Egg puncture allows shiny cowbirds to assess host egg development and suitability for parasitism

Abstract

Parasitic cowbirds and cuckoos generally reduce the clutch size of the hosts they parasitize by removing or destroying some of their eggs. Shiny cowbirds (Molothrus bonariensis) puncture their hosts' eggs both when parasitizing the nests and also when they do not parasitize them. We propose that, by puncturing the host's eggs, shiny cowbirds gain an informational bene¢t. They assess the degree of development of the host's embryos and so avoid laying in nests that would not provide enough incubation time for the parasitic eggs to hatch. Two predictions follow: (i) punctures should occur in advance or immediately before parasitic events, and (ii) the occurrence of parasitism should depend on the degree of development of the host's embryos when punctures occurred, i.e. on the stage of incubation. Both predictions are supported by our data of shiny cowbirds parasitizing yellow-winged blackbirds (Agelaius thilius). Egg punctures are not used to reset the host's nesting attempt when shiny cowbirds do not parasitize the nests. We discuss the potential mechanisms implicated in egg development assessment and propose a critical experiment to test this hypothesis.

Cuckoo adaptations: trickery and tuning

“…If a host can prevent parasitism through nest defence, it saves its entire clutch. If it delays defence until the egg stage, it will have lost some of its clutch because the cuckoo usually removes one or more host eggs before it lays (Davies & Brooke, 1988) or it may puncture host eggs (Soler, Soler & Martinez, 1997; see also Massoni & Reboreda, 1999; Spottiswoode & Colebrook‐Robjent, 2007). If the host delays further, until the chick stage, it will have probably lost its entire clutch (ejector cuckoos) or at least some of its clutch or brood (non‐ejector cuckoos).…”

“…She also depredates host clutches too late to parasitize (where incubation has already begun), which then makes the host's replacement clutch available for parasitism (Gärtner, 1981; Gehringer, 1979). It would be interesting to know whether female cuckoos identify these advanced clutches by pecking eggs to test their stage of development (as cowbirds do, Massoni & Reboreda, 1999), or whether they use cues from host behaviour (e.g. incubation).…”

Can nest predation explain the lack of defenses against cowbird brood parasitism in the Rufous-collared Sparrow (Zonotrichia capensis)?

“…The hatching and fledging success of nests in which we removed cowbirds eggs were similar to those of parasitized nests. This result is not totally surprising, given that one of the main costs suffered by hosts of the cowbird is egg puncture, which occurs before or during the parasitism event (Mermoz and Reboreda 1994, Massoni and Reboreda 1999. On the other hand, nest desertion was also associated with cowbird parasitism, but this behavior is likely a response to an abnormal clutch size due to multiple parasitism events (i.e., the nest received >1 cowbird egg), to egg losses due to egg punctures (Rothstein 1975;Fraga 1978Fraga , 1983, or to disturbance resulting from increased activity of cowbirds at the nest, rather than constituting a specific antiparasite response.…”

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

“…Similarly, Peer and Sealy (1999) suggested that Bronzed Cowbirds puncture host eggs to force renesting. Yet another hypothesis to explain egg punctures in nests that are not subsequently parasitized is that cowbirds do so to assess the degree of embryonic development of the egg, and use this information to decide whether to parasitize a nest (test‐incubation hypothesis; Livesey 1936, Massoni and Reboreda 1999).…”