Incubation behaviour of Great Tits Parus major in response to ambient temperature in three contrasting Mediterranean habitats

Abstract

Avian incubation is a complex behaviour that, in female-only incubator species, entails key trade-offs between egg warming periods and time off the nest for female self-maintenance. The dynamics between on- and off-bouts are thought to be mainly influenced by ambient temperature, because changes in egg cooling rates would influence how females allocate their time during the incubation period. Incubating females need to keep an adequate and narrow thermal environment for the egg, with small deviations causing long-term effects on survival and reproductive success. Females would need to adjust their bout duration to ambient temperatures. Both on- and off-bouts are expected to lengthen when temperatures increase because incubation constraints are eased. Females usually lengthen on-bouts at a higher rate, thus increasing incubation effort. The opposite response, i.e., increasing self-maintenance time with increasing ambient temperature, has been also reported in both different species and different populations of the same species. While these opposite behavioural responses might be the result of different breeding strategies adapted to habitat conditions, it might also be that they arise as artefacts of limited datasets, different methodological approaches, or the timescale at which incubation behaviour is measured. Determining the onset of incubation also implies certain complexities derived from trying to delimit a progressive behaviour that is gradually settled during the egg-laying period. Despite classic studies, performed forty years ago in Great Tits Parus major describing both diurnal and nocturnal incubation behaviours and their respective periods of partial and full incubation, little is known about how the onset of incubation relates to ambient temperature and its effects on hatching asynchrony. Ambient temperature might have a major role in the onset of incubation if incubating females use it as a cue to synchronize their hatchlings maximum growth period with the expected prey peak. Prey development, mainly caterpillars for Great Tits, accelerates with increasing temperatures. Once females start laying eggs, they could only keep track of faster prey development by an earlier onset of incubation. But an earlier onset also implies that incubation happens before the clutch is complete, which could cause hatching asynchrony. Hatching asynchrony could still occur even if incubation is delayed after clutch completion. Thermal gradients within the clutch during incubation might be a potential factor behind this residual hatching asynchrony. Females keep eggs under their brood patch warmer than peripheral ones, because they are not able to cover the whole clutch. If females do not distribute the heat properly by repositioning eggs within the nest-cup it could cause a differential embryo development, potentially resulting in hatching asynchrony.

In this thesis project I have chosen a commonly studied species, the Great Tit, and collected high-quality incubation data during three consecutive years in three different breeding populations. The main aim of the project was a better understanding of the dynamics of incubation behaviour, its onset, and the consequences on hatching asynchrony in relation to ambient temperature. During an additional fourth breeding season, I also investigated whether differential egg repositioning within the clutch, a rarely studied behaviour, had a role in hatching asynchrony. Incubation behaviour was recorded using temperature data loggers placed in the nest-cup and egg repositioning was calculated from photographed clutches during the incubation period. First, I delimited the different incubation behaviours (diurnal and nocturnal, and partial and full incubation), quantified them, and assessed how ambient temperature affected their onset relative to the egg-laying sequence. I also assessed the association between the onset and duration of these incubation behaviours and the extent of hatching asynchrony. Secondly, I tried to comprehend incubation rhythms (i.e., the relation between on- and off- bouts during diurnal full incubation), their number and duration, in relation to ambient temperature. I aimed to investigate whether divergent patterns of nest attentiveness were a result of local adaptations or the consequence of incomplete datasets and different analysed timescales (hourly, daily, overall incubation period). Finally, by using an experimental approach, I deterred clutches from being partially incubated and assessed the effect on hatching asynchrony. I also investigated if residual hatching asynchrony could be endorsed to differential egg repositioning within the clutch by taking twice-a-day photographs of marked eggs during the diurnal full incubation period. The duration of the diurnal full incubation was analysed under the effect of different factors related to each objective. I found that both ambient temperature and clutch size affected the onset of incubation behaviour (Chapter 1). Increasing ambient temperature during the egg-laying period advanced diurnal partial incubation relative to the laying sequence, but larger clutches delayed the onset of both nocturnal full incubation and diurnal partial incubation. Only diurnal incubation affected hatching asynchrony despite nocturnal periods being longer. Both partial incubation and full incubation occurring before clutch completion increased hatching asynchrony. In Chapter 2 I showed that incubating females allocated time into self-maintenance at higher ambient temperatures, i.e., reducing nest attentiveness when constraints alleviate, as a generalized response among populations. Females maximised the duration of incubation bouts based on local temperatures and not absolute values as suggested in previous studies. This behaviour translated into different nest attentiveness patterns depending on the timescale, even showing contrary incubating behaviours.Going beyond correlational studies, in Chapter 3 clutches where partial incubation was prevented showed longer incubation periods and reduced hatching asynchrony. However, egg repositioning within the clutch did not seem to be the cause behind the observed residual hatching asynchrony. Ambient temperature is a key variable for incubation behaviour, both for its onset and rhythms. It affects the onset of incubation asymmetrically, advancing only diurnal partial incubation. Female incubation rhythms differ among populations because they maximise onbout duration at different local ambient temperatures. Ambient temperature, both during the egg-laying and full incubation period, is indirectly associated with the duration of the full incubation period and the extent of hatching asynchrony.