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Rooftop-nesting European Herring Gulls Took Fewer Incubation Breaks but Had Lower Hatching Success

European herring gull sitting on a rooftop nest
Research · Reproduction & development

European herring gulls (Larus argentatus) nesting on urban rooftops left their eggs unattended less often than ground-nesting birds, yet still had substantially poorer hatching success, according to a Belgian study published on 25 March 2026.

Researchers compared rooftop nests on flat buildings in Ostend with ground nests in a gull colony at the outer port of Zeebrugge over two breeding seasons. Temperature loggers placed inside artificial eggs were used to reconstruct incubation breaks in 78 nests: 26 nests in 2023 and 52 in 2024.

Rooftop pairs reduced the number of incubation breaks

Pairs nesting on roofs took fewer daily off-bouts, meaning periods when the eggs were left without an incubating parent. The difference was especially strong in 2024, when ground-nesting pairs took about 3.5 times as many off-bouts as rooftop pairs. The average duration of each break, however, did not differ significantly between the two nesting environments.

The authors interpret the lower off-bout frequency as a behavioural adjustment that may reduce the time rooftop eggs are directly exposed to challenging environmental conditions.

Fewer breaks did not prevent lower hatching success

Despite spending more continuous time on the eggs, rooftop-nesting pairs had much lower hatching success. This difference remained after eggs lost through predation or breakage were excluded from the analysis.

Hatchlings that did emerge were similar in size in both environments. Mean combined head-and-bill length was almost identical in rooftop and ground-nest chicks. The researchers therefore concluded that the main difference in breeding outcome arose from embryo mortality rather than reduced growth of embryos that survived to hatching.

The authors describe urban rooftops as potentially suboptimal incubation habitat: fewer incubation breaks did not prevent substantially lower hatching success.

Sources and image credit

  1. Van Malderen, J., Stienen, E. W. M., Verbruggen, F., Müller, W., & Lens, L. (2026). Urban rooftops act as suboptimal incubation habitats: Fewer off-bouts yet poorer hatching success in Larus argentatus (Herring Gull). Ornithological Applications, 128(3), 1–12. https://doi.org/10.1093/ornithapp/duag036Primary peer-reviewed study published 25 March 2026. Incubation was monitored with temperature loggers in 78 nests across two Belgian breeding seasons. Rooftop pairs took fewer daily off-bouts, especially in 2024 (ground/rooftop ratio 3.49), but hatching success was much lower on rooftops (odds ratio 0.05 after excluding predation/breakage); hatchling size was similar between environments.

Image: Partonez — European herring gull on a nest built on a caravan roof in Burnham-on-Sea, Somerset, England, 20 June 2017; Wikimedia Commons; representative rooftop-nesting image, not a bird from the Belgian study · CC BY-SA 4.0 · Source

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