Biology Reference
In-Depth Information
mimetic egg, timing of laying and unusually fast laying are all adapted to increase the
success of parasitism (Davies & Brooke, 1988).
Secondly, egg mimicry by cuckoos evolves in response to host discrimination. To
quantify egg mimicry it is important to do so as through a bird's eye. Birds have
better colour vision than humans, with four cone types sensitive, respectively, to
very short (ultraviolet), short (blue), medium (green) and long (red) wavelengths.
Mary Caswell Stoddard and Martin Stevens (2011) measured the reflectance spectra
of cuckoo and host eggs from the various cuckoo host races (Fig. 4.20a), calculated
how these wavelengths would be captured by the four cone types, and then plotted
the colours in tetrahedral colour space (Fig. 4.20b). They found that the match
between cuckoo and host eggs varied across the different host races of cuckoo
(Fig. 4.20c), with a better match when the hosts were stronger discriminators
(Fig. 4.20d).
A question that is still unresolved is whether these differences in host discrimination
represent: (i) different equilibria set by differences in the costs and benefits of egg
rejection or (ii) evolution in progress, with older hosts having stronger rejection. For
example, dunnock hosts may reject foreign eggs less than great reed warbler hosts
either because: (i) rejection is more costly or of less benefit (lower parasitism risk) or
(ii) because dunnocks are more recent hosts and have not had sufficient time to evolve
defences.
Cuckoos have
evolved host-egg
mimicry
Hosts have evolved in response to cuckoos
If host discrimination evolves specifically in response to brood parasitism, then
we would predict that species untainted by cuckoos would show no rejection of
odd eggs. This is indeed what is found; species that are unsuitable as hosts,
either because of a seed diet (young cuckoos need to be raised on invertebrates) or
because they nest in holes (inaccessible to a female cuckoo), show little, if any,
rejection of eggs unlike their own and, in contrast to hosts, they also show little
aggression to adult cuckoos near their nests (Davies & Brooke, 1989a, 1989b;
Moksnes
et al
., 1991).
Hosts not only evolve egg rejection as a defence, their egg patterns evolve, too,
providing more distinctive 'signatures' to signify 'this is my egg'. Compared to
species with no history of brood parasitism, species exploited by cuckoos have less
variation in the appearance of eggs within a clutch and more variation between
clutches of different females (Stokke
et al
., 2002). This makes life harder for the
cuckoo, since it is easier for a host to spot a foreign egg if all its own eggs look exactly
the same, and distinctive markings for individual host females makes it harder for
the cuckoo to evolve a convincing forgery of that species' eggs.
The diversity of egg markings within one host species is especially remarkable in an
African warbler, the tawny-flanked prinia,
Prinia subflava
, which is parasitized by the
cuckoo finch,
Anomalospiza imberbis
(Fig. 4.21). The host eggs vary in background
colour, marking size, variation in markings and in marking dispersion. These four
parameters vary independently, which is exactly what would be predicted if hosts had
been selected to maximize the individual distinctiveness of their egg signatures.
Defences evolve
in response to
cuckoo parasitism
Host egg
signatures and
cuckoo forgeries
