Our results suggest that the UV/yellow colouration of nestlings does not affect parental feeding rates or sibling competition (i.e., parent-absent begging) according to the availability of lutein during egg laying. However, males of lutein-supplemented nests prey-tested UV-blocked nestlings less often than their non-UV-blocked siblings, which was mirrored in the body mass change, since UV-blocked nestlings gained more body mass than their siblings but only in lutein-supplemented nests. Therefore, nestling UV/yellow colour modulated certain intra-family interactions according to the quality of the prenatal environment (and thus to the females’ rearing capacity; 30), which ultimately affected offspring growth.
There were no significant differences in parental feeding behaviour between UV-blocked and non-UV-blocked nestlings, independently of lutein availability in the prenatal environment. Thus, neither males nor females favoured high quality offspring (i.e., non-UV-blocked nestlings), while we expected such parental favouritism to arise when rearing capacity was insufficient to raise all the offspring (4) (here, in control broods in which females did not receive the lutein supplementation prior to laying). Although positive effects of lutein supplementation had been observed during egg laying (30), it is possible that the environmental conditions at later stages were too favourable to detect differences. Perhaps a handicapping manipulation such as feather clipping or a more drastic food deprivation experiment (e.g., by temporarily closing the nest-box entrance) (44–45) would have resulted in significant differences. Besides, we only observed thirty minutes of behaviour, which is a snapshot of intra-family interactions during the two days elapsed since UV manipulation took place.
Yet, half an hour of observation was enough to detect significant differences in the prey-testings performed by male parents. In lutein-supplemented nests males tested UV-blocked nestlings (signalling poor condition) less often than their non-UV-blocked siblings, whereas there were no significant differences in control nests. Recently, prey-testings in blue tits have been interpreted as a parental strategy to evaluate nestling hunger levels (29Similar costly “hunger tests” have been found in other avian, mammal and insect species (46–49) raising more than one offspring at a time and as result of parent-offspring conflict over parental care. Such tests are costly for the offspring since they commonly trigger offspring begging, usually through the expression of signals of parental quality (i.e., the bill red spot in some gull species; 49–50) or active behaviours (i.e., feeding races in penguins; 47, 51). Hence, parents can evaluate the offspring true motivation of being fed and be more efficient in optimizing their investment (e.g., by shifting their care to the neediest sibling when rearing capacity is high). Forced energy expenditure could also explain why more prey-testings impose a growth cost to blue tit nestlings (29). A non-exclusive possibility is that prey-testings occur when nestlings have gapes not large enough to swallow big preys (41). However, this interpretation cannot explain our results, since the occurrence of prey-testings and prey size were not correlated, neither prior to UV-blocking (r32 = -0.21; P = 0.25) nor after it (r20 = 0.16; P = 0.53). Our results rather suggest that, under conditions of high resource availability, at least fathers were more inclined to favour UV-blocked nestlings without testing them, suggesting parental preference based on quality signals. Interestingly, previous results in the same study population suggested that only males but not females modified prey-testings according to both nestling UV colour and food availability at the end of the nestling period (29). Our results together with previous evidence points to the possibility that males are more responsive to nestling UV colouration than females.
We also hypothesized that UV-blocked nestlings - those signalling poor condition - should beg more than their non-UV-blocked siblings when parents are absent, since they should try to discourage their siblings from competing for the next parental feeding (“sibling negotiation hypothesis”; 31). However, we did not detect significant effects on sibling competition, in contrast to a previous study (31). As discussed above, it is possible that environmental conditions were too favourable in the postnatal environment (well after lutein supplementation) so that differences on nestling need among UV treatments diminished.
Interestingly, however, in lutein-supplemented nests, UV-blocked nestlings gained more body mass than their non-UV-blocked siblings, but in control nests, offspring did not differ in body mass change according to UV treatment.
Thus, when rearing capacity was low, UV-blocked offspring were not favoured over high-quality offspring while the opposite was true when rearing capacity was high. Since all the offspring included in the experiment belonged to a clutch laid by a control female, we can be sure that body mass change was not affected by early maternal effects but rather by behavioural interactions among family members. One likely possibility is that the father’s prey-testings mediated body mass change, and thus low-quality chicks gained more body mass because they were prey-tested less often in lutein-supplemented nests (see the contrasting patterns in Figs. 2
). Besides, in control nests, UV-blocked offspring tended (not significantly) to gain less body mass and to receive more prey-testings than their non-UV-blocked siblings. Thus, by reducing the prey-testings to UV-blocked nestlings in lutein-supplemented nests, fathers would be compensating for their low quality and facilitating a brood survival strategy. Moreover, even small differences in behaviour would become relevant since significant effects were found for nestling body mass, which is a more integrative measure
In conclusion, our results thus suggest that conditions in the prenatal environment determine the importance of offspring UV signals in mediating behavioural intra-family interactions. Finally, males were more responsive than females to nestling UV colouration when rearing capacity was high, thus supporting previous findings on sex-specific care strategies.