Brian D. Peer

CORRELATES OF EGG REJECTION IN HOSTS OF THE BROWN-HEADED COWBIRD

Abstract We conducted a comparative analysis of eight potential correlates of egg rejection in hosts of the parasitic Brown-headed Cowbird (Molothrus ater) to test the evolutionary equilibrium and evolutionary lag hypotheses as explanations for the acceptance of cowbird parasitism. The analyses generally supported evolutionary lag. Historic contact with cowbirds may explain why hosts that have recently come into contact with cowbirds accept parasitism, but it does not account for acceptance by hosts with long histories of contact with cowbirds. Egg predation by hosts, nest sanitation, population size, and egg appearance were not correlated with rejection. Larger species that typically build larger nests were more likely to reject. Large hosts may have been parasitized more frequently in the past, possibly due to their more easily found nests or superiority as hosts, and as a result, may have had more opportunity to evolve rejection. Rejection was also correlated with taxonomic affiliation, suggesting that once rejection evolves it is maintained, which implies that rejection is not costly and thus argues against an evolutionary equilibrium. Not surprisingly, hosts with large bills were more likely to reject. This may be a corollary of the tendency for large hosts, which tend to have larger bills, to reject. An evolutionary equilibrium may exist for hosts with eggs that resemble cowbird eggs, depending on the costs to host reproductive success and the likelihood of committing recognition errors. Nevertheless, some hosts have been in contact with cowbirds for a long time, build large nests, have large bills, have a “favorable” phylogeny, and lay eggs that differ from cowbird eggs, yet accept cowbird parasitism. Chance may play a role in the accumulation of the necessary recombinants and mutations necessary for the evolution of rejection. Factores Correlacionados con el Rechazo de Huevos por parte de Hospederos de Molothrus ater Resumen. Realizamos un análisis comparativo de ocho factores que potencialmente podrían estar correlacionados con el rechazo de huevos por parte de hospederos del parásito de cría Molothrus ater para poner a prueba las hipótesis de equilibrio evolutivo y de demora en la respuesta evolutiva propuestas para explicar la aceptación del parasitismo. Los análisis generalmente apoyaron la hipótesis de la demora en la respuesta evolutiva. El contacto histórico con Molothrus podría explicar por qué hospederos que sólo recientemente han entrado en contacto con estas aves aceptan el parasitismo, pero no explica por qué hay hospederos que tienen largas historias de contacto con el parásito y aceptan ser parasitadas. La depredación de huevos por parte del hospedero, el comportamiento de aseo del nido, el tamaño poblacional y la apariencia de los huevos no estuvieron correlacionados con el rechazo de huevos parásitos. Las especies grandes, que típicamente construyen nidos más grandes, presentaron una probabilidad más alta de rechazar huevos. Los hospederos más grandes podrían haber sido parasitados más frecuentemente en el pasado, posiblemente debido a que sus nidos se encuentran con mayor facilidad o a que son mejores hospederos y, como resultado, podrían haber tenido mayores oportunidades evolutivas para desarrollar el comportamiento de rechazar huevos. El comportamiento de rechazo también estuvo correlacionado con la filiación taxonómica, lo que sugiere que una vez que éste evoluciona se mantiene, implicando que no es costoso, lo que a su vez es un argumento en contra de la hipótesis del equilibrio evolutivo. De forma poco sorprendente, los hospederos con picos grandes fueron más propensos a rechazar huevos. Esto podría ser un corolario de la tendencia de los hospederos grandes (los cuales tienden a tener picos más grandes) a rechazar huevos. Es posible que exista un equilibrio evolutivo para los casos de hospederos que tienen huevos que se asemejan a los de Molothrus, dependiendo de los costos sobre el éxito reproductivo del hospedero y la probabilidad de cometer errores de reconocimiento. Sin embargo, algunos hospederos han estado en contacto con Molothrus por períodos prolongados, construyen nidos grandes, tienen picos grandes, tienen afinidades filogenéticas “favorables” y ponen huevos que difieren de los del parásito, y aún así aceptan el parasitismo. El azar podría jugar un papel importante en la acumulación de los recombinantes y mutaciones necesarios para que el comportamiento de rechazo pueda evolucionar.

IMPACT OF BLACKBIRD DAMAGE TO SUNFLOWER BIOENERGETIC AND ECONOMIC MODELS

We constructed bioenergetic and economic models to estimate the potential impact of Red-winged Blackbirds (Agelaius phoeniceus), Common Grackles (Quiscalus quiscula), and Yellow-headed Blackbirds (Xanthocephalus xanthocephalus) on production yields of sunflower in the northern Great Plains of North America. The amount of sunflower consumed annually by males and females, after considering field metabolic rates, energy value and moisture content of achenes, and percentage of sunflower in diets was, respec- tively: Red-winged Blackbirds 277 g and 168 g; Common Grackles 267 g and 230 g; and Yellow-headed Blackbirds 248 g and 139 g. The per capita annual economic damage was: male Red-winged Blackbirds

FATE OF GRACKLE (QUISCALUS SPP.) DEFENSES IN THE ABSENCE OF BROOD PARASITISM: IMPLICATIONS FOR LONG-TERM PARASITE-HOST COEVOLUTION

0.09 (U.S. dollars), females

EGG DESTRUCTION AND EGG REMOVAL BY AVIAN BROOD PARASITES: ADAPTIVENESS AND CONSEQUENCES

0.05; male Common Grackles

Persistence of host defence behaviour in the absence of avian brood parasitism

0.09, females

Phenotypic Plasticity in Common Crackles (Quiscalus quiscula) in Response to Repeated Brood Parasitism

0.07; and male Yellow-headed Blackbirds

Intermediate Frequencies of Egg Ejection by Northern Mockingbirds (Mimus polyglottos) Sympatric with Two Cowbird Species

0.08, females

Variation in host response to brood parasitism reflects evolutionary differences and not phenotypic plasticity

0.05. Annual loss was

Intraclutch variation in egg appearance of Brown-headed Cowbird hosts

5.4 + 1.3 X 106 for all three species in aggregate, with Red-winged Blackbirds accounting for 52% of the loss. Blackbird damage represented 1.7% of the dollar value of the 1999 sunflower harvest in the northern Great Plains. This loss would be inconsequential if damage were distributed evenly; however, bird damage is often localized around wetlands and can be economically debilitating to individual producers. Although our model was based on regional population estimates, it should perform well at local scales, provided that a local population can be defined, accurately estimated, and remains stable in size over the six-week length of the damage period. Because of the large numbers of blackbirds that congregate in the region during August and September prior to migration, sunflower pro- ducers should expect some crop losses. The solution to the conflict appears to be one that focuses not on eliminating all damage, but on preventing it from exceeding 5% per field.

Effect of parasite-to-host egg ratio on egg rejection by a Brown-headed Cowbird host

Abstract We tested grackles (Quiscalus spp.) to determine whether they retain egg rejection behavior in the absence of the selection pressure of brood parasitism. Neither Bronzed Cowbird (Molothrus aeneus) nor Brown-headed Cowbird (M. ater) parasitism was recorded in 797 Great-tailed Grackle (Q. mexicanus) nests. Cross-fostered Bronzed Cowbird nestlings, but not Brown-headed Cowbird nestlings, fledged from Great-tailed Grackle nests, indicating that Brown-headed Cowbird parasitism does not select for rejection in these grackles. Great-tailed Grackle populations sympatric and allopatric with Bronzed Cowbirds rejected 100% of model cowbird eggs. An allopatric population of Boat-tailed Grackle (Q. major), a sister species of the Great-tailed Grackle, also rejected 100% of model eggs. Egg rejection in the Boat-tailed Grackle has apparently been retained in the absence of parasitism for as long as 800,000 years since it split from the Great-tailed Grackle. The Common Grackle (Q. quiscula), which lays the most variable eggs among the grackles, also has the lowest level of egg rejection—which is consistent with the argument that it may have lost most of its rejection behavior in the absence of parasitism. With extreme intraclutch egg-variation, Common Grackles may be more likely to reject their own oddly colored eggs, which would select against rejection behavior in the absence of parasitism. Those results have significant implications for long-term parasite-host coevolution, because they suggest that egg rejection has been retained in most species of Quiscalus in the absence of parasitism. If typical of the worlds avifauna, such retention may force brood parasites to specialize on a few host species and to rarely return to using old hosts, which would readily reject their eggs.