Therefore, the predicted positive relationship between the duration of nestling period and sibling competitive aggression may not apply when trying to explain conspicuousness of nestlings gapes as consequence of sibling competition for parental attention.
From Freud's psychoanalytic ideas about sibling competition to his disciple, Adler, and his work on sibling birth order, traditional psychological theory has plenty to say about the dark side of the sibling relationship.
When these subregions meet, sibling competition starts and spurious boundaries may form.
It, thus, follows that conspicuousness should positively covary with level of sibling competition.
However, parents might be selected to influence the outcome of sibling competition through differential investment.
Moreover, the merely use of clutch size as a surrogate for sibling competition is debatable.
Furthermore, conspicuousness of nestling gapes has likely evolved as a result of sibling competition for parental resources by appealing to some aspect of parental psychology, but not for sibling competition for establishing within nest hierarchy [1].
Clutch size was used as a species-specific proxy of sibling competition in the nest based on the assumption that for a similar amount of parental feeding effort sibling competition would be higher in nests with larger broods.
In a previous study we have controlled for a possible role of sibling competition on the relationship between nestling conspicuousness and nest type by using clutch size as a proxy for the level of sibling competition [26].
On the other hand, the low covariation between variables hypothetically related to intensity of sibling competition (see above) would suggest that these factors might explain different portions of whole variance in sibling competition.
No more than 4 nestlings were placed in any one box and nestlings were paired according to age to reduce sibling competition.
