Egg rejection in blackbirds Turdus merula: a by-product of conspecific parasitism or successful resistance against interspecific brood parasites?

Francisco Ruiz-Raya,Gianluca Roncalli,Teresa Abaurrea,Juan Diego Ibáñez-Álamo,Manuel Soler

doi:10.1186/s12983-016-0148-y

Abstract

BackgroundTraditional theory assumes that egg recognition and rejection abilities arise as a response against interspecific brood parasitism (IBP). However, rejection also appears in some species that are currently not exploited by interspecific parasites, such as Turdus thrushes. Recent evidences suggest that rejection abilities evolved in these species as a response to conspecific brood parasitism (CBP). To test these two alternative hypotheses, we performed an experimental study by parasitizing nests of the common blackbird (Turdus merula) with conspecifics or heterospecific eggs under different risk of parasitism (presence of interspecific or conspecific parasites near the nest). Common blackbird is a potential host of the common cuckoo (Cuculus canorus) but suffers low levels of CBP too.ResultsWe found that blackbirds were able to recognize and eject heterospecific eggs at high rates whereas most of conspecifics eggs were not recognized and, therefore, accepted. Ejection rates of conspecific eggs did not exceed 13 %, even in situations of high risk of CBP (blackbird female placed near the nest), which contradict the main prediction derived from the CBP hypothesis. Conversely, ejection rates of experimental eggs simulating IBP were much higher (80–100 %). Furthermore, female blackbirds were more aggressive towards cuckoos than towards blackbird dummies.ConclusionsOur results considered together support the IBP hypothesis, indicating that recognition and rejection of parasitic eggs in blackbirds have probably evolved due to previous cuckoo parasitism. The current absence of IBP in blackbirds may be due to the highly efficient rejection abilities in this species. Thus, these abilities have been retained in absence of brood parasitism as a consequence of the low costs involved for blackbirds, resulting in a successful resistance against interspecific brood parasitism.

Highlights

Traditional theory assumes that egg recognition and rejection abilities arise as a response against interspecific brood parasitism (IBP)
Samas et al [14], in an experimental study with two species of Turdus thrushes: the common blackbird (Turdus merula; blackbird hereafter) and the song thrush (Turdus philomelos), concluded that egg discrimination in thrushes has evolved as a response to conspecific brood parasitism (CBP) instead of IBP based on the ejection rate of conspecific eggs found in their study (~20–40 %) and the existence of CBP in their blackbird populations (CBP rates of 3.1 and 0 % in the areas of sympatry and allopatry with the common cuckoo - Cuculus canorus; cuckoo hereafter, respectively)
There are no reasons to think that the existence of rejection abilities in blackbirds could not have evolved in response to IBP because the maintenance of rejection abilities in the absence of brood parasitism is a frequent long-term outcome of the relationships between interspecific brood parasites and their hosts [3, 18, 25, 26]

Summary

Introduction

Traditional theory assumes that egg recognition and rejection abilities arise as a response against interspecific brood parasitism (IBP). Samas et al [14], in an experimental study with two species of Turdus thrushes: the common blackbird (Turdus merula; blackbird hereafter) and the song thrush (Turdus philomelos), concluded that egg discrimination in thrushes has evolved as a response to CBP instead of IBP based on the ejection rate of conspecific eggs found in their study (~20–40 %) and the existence of CBP in their blackbird populations (CBP rates of 3.1 and 0 % in the areas of sympatry and allopatry with the common cuckoo - Cuculus canorus; cuckoo hereafter -, respectively). Based on these two points, they proposed that IBP is unlikely to be the factor responsible of the evolution of egg rejection in thrushes and suggested that CBP constitutes an evolutionary scenario comparable to IBP that could produce the same antiparasitic adaptations in hosts This is an important conclusion with great impact in the field of brood parasitism that deserves to be studied in detail. According to calculations in Soler [3], 29.7 % of potential host species that are not currently parasitized reject nearly 100 % of nonmimetic eggs

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