Effects of brown-headed cowbirds on the nesting success of chipping sparrows in southwest Colorado

We documented effects of Brown-headed Cowbird (Molothrus ater) parasitism o n Chipping Sparrows (Spizella passerina) in southwest Colorado. Overall, 17 of 77 nests (22%) were parasitized. Abandonment was significantly higher a mong parasitized nests (29%) than unparasitized nests (5%). More Chipping S parrows hatched and hedged per unparasitized nest than per parasitized nest . Reduction of host hatching and fledging rates in parasitized nests was at tributable to smaller clutch size and higher abandonment. The major source of nest failure was predation, not parasitism: overall, 53% failed from pre dation, and only 6% failed from parasitism. There were no differences in we ight, ulna length, tarsometatarsus length, or length of outermost primary b etween sparrow nestlings in parasitized and unparasitized nests. Only 18% o f cowbird eggs laid resulted in a hedged cowbird. We observed no difference s in nest placement or nest concealment between unparasitized and parasitiz ed nests.