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rather than the parasite's egg (Box 4.2). Therefore, as for prey, host defences are costly.
However, the costs of misimprinting would become prohibitively large at the chick
stage if hosts are faced with parasitic chicks, like the common cuckoo, which hatch
early and eject the host eggs. Then, if hosts are parasitized in their first breeding
attempt, all they see in the nest at the chick stage is a cuckoo chick. If they imprint on
this, then they'd reject their own chicks in future, unparasitized clutches. Faced with
BOX 4.2 SIGNAL DETECTION (REEVE 1989)
Imagine a world with both 'desirable' and 'undesirable' signallers. For a cuckoo
host, desirable signallers are their own eggs, undesirable signallers are cuckoo
eggs. For a predator, desirable signallers are tasty prey, undesirable signallers
are inedible objects or noxious prey. Now imagine there is some overlap in the
signal from desirable and undesirable signallers (host eggs are mimicked by
cuckoo eggs; tasty prey resemble inedible objects or noxious prey), as shown
in Fig. B4.2.1.
Desirable
Undesirable
A
B
Signal characteristics
Fig. B4.2.1
Where should the host or predator set its rejection threshold? If it rejected
signals to the right of A (a strict threshold), it would reject all undesirable
signallers but at the cost of rejecting many desirable signallers too. At the other
extreme, if it rejected signals to the right of B (a lax threshold), all desirable
signallers would now be accepted but at the cost of accepting many undesirable
signallers too. The optimal threshold depends on: (i) the relative frequency of
desirable and undesirable signallers, and (ii) the economics of the four
outcomes: accepting versus rejecting desirable signallers and accepting versus
rejecting undesirable signallers.
For reed warbler hosts whose clutch may be parasitized by a common cuckoo,
the following pay-offs (explained in the footnotes) apply (modified from Davies
et al ., 1996a):
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