Evolutionary lag versus bill-size constraints: a comparative study of the acceptance of cowbird eggs by old hosts

SummaryEvolutionary lag and resistance to puncture-ejection are two hypotheses explaining why hosts long exposed to brood parasitism by molothrine cowbirds accept their eggs. The former hypothesis assumes no physical constraints while the latter requires them. We show (1) that most acceptors of Brown-headed Cowbird eggs have small bills, (2) that the single species known to puncture cowbird eggs for ejection also has a small bill, and (3) that all grasp ejectors of Brown-headed Cowbird eggs have large bills. We propose that most acceptors cannot remove cowbird eggs by grasp ejection because their bills are too small. Small hosts probably cannot puncture cowbird eggs for ejection because their unusually thick shells make the costs of puncturing them higher than the benefits of ejection. This hypothesis is capable of explaining the success of cowbirds with scores of host species in the puzzling absence of egg mimicry.

[1]  R. N. Mack,et al.  Evolution in Steppe with Few Large, Hooved Mammals , 1982, The American Naturalist.

[2]  R. B. Payne,et al.  The Ecology of Brood Parasitism in Birds , 1977 .

[3]  J. Picman.  Destruction of eggs by the long-billed marsh wren (Telmatodytes palustris palustris) , 1977 .

[4]  H. Friedmann.  Further Information of the Host Relations of the Parasitic Cowbirds , 1971 .

[5]  D. Lack,et al.  Ecological adaptations for breeding in birds , 1969 .

[6]  M. Brittingham,et al.  Have Cowbirds Caused Forest Songbirds to Decline , 1983 .

[7]  R. J. Robertson,et al.  The function and evolution of aggressive host behavior towards the brown-headed cowbird (Molothrus ater) , 1977 .

[8]  James N. M. Smith.  Cowbird Parasitism, Host Fitness, and Age of the Host Female in an Island Song Sparrow Population , 1981 .

[9]  P. F. Elliott.  Cowbird Parasitism in the Kansas Tallgrass Prairie , 1978 .

[10]  S. Rohwer,et al.  A comparative study of eggshell thickness in cowbirds and other passerines , 1987 .

[11]  L. H. Walkinshaw.  The Effect of Parasitism by the Brown-Headed Cowbird on Empidonax Flycatchers in Michigan , 1961 .

[12]  R. J. Robertson,et al.  Cowbird parasitism and evolution of anti-parasite strategies in the yellow warbler , 1981 .

[13]  R. D. Guthrie.  Bison Evolution and Zoogeography in North America During the Pleistocene , 1970, The Quarterly Review of Biology.

[14]  H. W. Hann.  Life History of the Oven-Bird in Southern Michigan , 1937 .

[15]  Robert M. May,et al.  Population Dynamics of Avian Brood Parasitism , 1985, The American Naturalist.

[16]  Scott,et al.  FECUNDITY OF THE BROWN-HEADED COWBIRD IN SOUTHERN ONTARIO , 2003 .

[17]  J. Graber.  Distribution, Habitat Requirements, and Life History of the Black-Capped Vireo (Vireo atricapilla) , 1961 .

[18]  Rothstein,et al.  COWBIRD PARASITISM AND EGG RECOGNITION OF THE NORTHERN ORIOLE , 2002 .

[19]  R. Hill.  HOST-PARASITE RELATIONSHIPS OF THE BROWN-HEADED COWBIRD IN A PRAIRIE HABITAT OF WEST-CENTRAL KANSAS , 2002 .

[20]  S. I. Rothstein.  Successes and Failures in Avian Egg and Nestling Recognition With Comments on the Utility of Optimality Reasoning , 1982 .

[21]  S. I. Rothstein,et al.  A further contribution of knowledge of the host relations of the parasitic cowbirds , 1977 .

[22]  S. I. Rothstein.  An Experimental and Teleonomic Investigation of Avian Brood Parasitism , 1975 .

[23]  D. Finch.  Rejection of Cowbird Eggs by Crissal Thrashers , 1982 .

[24]  G. Orians,et al.  Blackbirds of the Americas , 1985 .