When parents play favourites: Brood demand shapes parental preference for offspring UV colour

Parents might initially produce more offspring than they might be able to
raise. However, when offspring demand exceeds their parents´ rearing
capacity, parents might shift care towards the offspring that yield
greater fitness returns to achieve their optimal brood size via
brood reduction. Such favoritism could rely on offspring signaling traits
if these inform parents about offspring quality and hence about the
pay-offs of their investment. Here we investigated whether favoritism of
blue tit (Cyanistes caeruleus) parents for an offspring signal (i.e.,
ultraviolet (UV) plumage coloration) varies with brood demand. To test
this, we experimentally blocked the UV reflectance of yellow breast
feathers in half of the nestlings of each brood, and then we sequentially
performed two opposing brood size manipulations to vary nestling demand
below or above parental rearing capacity. In reduced broods, nestlings
begged overall less intensely and gained more body mass, supporting that
parental rearing capacities sufficed to satisfy brood demand. Moreover, in
reduced broods, UV-blocked nestlings (i.e., low-quality offspring) were
fed and prey-tested more often. Yet, they begged more than control
nestlings, suggesting that they were perhaps treated differently by other
family members or that they may exploit parental preferences beyond actual
need (at least in reduced nests). Parents flexibly shifted their feeding
rate and favoritism in response to short-term changes in family size, as
there was no parental preference in enlarged broods. Such flexible
parental feeding rules may allow parents to gain the upper hand in
parent-offspring conflict. However, we did not find evidence that parental
favoritism facilitated brood reduction, at least in conditions were demand
was temporally enhanced.