Costs of egg punctures and parasitism by Shiny Cowbird (Molothrus bonariensis) at Creamy-bellied Thrush (Turdus amaurochalinus)nests

ASTIÉ, ANDREA ALEJANDRA; REBOREDA JUAN CARLOS

AUK

The American Ornithologists Union

Año: 2006 vol. 123 p. 23 - 32

0004-8038

Costs of egg punctures and Parasitism by Shiny Cowbirds at Creamy-bellied Thrushes nests Andrea A. Astie and Juan Carlos Reboreda Departamento de Ecología, Genética y Evolución, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Pabellón II Ciudad Universitaria, C1428EHA Buenos Aires, Argentina. Abstract.–Most studies on cowbird parasitism have focused on the impact produced by cowbirds at parasitized nests, but few of them have dealt with the costs at nests that cowbirds visit but at which they do not lay eggs. Shiny Cowbirds (Molothrus bonariensis) peck and puncture host eggs in nests where they lay, but also at unparasitized nests. In this study we analyze the impact of egg punctures in unparasitized and parasitized nests of a large host, the Creamy-bellied Thrush (Turdus amaurochalinus) as well as the costs of a cowbird egg and/or chick in this host´s nests. We determined thrush egg survival, hatching success, and chick survival in successful nests, and nest survival during the eggs and nestlings stages. The frequency of parasitism was 60% and its intensity 1.6 ± 0.1 eggs/nest. The number of host eggs punctured was positively associated with intensity of parasitism. Host´s eggs were frequently punctured in parasitized nests (71%) as well as in unparasitized nests (42%). Egg punctures reduced the number of eggs at hatching in 23% and 49% in unparasitized and parasitized nests, respectively. Nests with egg punctures had a lower survival than nests without them, but nest survival was not associated with parasitism. The presence of a cowbird egg was associated with a decrease in the hatching success of host eggs, but the presence of a cowbird chick did not have any detrimental effect on either the survival and growth rate of host chicks in successful nests, or the survival of the whole nest. Our results show that egg punctures were the primary determinant of thrush reproductive success. Consequently, comparison of unparasitized and parasitized nests gives an incomplete estimation of the impact that Shiny Cowbirds produce on host reproductive success, as the cost produced by egg punctures is also important in nests where there is no cowbird egg-laying.Abstract.–Most studies on cowbird parasitism have focused on the impact produced by cowbirds at parasitized nests, but few of them have dealt with the costs at nests that cowbirds visit but at which they do not lay eggs. Shiny Cowbirds (Molothrus bonariensis) peck and puncture host eggs in nests where they lay, but also at unparasitized nests. In this study we analyze the impact of egg punctures in unparasitized and parasitized nests of a large host, the Creamy-bellied Thrush (Turdus amaurochalinus) as well as the costs of a cowbird egg and/or chick in this host´s nests. We determined thrush egg survival, hatching success, and chick survival in successful nests, and nest survival during the eggs and nestlings stages. The frequency of parasitism was 60% and its intensity 1.6 ± 0.1 eggs/nest. The number of host eggs punctured was positively associated with intensity of parasitism. Host´s eggs were frequently punctured in parasitized nests (71%) as well as in unparasitized nests (42%). Egg punctures reduced the number of eggs at hatching in 23% and 49% in unparasitized and parasitized nests, respectively. Nests with egg punctures had a lower survival than nests without them, but nest survival was not associated with parasitism. The presence of a cowbird egg was associated with a decrease in the hatching success of host eggs, but the presence of a cowbird chick did not have any detrimental effect on either the survival and growth rate of host chicks in successful nests, or the survival of the whole nest. Our results show that egg punctures were the primary determinant of thrush reproductive success. Consequently, comparison of unparasitized and parasitized nests gives an incomplete estimation of the impact that Shiny Cowbirds produce on host reproductive success, as the cost produced by egg punctures is also important in nests where there is no cowbird egg-laying. Resumen.– La mayoría de los estudios sobre el parasitismo de los tordos se ha focalizado en el impacto producido por éstos en nidos parasitados, pero pocos trabajos han tratado los costos producidos en los nidos que los tordos visitan pero donde el parásito no pone huevos. El Tordo Renegrido, (Molothrus bonariensis) pica y perfora los huevos del hospedador en nidos donde pone sus huevos, pero también en nidos no parasitados. En este estudio analizamos el impacto de las perforaciones de huevos en nidos no parasitados y parasitados de un hospedador grande, el Zorzal Chalchalero (Turdus amaurochalinus) así como los costos producidos por huevos y pichones de tordo en nidos de este hospedador. Se determinó la supervivencia de huevos, el éxito de eclosión y la supervivencia de pichones de zorzal en nidos exitosos, y la supervivencia de los nidos durante los estadíos de huevos y pichones. La frecuencia de parasitismo fue 60% y su intensidad 1.6 ± 0.1 huevos/nido. El número de huevos del hospedador con perforaciones estuvo positivamente asociado con la intensidad de parasitismo. Los huevos del hospedador fueron frecuentemente perforados en nidos parasitados (71%) así como en nidos no parasitados (42%). Las perforaciones de huevos disminuyeron el número de huevos antes de la eclosión en 23% y 49% en nidos no parasitados y parasitados respectivamente. Los nidos con huevos perforados tuvieron una menor supervivencia que los nidos con huevos sin perforaciones, pero la supervivencia de los nidos no estuvo asociada al parasitismo. La presencia de un huevo de tordo estuvo asociada con un descenso del éxito de eclosión de los huevos del hospedador, pero la presencia de un pichón de tordo no tuvo efectos negativos sobre la supervivencia y el crecimiento de los pichones del hospedador en nidos exitosos, o sobre la supervivencia del nido. Nuestros resultados muestran que la perforación de huevos fueron el determinante primario del éxito reproductivo de los zorzales. Consecuentemente, comparar nidos no parasitados y parasitados brinda una estimación incompleta del impacto que el Tordo Renegrido produce sobre el éxito reproductivo del hospedador ya que el costo de perforación de huevos es también importante en nidos donde no se hay puesta de huevos del parásito.