Strategic egg destruction by brood parasitic cowbirds?

FIORINI, V.D.; GLOAG, R.; KACELNIK, A.; REBOREDA, J.C.

ANIMAL BEHAVIOUR

ACADEMIC PRESS LTD-ELSEVIER SCIENCE LTD

Lugar: Amsterdam; Año: 2014 vol. 93 p. 229 - 235

0003-3472

Obligate avian brood parasites do not provide direct care to their young but can indirectly increase their offspring?s success in host nests. One way in which parasitic cowbirds (Molothrus sp.) could achieve this is through egg-puncturing whereby, prior to laying in a nest, females puncture holes in the eggs already present so as to reduce the competition that their offspring will later face for food. In this study we investigated whether cowbirds strategically increase their puncturing effort with increasing competitiveness of the future brood. We filmed egg-puncturing behaviour by shiny cowbirds (M. bonariensis) at nests of chalk-browed mockingbirds (Mimus saturninus), a large host whose nests often receive multiple cowbird eggs. We presented cowbirds with large (4-eggs) or small (1-egg) clutches of either mockingbird or cowbird eggs, where large clutch sizes predict greater intrabrood competition than small clutch sizes, and mockingbird eggs (which are larger) predict greater competition than other cowbird eggs. The number of pecks made per visit by cowbirds and the number of eggs they successfully punctured was higher in larger clutches, while mockingbird eggs were broken more than cowbird eggs, but pecked less per visit. The higher number of pecks aimed at cowbird eggs, despite these producing less-competitive nestmates, could reflect responses to eggshell strength rather than egg size, as cowbird eggs are harder to break because of their thicker shells and so require more effort to puncture. Our results show that cowbird puncturing behaviour is not rigid and varies with nest contents. We suggest this variation is consistent with females increasing their offspring?s chance of survival.