost certainly mimetic. Langmore et al, 2008;De Mársico et al, 2012), but since adult cuckoos can use visual mimicry to gain access to host nests (Welbergen & Davies, 2011), it is possible that in some species adults may also use vocal mimicry to manipulate hosts. By contrast, a single chick of a blackbird, Turdus merula, or song thrush, T. philomelos, with the same mean mass as a cuckoo nestling, was fed at a significantly lower rate.…”

Russet Sparrows spot alien chicks from their nests

“…This selects for the evolution of defenses in hosts, which in turn selects for reciprocal counter-adaptations in the parasite. Although there is a vast literature on the reciprocal adaptations of brood parasites and their hosts at the pre-laying (Davies and Welbergen 2009;Feeney et al 2012), egg (Davies and Brooke 1988;Starling et al 2006;Spottiswoode and Stevens 2010;Yang et al 2010Yang et al , 2016aStoddard and Stevens 2011) and chick stages (Langmore et al 2003;de Mársico et al 2012;Yang et al 2015b), less is known about how the coevolution between parasites and hosts reach the current outcome of these arms races (Kilner and Langmore 2011). In particular, relatively few studies have investigated whether host defenses can result in escaping from brood parasitism.…”