Structural (UV) and carotenoid-based plumage coloration - signals for parental investment?

Experimental evidence that adult UV/yellow colouration functions as a signal in blue tit families — but only for parents

Abstract

In bi-parental species, reproduction is not only a crucial life-history stage where individuals must take fitness-related decisions, but these decisions also need to be adjusted to the behavioural strategies of other individuals. Hence, communication is required, which could be facilitated by informative signals. Yet, these signalling traits might have (co-)evolved in multiple contexts, as various family members usually meet and interact during reproduction. In this study, we experimentally explored for the first time whether a colourful plumage trait in adults acts as a signal that regulates multiple intra-family interactions in a bird species, the blue tit (Cyanistes caeruleus). We expected that an experimental reduction of adults’ UV/yellow reflectance (i.e. a reduction of apparent individual quality) should affect the behavioural strategies of all family members. We found evidence for this at least in adults, since the partners of UV-blocked individuals (either males or females) increased their parental investment — perhaps to compensate for the apparent lower condition of their mates. As the UV-blocked adult did not change its provisioning behaviour, the partner presumably responded to the manipulated signal and not to a behavioural change. However, the offspring did not co-adjust their begging intensity to the experimental treatment. It is thus possible that they responded to overall parental care rather than the signal. These results suggest that UV/yellow colouration of adult blue tits may act as quality signal revealing the rearing capacity to mates.

Significance statement

How parents respond to signals of genetic or phenotypic quality of their mates has received significant attention. However, previous studies have primarily focused on the receiver’s response and have not always controlled for the signaller’s behaviour and its investment in reproduction. Our results provide the first experimental evidence that ultraviolet (UV)/yellow colouration acts as a signal of parental quality in the blue tit. Parents responded by increasing their effort when paired with UV-blocked (low-quality) mates, while controlling for the mate’s behaviour. We argue that the reduced expression of the signal triggered a compensatory response in the mate. Interestingly, both males and females responded similarly to changes in mate’s UV/yellow reflectance, suggesting similar rules over investment in response to this trait. However, nestlings, a potential (and often neglected) set of observers of parental signals, did not change their behaviour when raised by an UV-blocked (= low-quality) parent.

Enduring rules of care within pairs - how blue tit parents resume provisioning behaviour after experimental disturbance

Sexual conflict over parental investment can result in suboptimal reproductive output. A recent hypothesis suggests that equality in investment, and hence conflict resolution, may be reached via coordination of parental activities like alternating nest visits. However, how robust patterns of care within couples are against temporal disturbances that create asymmetries in parental investment remains as yet to be shown. We here experimentally created such a social disturbance in a wild population of biparental blue tits (Cyanistes caeruleus) when provisioning their nestlings. We randomly caught and subsequently released one of the parents when nestlings were 6 and 12 days old respectively. On average, the parent that was caught did not resume care for nearly two hours. We then compared the levels of individual investment and within-pair coordination before, during and after the absence of the disturbed parent. We show that the remaining parent partially compensated by increasing its provisioning rate, but this compensatory response was strongest in females when nestlings were 6 days old. Once the caught parent returned to feed its nestlings, both parents resumed provisioning at a similar rate as before the disturbance. Likewise, the within-pair alternation level quickly resembled the pre-manipulated level, independent of nestling age or which sex was caught. Thus our experiment highlights the resilience of parental behaviour against temporal disturbances of individual parents.

How to quantify animal activity from radio-frequency identification (RFID) recordings

Automated animal monitoring via radio-frequency identification (RFID) technology allows efficient and extensive data sampling of individual activity levels and is therefore commonly used for ecological research. However, processing RFID data is still a largely unresolved problem, which potentially leads to inaccurate estimates for behavioral activity. One of the major challenges during data processing is to isolate independent behavioral actions from a set of superfluous, nonindependent detections. As a case study, individual blue tits (Cyanistes caeruleus) were simultaneously monitored during reproduction with both video recordings and RFID technology. We demonstrated how RFID data can be processed based on the time spent in- and outside a nest box. We then validated the number and timing of nest visits obtained from the processed RFID dataset by calibration against video recordings. The video observations revealed a limited overlap between the time spent in- and outside the nest box, with the least overlap at 23 s for both sexes. We then isolated exact arrival times from redundant RFID registrations by erasing all successive registrations within 23 s after the preceding registration. After aligning the processed RFID data with the corresponding video recordings, we observed a high accuracy in three behavioral estimates of parental care (individual nest visit rates, within-pair alternation and synchronization of nest visits). We provide a clear guideline for future studies that aim to implement RFID technology in their research. We argue that our suggested RFID data processing procedure improves the precision of behavioral estimates, despite some inevitable drawbacks inherent to the technology. Our method is useful, not only for other cavity breeding birds, but for a wide range of (in)vertebrate species that are large enough to be fitted with a tag and that regularly pass near or through a fixed antenna.

Size matters but hunger prevails-begging and provisioning rules in blue tit families

It is commonly observed in many bird species that dependent offspring vigorously solicit for food transfers provided by their parents. However, the likelihood of receiving food does not only depend on the parental response, but also on the degree of sibling competition, at least in species where parents raise several offspring simultaneously. To date, little is known about whether and how individual offspring adjusts its begging strategy according to the entwined effects of need, state and competitive ability of itself and its siblings. We here manipulated the hunger levels of either the two heaviest or the two lightest blue tit (Cyanistes caeruleus) nestlings in a short-term food deprivation experiment. Our results showed that the lightest nestlings consistently begged more than the heaviest nestlings, an effect that was overruled by the tremendous increase in begging behaviour after food deprivation. Meanwhile, the amplified begging signals after food deprivation were the only cue for providing parents in their decision process. Furthermore, we observed flexible but state-independent begging behaviour in response to changes in sibling need. As opposed to our expectations, nestlings consistently increased their begging behaviour when confronted with food deprived siblings. Overall, our study highlights that individual begging primarily aims at increasing direct benefits, but nevertheless reflects the complexity of a young birds’ family life, in addition to aspects of intrinsic need and state.

Decreasing parental task specialization promotes conditional cooperation

How much to invest in parental care and by who remain puzzling questions fomented by a sexual conflict between parents. Negotiation that facilitates coordinated parental behaviour may be key to ease this costly conflict. However, understanding cooperation requires that the temporal and sex-specific variation in parental care, as well as its multivariate nature is considered. Using a biparental bird species and repeated sampling of behavioural activities throughout a major part of reproduction, we show a clear division of tasks between males and females in provisioning, brooding and foraging. Such behavioural specializations fade with increasing nestling age, which stimulates the degree of alternated feeding visits, as a recently promoted form of conditional cooperation. However, such cooperation is thought to benefit offspring development, which is not supported by our data. Thus, from a proximate point of view, conditional cooperation via alternation critically depends on the division of parental tasks, while the ultimate benefits have yet to be shown.