Explanations for the Infrequent Cowbird Parasitism on Common Grackles

Abstract

Abstract. We determined the factors responsible for the lack of parasitism on Common Grackles (Quiscalus quiscula) by Brown-headed Cowbirds (Molothrus ater). We found no evidence of parasitism on the 401 grackle nests we monitored in east-central Illinois. By the time cowbirds began laying eggs, 88.5% of all grackle nests were beyond the point of successful parasitism. Grackles rejected cowbird eggs more frequently during the prelaying stage of the nesting cycle (38.2%) compared to later stages (12.3%). Thirty-three cowbird eggs and nestlings were cross-fostered into grackle nests. Data were collected on 15 cowbird nestlings, of which three fledged. The cross-fostered cowbird eggs that did not hatch appeared to have been incubated ineffectively. These eggs were in clutches that contained four to six eggs, whereas the cowbird eggs that hatched were in clutches of three eggs or less. Grackles also responded more aggressively toward female cowbird models than to Fox Sparrow (Passerella iliaca) models in five of seven response categories. Cowbirds avoid parasitizing grackles even though the opportunity existed. Nest abandonment and infestation of grackle nests with ectoparasitic mites may decrease the suitability of grackles as hosts; however the primary reason for the infrequent cowbird parasitism on grackles appears to be past rejection behavior. Grackles likely rejected cowbird eggs at a higher frequency in the past and as a consequence grackles were avoided by cowbirds. Rejection apparently decreased in the absence of parasitism due to the high degree of intraclutch egg variability exhibited by grackles which would increase the chances of them mistakenly rejecting their own discordant eggs.

Host use strategies of individual female brown‐headed cowbirdsMolothrus aterin a diverse avian community

“…It is unlikely that unsuitable host species completely prevented parasitism because even highly aggressive species are often successfully parasitized (Strausberger and Horning 1998, Strausberger 2001). Female cowbirds consistently and nearly completely avoid such unsuitable host species such as mourning doves, common grackles and barn swallows throughout their range (Friedmann 1963, Friedmann et al 1977, Strausberger and Ashley 1997, Peer and Bollinger 1997Peer and Bollinger 1998). On the other hand, the occasional parasitism of house finches is a case that appears to be maladaptive and possibly reflects limitations in cowbirds’ host avoidance.…”

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

“…Likewise, screaming cowbirds may avoid currently parasitizing host species that have evolved effective defences against parasitism. Currently, unused hosts could have defences against screaming cowbirds if interactions between them occurred in the past (Peer & Bollinger 1997;Arias de Reyna 1998;Rothstein 2001; but see Soler et al 2003), or if they evolved antiparasitic defences in response to other brood parasites (i.e. shiny cowbirds) that also serve to protect them against screaming cowbirds (Rothstein et al 2002).…”

The Brown-headed Cowbird: North America’s avian brood parasite

“…Perhaps as a result, the Brown-headed Cowbird lacks some adaptations possessed by other brood parasites (Mermoz and Ornelas, 2004) including mimicry of host eggs (Rothstein and Robinson, 1998; but see Peer et al 2002), or evidence of specialized nestling adaptations such as directly killing nestmates (Lichtenstein and Sealy, 1998;Peer et al, 2013). Among the adaptations possessed by cowbird are that females have a larger hippocampus to remember where host nests are located (Sherry et al, 2003); laying eggs rapidly and before sunrise to avoid detection by hosts (Scott, 1991;Sealy et al, 1995; see also Peer and Sealy, 1999b); thick eggshells presumably to withstand puncture-ejection by hosts (Picman, 1989); greater pore diameter in their eggshells for increased embryonic respiration and shorter incubation periods (Jaeckle et al, 2012; see also Briskie and Sealy, 1990); removal of host eggs by females in conjunction with parasitism to enhance incubation efficiency (Peer and Bollinger, 1997; and for nutrition (Sealy, 1992); egg puncture and killing nestlings to force hosts to renest providing additional chances for parasitism (Arcese et al, 1996;Elliott, 1999;Hoover and Robinson, 2007;Dubina and Peer, 2013); and possibly forcing hosts to accept parasitism through mafia enforcement tactics (Hoover and Robinson, 2007).…”