Dc. Dearborn et al., EFFECTS OF COWBIRD PARASITISM ON PARENTAL PROVISIONING AND NESTLING FOOD ACQUISITION AND GROWTH, The Condor, 100(2), 1998, pp. 326-334
Brood parasitism by Brown-headed Cowbirds (Molothrus ater) is known to
affect the fitness of many hosts by causing a reduction in the number
of chicks that fledge from parasitized nests. However, little is know
n about less immediate effects on host fitness. We studied nestling gr
owth and food acquisition and parental provisioning in parasitized and
unparasitized nests of the Indigo Bunting (Passerina cyanea). Indigo
Bunting nestlings in parasitized nests exhibited reduced rates of mass
gain, but not tarsus growth, relative to bunting chicks in unparasiti
zed nests. Bunting nestlings in parasitized nests received less food t
han did buntings in unparasitized nests. Buntings in parasitized nests
spent more time begging than did those in unparasitized nests, but en
ergy expended in this behavior may not have detracted greatly from the
amount of energy available for growth. Adults at parasitized nests ex
hibited a higher provisioning rate than those at unparasitized nests.
Increased provisioning by adult buntings at parasitized nests did not
come at the expense of time spent brooding nestlings, but increased pr
ovisioning has the potential to affect the survival and future reprodu
ctive success of host adults. Because cowbird parasitism appears to im
pose substantial costs on Indigo Bunting nestlings and adults, concern
over the conservation implications of parasitism should not be limite
d to species that suffer total reproductive failure when parasitized.