Biology Reference
In-Depth Information
Reproductive success of host
Host response
Not parasitized
Parasitized
(4 host eggs)
(3 host + 1 cuckoo egg)
a
Accept
Correct acceptance
b
Missed rejection
c
(4)
(0)
Reject
False alarm
d
True alarm
e
(3)
(0.7
×
3)
+
(0.3
×
0)
=
2.1
a
Average clutch is four eggs. The cuckoo removes one host egg and replaces it with her
own egg.
b
Hosts raise all four eggs.
c
The cuckoo egg remains and the cuckoo chick
ejects all host eggs.
d
Assume hosts reject one egg.
e
Hosts again reject one egg but field
observations show that they make recognition errors: in 70% cases they reject the
cuckoo egg and so save their three remaining eggs but in 30% cases they reject one of
their own eggs, so the cuckoo egg remains and the cuckoo chick ejects their eggs.
It's clearly better to reject if parasitized (2.1>0) but to accept if not parasitized
(4>3). The critical parasitism frequency at which rejection does better than
acceptance is 32.2%. In fact, reed warblers accept most cuckoo eggs (parasitism
is usually much less than this critical frequency) unless they have seen a cuckoo
at their nest, in which case they are more likely to reject an egg.
The key point is that recognition systems are not perfect, so the best response
will be influenced by costs. A more sophisticated analysis of rejection of
cuckoo eggs is given in Rodriguez-Girones & Lotem (1999). For a signal-
detection approach to cuckoo chick rejection see Langmore
et al
. (2009),
and for alarm calling by prey (where predator recognition may not be perfect)
see Getty (2002).
this conundrum, the rule 'accept everything' is better at the chick stage than 'imprint
and reject' (Lotem, 1993).
This clever argument may help to explain why rejection of common cuckoo chicks
is rare. However, misimprinting cannot be a general constraint because in Australia,
hosts of bronze-cuckoos
Chalcites
spp. (where the cuckoo chick also ejects host eggs)
do regularly reject a cuckoo chick, either by abandoning it (superb fairy wrens
abandon 38% of Horsfield bronze cuckoos; Langmore
et al
., 2003) or by picking it up
and tossing it out of the nest (Sato
et al
., 2010). Bronze-cuckoo chicks have evolved
visual mimicry of their host's chicks, presumably in response to such host rejection
(Fig. 4.22).
Experiments show that superb fairy-wrens do not reject their own young after raising
a cuckoo, and with increased experience of raising their own young they are more likely
but, some
Australian hosts
do reject foreign
chicks … and their
cuckoos have
evolved mimetic
chicks
