Biology Reference
In-Depth Information
0.75
6
0.7
5
0.65
4
0.6
3
0.55
2
0.5
1
0.45
0.40
0
0
0.5
1
1.5
2
2.5
3
3.5
4
4
8
Mean number of host young
fledged per parasitized nest
Days after hatching
Fig. 8.13
(a) A brown-headed cowbird chick grows best when it shares the nest with two host young. Each
point refers to a different host species. The curve is the fitted polynomial regression. (b) An experiment in
which a cowbird chick is raised by eastern phoebe host parents, either on its own (light blue bars) or together
with two host young (dark blue bars). The cowbird grows best when it is with host young. From Kilner
et al
.
(2004). Reprinted with permission from AAAS. The photograph shows an eastern phoebe with two of its own
chicks (yellow gapes) and a brown-headed cowbird chick. Photo © Marie Read.
cowbird chick can then exploit by grabbing more than its fair share in competition with
the host young. Comparisons of the growth rate of a cowbird chick when raised in the
nests of various host species showed that the cowbird did best when it had the company
of two host young (Fig. 8.13a). Experiments with cowbirds raised in the nests of Eastern
phoebes,
Sayornis phoebe
, also revealed that the cowbird grew better when it shared the
nest with two host young than when it was reared alone (Fig. 8.13b). Therefore, the
cowbird uses the host young to help it procure resources from the host parents. The
optimal number of host young is two; presumably a larger number would take too much
of the additional food collectively solicited by the brood (Kilner
et al
., 2004).
How, then, does a common cuckoo chick manage on its own (see back to Fig. 4.19d)?
By ejecting all the host young from the nest, it benefits by removing the competition, but
then faces the cost of having to do all the work in soliciting food. The common cuckoo's
trick is a remarkable rapid begging call, which sounds like many hungry host young
(Fig. 8.14a). The way this works is subtle (Kilner
et al
., 1999). Experiments with reed
warbler
Acrocephalus scirpaceus
hosts show that when parents provision a brood of their
own young, they respond both to the total gape area on view (a visual cue) and the
begging rate of the brood (a vocal cue). The visual cue gives the parents a rough guide
of how much food to bring, because it is related to chick number (more chicks, more
gapes) and chick age (older chicks, larger gapes). The vocal cue enables parents to fine-
tune their provisioning in relation to chick hunger (hungrier chicks beg more rapidly).
So if host parents are given more chicks, or older chicks, they work harder and if the
brood's begging calls are augmented by playback of extra calls, they work harder still.
How does the common cuckoo chick exploit this system? The cuckoo needs as much
food as a brood of four reed warblers. Its problem is that although it is larger than any
one reed warbler chick, its gape area cannot match that of a whole brood of reed
warblers. To compensate for this deficient visual component of the begging display, the
Vocal trickery by
common cuckoo
chicks
