Abstract
Animals rely on an array of environmental triggers or cues to make their
behavioural and life-history decisions. In the case of brood parasites,
it is adaptive to use a suitable host to guarantee maximum fitness.
Despite the fact that the Eurasian reed warbler (Acrocephalus
scirpaceus) is the most frequent host of the common cuckoo (Cuculus
canorus), we video recorded that cuckoo young often fall out of the
nests of this host before fledging time and drown. To establish whether
these events are influenced by the size of the nest, we replaced
original Eurasian reed warbler nests for much bigger great reed warbler
(Acrocephalus arundinaceus) nests and transferred cuckoo chicks into
them. Subsequently, we video-recorded the fate of these cuckoo chicks.
In these bigger nests, we recorded a significantly lower falling-out
rate (4 of 29) compared with those remained in the real reed warbler
nests (12 of 32). This result suggests that host nest size plays an
important role in the suitability of reed warbler host species and that
Eurasian reed warbler nests have a high mortality risk to the relatively
big cuckoo chicks. Moreover, we found that cuckoo chicks that fell out
of the nests were significantly less fed by foster parents during the
three hours before falling-out event than chicks that stayed in the
nests. This suggests that also insufficient parental care in terms of
lower feeding intensity may facilitate falling-out of cuckoo chicks,
probably because hungry chicks are more restless in host nests. The
relatively high falling-out rate represents a substantial and
underestimated cost of brood parasites. Moreover, without video
surveillance it can be incorrectly confused with predation. Therefore,
we strongly recommend using video-recording for estimating predation
costs in birds.