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Earlier-Hatched Black-headed Gull Chicks Survived Better

Adult black-headed gulls with newly hatched chicks beside water
Research · Ecology

Black-headed gull chicks that hatched earlier in the breeding season had a higher probability of surviving their first year than later-hatched chicks, even though the later birds had higher body mass, haemoglobin and a stronger measured immune response while still in the colony. The Polish study was published on 17 March 2026.

Researchers measured 530 colour-marked chicks that successfully fledged from four breeding colonies. They recorded size-corrected body mass, blood haemoglobin and a skin-swelling response to phytohaemagglutinin (PHA) as an indicator of immune responsiveness. PHA stimulates local swelling through a combination of innate and acquired immune pathways. Resightings from across Europe were then analysed with capture–recapture models that accounted for incomplete detection.

Earlier hatching predicted higher first-year survival

For reference, the constant model corresponded to an estimated first-year survival probability of 0.28, or about 28%. Survival varied among years, but the best-supported models consistently showed lower survival as hatching date became later.

Association between hatching date and first-year monthly survival in black-headed gulls
Monthly post-fledging survival declined with later hatching date in the study population. Figure 2 from Indykiewicz et al. (2026), Frontiers in Zoology, CC BY 4.0.

Body condition and PHA response did not improve the survival models. The result therefore separated the timing of hatching from several traits that might otherwise be expected to identify the chicks most likely to survive.

Later chicks were heavier and showed stronger measured immune responses

Later-hatched chicks had higher size-corrected mass, higher haemoglobin concentrations and stronger PHA responses. Those apparent advantages during rearing did not translate into higher post-fledging survival. Earlier chicks showed the opposite combination: lower values for the measured condition traits, but better survival through the first year.

PHA response represents one standardized component of immune responsiveness rather than the whole immune system. The lack of a survival relationship therefore does not show that immunity was unimportant; it shows that this particular early-life measure did not predict first-year survival.

The mechanism behind the hatching-date effect was not measured

The authors suggest that early chicks may become independent closer to a seasonal period of high food availability, but the study did not measure the relevant food peak. Late-hatched chicks may also have less time to develop flight and foraging skills and prepare for their first migration, potentially increasing migration costs and delaying arrival at wintering areas. Parental quality could also contribute if higher-quality adults tend to breed earlier.

Because hatching date was observed rather than experimentally manipulated, the study establishes an association rather than a causal effect of date itself. Within that limitation, hatching date was the strongest measured predictor of first-year survival in these marked black-headed gull chicks.

Sources and image credit

  1. Indykiewicz, P., Barczyk, J., Kowalski, J., & Minias, P. (2026). Earlier hatching date rather than chick phenotypic quality enhances first-year survival in the black-headed gull. Frontiers in Zoology, 23, 16. https://doi.org/10.1186/s12983-026-00605-6Primary open-access research article published 17 March 2026. More than 500 colour-marked chicks from four Polish colonies were followed with European resightings; earlier hatching date predicted higher first-year survival, while body condition and PHA response did not improve survival models.

Image: Charles J. Sharp — black-headed gulls (Chroicocephalus ridibundus) with newly hatched chicks at Snettisham Beach, Norfolk, photographed 18 May 2022; Wikimedia Commons; cropped to 16:9; representative breeding image, not birds from the Polish study · CC BY-SA 4.0 · Source

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