PARASITISM OF HOUSE WREN NESTS BY BROWN-HEADED COWBIRDS - WHY IS IT SO RARE

We tested five hypotheses that may explain why House Wren (Troglodytes aedon) nests are rarely parasitized by Brown-headed Cowbirds (Molothr us ater). House Wrens may prevent parasitism in five ways: (1) by choo sing to nest in cavities with small entrances (inaccessible-entrance h ypothesis), (2) by restricting the size of the entrance with nest mate rial (nest-structure hypothesis). (3) by puncturing and ejecting paras itic eggs (puncture-ejection hypothesis), (4) by burying the parasitiz ed clutch under a new nest (egg-burial hypothesis), or (5) by abandoni ng the parasitized nest altogether (nest-desertion hypothesis). We tes ted these hypotheses in field experiments and found that (i) female co wbirds cannot enter circular entrances smaller than 38 mm in diameter, (ii) wrens prefer cavities with small entrances (inaccessible to cowb irds) to those with large entrances (accessible to cowbirds), (iii) wh en forced to breed in cavities with large entrances, wrens do not redu ce the entrance size with nest material, (iv) despite the unusual stre ngth of cowbird eggs, wrens are physically capable of puncture-ejectin g them, (v) wrens do not puncture-eject cowbird eggs from their own ne sts, (vi) wrens do nor abandon parasitized nests or bury the parasitiz ed clutches under new nests. These results are consistent with the ina ccessible-entrance hypothesis. We propose that additional nesting adap tations, as well as active cowbird avoidance of House Wrens, may contr ibute to the low frequency of cowbird parasitism.