Urban rooftops act as suboptimal incubation habitats: Fewer off-bouts yet poorer hatching success in <i>Larus argentatus</i> (Herring Gull)

Abstract Urbanization is transforming the breeding habitats of many bird species. Key urban features, such as elevated temperatures, poor water drainage, altered predator communities, including cats, and the presence of anthropogenic food sources, such as human waste, can influence the risk of embryo mortality and alter parental care, particularly in open-nesting species. Parents may adjust incubation behavior to buffer embryos against these conditions, but whether this suffices to maintain reproductive success remains unclear. We investigated how rooftop breeding affects incubation behavior and reproductive success in Larus argentatus (Herring Gull) by comparing rooftop- and ground-nesting pairs across 2 breeding seasons in Belgium. Using temperature loggers placed in artificial eggs, we quantified daily off-bout frequency and duration with the incR package in 78 nests (2023: 11 ground, 15 rooftop; 2024: 22 ground, 30 rooftop). We predicted that rooftop-nesting pairs would reduce off-bout frequency and/or duration relative to ground-nesting pairs, thereby limiting egg exposure to environmental conditions. We further predicted that, if these adjustments effectively buffer embryos from rooftop stressors, hatching success would be comparable between environments. In line with our behavioral predictions, rooftop-nesting pairs took fewer off-bouts per day than ground-nesting pairs, particularly in 2024 (2024 ground/rooftop ratio = 3.49, 95% CI: 2.28–5.34), whereas off-bout duration did not differ. However, hatching success was lower in rooftop nests, even after excluding egg predation or breakage (odds ratio = 0.05, 95% CI: 0.01–0.35), suggesting that the observed behavioral changes did not fully mitigate environmental stressors. Hatchling size was comparable across environments (mean head + bill size: rooftop = 47.77 ± 1.77 mm, ground = 47.84 ± 1.06 mm), indicating that differences in reproductive success were driven mainly by embryo mortality rather than altered embryonic growth. These findings suggest that urban rooftops impose incubation-stage challenges, altering incubation dynamics and breeding outcomes in open-nesting birds.