Biology Reference
In-Depth Information
(a)
(b)
-10
-20
45
40
35
30
25
20
15
10
5
yellow
-30
-40
red
-50
0
20
40
60
100
0 .1.2.3.4.5.6
% extra-pair young
% extra-pair young
Fig. 8.9
Sibling conflict within broods of nestling birds increases as relatedness declines.
(a) Nestlings beg more loudly in species where there is higher extra-pair parentage (i.e. lower
average relatedness between siblings). This significant relationship still holds when
controlling statistically for phylogeny, brood size and body mass. The species with 100%
extra-pair parentage is the brown-headed cowbird, a brood parasite unrelated to the
host young (Briskie
et al
. 1994). (b) Nestlings also have redder mouths in species with
higher extra-pair parentage, but only in species nesting in open nests (solid symbols; solid
line), not in those nesting in dark nests (open symbols: dashed line) (Kilner, 1999).
In precocial birds, the newly-hatched chicks have downy feathers and can run or (in
waterbirds) swim soon after hatching. In the rail family (Rallidae), 36 of the 97 species
studied had ornamented chicks, in the form of brightly coloured bills, fleshy patches or
plumes. A phylogenetic analysis revealed that chick ornamentation has evolved multiple
times within this family and is associated with increased sibling competition, as
measured by larger brood sizes and mating systems involving multiple parentage (hence
lower relatedness between siblings; Krebs & Putland, 2004).
So far, parental feeding preferences have been studied in just one species of this family,
the American coot
Fulica americana
, in which newly-hatched chicks have long, bright
orange tips to their black body feathers (Fig. 8.10a). Bruce Lyon and colleagues (1994)
performed an elegant experiment in central British Columbia, Canada. When whole
broods of chicks had their orange plumes trimmed, so they became black in colour, they
were fed by their parents just as well, and grew just as well as whole broods left with their
normal plumes intact (Fig. 8.10b). This shows that these plumes were entirely ornamental
and did not influence chick viability directly, for example by improving warmth. However,
when broods were manipulated so that half of the chicks had their orange plumes intact
and half were trimmed, then parents showed a clear preference for feeding the
ornamented chicks and the black chicks grew less well (Fig. 8.10b). Therefore, parental
preference is relative, a key element in the evolution of exaggerated traits (Chapter 7
gives analogous examples where female mate choice selects for ornaments in males).
Further studies are needed to investigate why parents prefer ornamented chicks;
ornaments may signal chick age or quality or may involve sensory exploitation of parental
Chick ornaments
charm parents
