Egg removal campaigns are a common practice in France and Europe, including in natural areas, to manage conflicts between large gulls and other species or humans. Few studies have evaluated the impact of this practice on the breeding site fidelity of targeted colonies. The Lilleau des Niges Nature Reserve hosts a mixed-species colony of gulls (Laridae), including European Herring Gull Larus argentatus, Lesser Black-backed Gull L. fuscus, and Great Black-backed Gull L. marinus. The gull colony was controlled between 1990 and 2017 using egg removal. A citizen-science led capture-recapture study began in 2010, and resighting of colour-marked individuals were obtained at the breeding colony between 2010 and 2022. By using a multi-event model, we assessed the colony fidelity and adult survival during the egg removal campaign (2010–17) and in the following years (2018–22). Breeding site fidelity was lower during the years of egg removal than in subsequent years for two species (a decrease of 0.07 for Herring Gull and 0.22 for Lesser Black-backed Gull). The probability of survival for all species declined during the study period but could not be directly linked to gull control measures. No difference between sexes was found for these two parameters. Moreover, colour ring resighting by citizen scientists showed that some individuals moved from natural colonies to urban colonies. These initial results suggest that the local population dynamics of gulls have changed over time, although a causal link to egg removal campaigns cannot be confirmed.

Since the 1950s, population sizes of several gull (Laridae) species have increased in Europe. This development is often attributed to the increasing availability of anthropogenic food (Coulson 2015), for example, from open landfill development (Pons & Migot 1995) and discards from industrial fishing (Oro et al. 2004; Hudson & Furness 2008). Another contributing factor is the introduction of better legal protection of gulls in France in 1962 (Henry & Monnat 1981). The population growth in gulls encouraged the emergence of large colonies, which can impact rare flora (Vidal et al. 1998) or compete with other colonial birds (Salathé 1983; Cadiou & Fortin 2010). It has therefore been necessary for some conservation managers to control numbers of gulls, despite their protection status (Paulet & Bioret 2021). In addition to the growth in natural colonies, since the 1970s gull populations have also been urbanising. Observations of gull colonisation of urban areas were first made in England (Cramp 1971) and Bulgaria (Nankinov 1992), then in France in 1975 (Camberlein & Floté 1979; Vincent 1986), but no study quantified this tendency for urbanisation. The recent proximity to humans in urban areas has caused new conflicts to emerge (Paulet & Bioret 2021) and, consequently, control measures have been implemented to reduce nuisance behaviours in gulls. Many methods are available to control gulls, including: culling of adults, egg-oiling, replacing eggs with fake eggs, euthanasia, use of narcotics, contraception, sound deterrents, or even the introduction of predators (Calladine 2006).

The population trends of European Herring Gull Larus argentatus (hereafter ‘Herring Gull’), Lesser Black-backed Gull L. fuscus and Great Black-backed Gull L. marinus in Europe have varied between 1950 and 2000, with increases reaching a plateau by the 1970s, followed largely by a declining trend. The British Herring Gull population deteriorated rapidly, with a decline of 60% between 1969 and 2015 (Eaton et al. 2015). In France, the breeding population of Herring Gulls declined moderately between 1999 and 2012 (Issa & Muller 2015), with the most recent estimate of 50,720 pairs of Herring Gulls made during the 2020–22 census (GISOM 2023). In Great Britain, the population of Lesser Black-backed Gulls declined between 2000 and 2013 (Ross-Smith et al. 2014), whereas in France, the breeding population trend was stable between 1989 and 2012 (Issa & Muller 2015), with the most recent census estimate at 13,705 pairs (GISOM 2023). European populations of Great Black-backed Gull declined by 28% between 1985 and 2021, with declines more pronounced in populations in northern Europe (Langlois Lopez et al. 2023). The French Great Black-backed Gull population increased strongly between 1989 and 2012 (Issa & Muller 2015) and was estimated to be 6,565 pairs during the most recent census (GISOM 2023). Although the three species are considered as ’Least concern’ on the IUCN’s European Red List of Threatened Species (Birdlife International 2020), this status is uncertain. The Herring Gull has recently moved to the ‘Least concern’ category of the Red List of the UK (Stanbury et al. 2024). Langlois Lopez et al. (2023) recommended the status of Great Black-backed Gull from ‘Least Concern’ to ‘Vulnerable’ on the world IUCN Red List categories. Over the last century, the reduction of available anthropogenic food resources is partly responsible for their decline (Pons 1992; Bicknell et al. 2013).

Few studies have investigated the importance of control measures on the population dynamics of gulls and the impacts these may have on breeding site fidelity or other demographic parameters (Coulson 2015). Due to the decline in European gull populations, non-urban colony management is being reconsidered to improve the conservation status of their population by stopping control measures such as egg removal or sterilisation campaigns (Paulet & Bioret 2021). Gull control measures exert a similar pressure to predation by reducing adult survival (Coulson et al. 1982) or reproductive success (Parsons et al. 1976; Monaghan 2008). Predation disrupts colony dynamics by reducing reproductive success and increasing adult dispersal away from the colony (Craik 1997; Oro & Pradel 2000). High reproductive success and the presence of conspecifics are two factors promoting colony fidelity for individuals in colonial birds (Smith & Peacock 1990; Danchin et al. 1998; Cam et al. 2004). For larids, breeding sites that do not produce many young in a given year tend to cause individuals to disperse to more productive colonies in the next year(Oro & Pradel 2000), while the most productive sites recruit more easily the following year (Danchin et al. 1998; Cam et al. 2004). Disturbing the reproduction of an individual can therefore have an impact on its reproduction over several years, through causing dispersal to another colony or non-reproduction in subsequent years (Fairweather & Coulson 1995; Salas et al. 2020). In the Double-crested Cormorant Nannopterum auritum, one of the rare examples for which the impact of regulation campaigns was investigated and published, adults showed greater fidelity to the breeding site when colonies were not sterilised (Chastant et al. 2014).

The gull colony of the national nature reserve of Lilleau des Niges was established in 1984 (Robreau 1999). It is one of the few natural non-urban colonies in the centre of the French Atlantic coast. It is a multi-species colony where Herring Gulls, Lesser Black-backed Gulls, Great Black-backed Gulls, and Yellow-legged Gulls L. michahellis nest. Gull management was requested in 1992 by local authorities to reduce predation on eggs and chicks of Pied Avocet Recurvirostra avosett and Black-winged Stilt Himantopus himantopus. These operations targeted Herring Gulls and Yellow-legged Gulls, which were suspected of predating wader nests at a higher rate than Lesser Black-backed Gulls and Great Black-backed Gulls. To assess the impact of egg removal campaigns on gull colony dynamics, a capture-recapture programme was launched in 2010. Long-term breeding season monitoring on the reserve, including participation by many citizen scientists, has resulted in numerous resightings of marked individuals within the reserve and outside its borders. Gull management was stopped in 2018, with the authorisation of local authorities, due to the risk the colony might disappear. In total, nearly 17,000 nests were destroyed during the regulation period.

This study aims to compare adult gull survival and colony fidelity during the egg removal campaigns at Lilleau des Niges National Nature Reserve (2011–17) and after the operations were stopped (2018–22). We predicted that egg removal campaigns promote the dispersal of adults away from their original colony.

We extend our gratitude to all contributors to this programme, including numerous ornithologists, volunteers, and professionals involved in resighting efforts. Special thanks go to Frédéric Robin for his initial involvement in the programme. The programme, titled “Long-term monitoring of demographic parameters of breeding larids in the Nature Reserve of Lilleau des Niges and the Charentais Shoals,” has been authorised by the Centre de Recherche sur la Biologie des Populations d’Oiseaux (CRBPO) under the project name PP533. We also acknowledge the team at the Centre d’études biologiques de Chizé (CEBC - UMR 7372 CNRS Univ. La Rochelle) for their assistance with molecular sexing and their research on the presence of contaminants in gulls, which may help explain variation in mortality over time. This programme has been made possible through the financial support of DREAL Nouvelle Aquitaine, CDC Île de Ré, and the French Ministry of the Environment. And we extend an acknowledgement to the editorial team of Seabird for manuscript support.

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