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 rea..., Statistical analyses (from the published article)
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 intera...,