Effects of experimental manipulation of nest temperatures and food supplementation on patterns of parental care in the Lesser Kestrel (Falco naumanni)

Parental care behaviours play a vital role in improving reproductive success by creating a favourable developmental environment for the offspring. However, the two sexes generally disagree over their relative contribution to joint parental effort, and species-specific care patterns are inherently shaped by the resolution of this evolutionary sexual conflict. Parenting strategies are also shaped by environmental pressures and plasticity in offspring care behaviours may be necessary to buffer offspring from anthropogenic climate change. The Lesser Kestrel (Falco naumanni), a cavity-nesting, sexually dimorphic raptor, serves as an ideal model for studying the complex patterns of parental care and their adaptive plasticity. While both parents contribute to offspring care throughout the reproductive cycle, they exhibit marked asymmetries in how each sex contributes to different parental tasks, both within and across breeding stages, offering the possibility to explore multiple dimensions of parental behaviour under controlled environmental variation. Mediterranean Lesser Kestrel populations are increasingly threatened by rising ambient temperatures and extreme weather events, with birds nesting in artificial structures being particularly vulnerable to catastrophic nestling mortality caused by nest overheating. Recent studies on Lesser Kestrels breeding in nest boxes in Matera, Southern Italy, demonstrated that shading nests from direct sunlight effectively protects nest-bound young from both sub-critical and lethal temperature increases, while the experimentally augmented food availability was able to offset the negative effects of sub-lethally high nest temperatures on nestling growth and condition. Yet parental responses underpinning the observed experimental outcomes remained largely unexplored. This study therefore set out to investigate how experimentally reducing nest temperatures affects egg incubation behaviour, and how nest cooling and augmented food availability during early nestling-rearing separately influence the division of labour between parents across competing parental tasks. Behavioural data was obtained using video-recordings while internal nest temperature was continuously monitored via thermal dataloggers, and regular nest visits allowed to track reproductive progress. In the late incubation period, daytime egg incubation routines and the daily scheduling of male and female effort were assessed in broods exposed to natural sunlight (control group) and in nest boxes shaded from the beginning of incubation. Upon hatching of the first egg in the clutch, nesting units were randomly assigned to one of three groups: nest boxes shaded from hatching (reduced nest temperature, natural food availability), food supplemented (natural thermal conditions, increased food availability), or controls (natural thermal and food availability); total daily parental time investment and sex-specific contributions to nest attendance, brooding, and nestling feeding were then quantified and compared across experimental groups. During late incubation, repeatable rhythmic temporal patterns of diurnal egg incubation were identified, which were unaffected by the experimental manipulation of nest temperature. During the early nestling-rearing stage, food supplemented pairs displayed a clear pattern of compensatory reallocation of sex-specific effort across care tasks, ultimately reducing the sex-based parental role specialization, while daily nest attendance and total nest-based care did not change relative to control conditions. Experimental nest cooling, by contrast, tended to increase female nest attendance but did not significantly affect male behaviour, or the relative contributions of each sex to brooding and nestling feeding. These findings advance our understanding of parental care strategies in Lesser Kestrels, which may extend to other role-specialized biparental species, and may help inform more effective conservation.