DOI: 10.5061/dryad.rv15dv4cp
title: When parents play favourites: Brood demand shapes parental preference for offspring UV colour

description: 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.
description: Statistical analyses (from the
 published article)<br> All analyses were performed with SAS 9.4. (SAS
 Inst., Cary, NC, USA), using the mean values per treatment in each nest.
 To analyze feeding rates, we performed a generalized linear mixed model
 with Poisson distribution, while the mean number of nestlings that were
 prey-tested and mean begging intensity were analyzed with mixed models
 with normal distribution. The mean number of nestlings that were
 prey-tested was “log10 (x+1)” transformed to fulfil the assumption of
 normality. For the three behavioral variables, we had four data per nest
 corresponding to non-UV-blocked and UV-blocked chicks in both the enlarged
 and the reduced brood size treatment. All models included dyad ID and nest
 ID (nested within dyad) as random factors, and two random slopes (nest
 ID*UV treatment and nest ID*brood-size treatment). As predictor variables,
 we included the UV treatment (non-UV-blocked/UV-blocked), brood-size
 manipulation (enlarged/reduced), the interaction between both treatments,
 the day of the video recording (day 12/13), the original brood size and
 the Julian hatching date. We also included the interactions between UV
 treatment and all other predictors, which were never significant (see
 Results). We did not include the interactions between brood size
 manipulation and the rest of variables to avoid over-parameterization, and
 because the brood size was manipulated in order to observe the effect of
 UV color on behavioral parameters. 
 To analyze the individual body
 mass change, we used a mixed model with normal distribution, which
 included dyad ID and nest ID (nested within dyad ID) as random factors,
 and the same two random slopes as above. Moreover, we included a new
 categorical variable (foster nestling yes/no) as a random factor to
 control for whether each nestling had been exchanged or not. We included
 the same predictor variables as in previous models, as well as nestling
 sex and all the interactions between UV treatment and predictor variables.