Data and code for: Urban rooftops act as suboptimal incubation habitats: fewer off-bouts yet poorer hatching success in <em>Larus argentatus</em> (Herring Gull)

Urbanization is transforming the breeding habitats of many bird species and may alter incubation conditions, particularly for open-nesting birds. Parents may adjust incubation behavior to buffer embryos against these conditions, but whether this suffices to maintain reproductive success remains unclear. We investigated the effects of rooftop breeding on incubation behavior and reproductive success in Larus argentatus by comparing rooftop- and ground-nesting pairs across two 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 also assessed reproductive output by comparing hatching success and chick size at hatching. Egg volume was compared between habitats as a proxy for parental quality, and local ambient temperatures around nests were measured to verify whether heating risks differed between environments. Roof..., , # Data and code for: Urban rooftops act as suboptimal incubation habitats: fewer off-bouts yet poorer hatching success in *Larus argentatus* (Herring Gull) Dataset DOI: [10.5061/dryad.08kprr5hf](https://doi.org/10.5061/dryad.08kprr5hf) ## Description of the data and file structure The data were collected during field studies conducted in the 2023 and 2024 breeding seasons (April–July) in coastal Belgium. The study investigated how breeding environment influences incubation behavior and reproductive success in *Larus argentatus* by comparing rooftop-nesting and ground-nesting pairs. Ground nests were monitored in a mixed-species gull colony at the outer port of Zeebrugge, while rooftop nests were located on flat roofs of public and commercial buildings in Ostend. Nests were visited three times per week to record laying dates, egg fate and chick measurements at hatching. Egg length and width were measured to calculate egg volume, and hatchlings were weighed and measured on the day of ..., ,