Hourly analyses reveal environment-linked between-parent dynamics in a biparentally incubating shorebird

Biparental incubation emerges from the balance between cooperation and conflict. Because only one parent can sit on the nest at a time, any environment-linked change in one parent's schedule compels a reallocation by the partner. Yet most field studies collapse incubation data to daily totals and rarely model continuous 24-h, sex-specific rhythms, masking how environmental context reshapes between-parent dynamics. We used minute-resolution incubation records (13,632 nest-hours from 45 nests) to examine temporal and spatial variations in incubation by Kentish Plovers (Anarhynchus alexandrinus) breeding in northern Bohai Bay, China. By pairing daily summaries with cosinor-based models of sex-specific hourly incubation, we revealed context-dependent diel timing shifts that coarser methods missed. Consistent with previous work, we found females dominated daytime incubation and males at night, yielding female-biased daily totals. Hourly analyses, however, revealed context-dependent retiming in our study. As season warmed, males increased midday attendance but delayed the onset of their night shift; while female attendance declined, producing a more even division between parents with male daily attendance remaining unchanged. Across habitats, total attendance was similar, but in rocky sites, where nocturnal predation was higher, males reallocated effort from night to day and exceeded females in daily totals. Increased male daytime involvement was accompanied by more frequent changeovers, which were more responsive to environmental context than daily attendance. These findings show that environmental variation primarily reorganises when parents incubate rather than simply how much they incubate, and that between-parent dynamics are best examined at hourly scales. Hourly sex-specific analyses thus provide a clearer bridge between environmental context and parental decision-making—insights that matter for forecasting reproductive outcomes under warming climates and changing predator communities.