This study is one of the few ones analysing local philopatry rates in a Yellow legged Gull population. Our colonies showed extremely high philopatry, with rates, close to 100%. It is true that this result might partly be biased by the fact that our sampling colonies were located close between each other, hence ignoring those birds which might recruit in colonies located at further distances (Coulson and Coulson 2008). However, during the period of 14 years in which this study was carried out, we were notified to have only 21 birds breeding in colonies found at further distances, and all of them located to the west of the western-most colony of Getaria: Lekeitio (25 km), Bermeo (44 km), Santander (130 km). Furthermore, even within our small sampling range in Gipuzkoa, an extremely low number of birds were found to breed in a colony different from the one where they hatched. Overall, evidence supports that the philopatry rates are truly high within the population.
In this research we were not able to estimate the first age of reproduction due to two main reasons: the orography and the fact that most gulls fly when we approach to the colony, which hampers us from confirming whether a certain bird is breeding or not. Age of recruitment, is an aspect that remains for future studies (Porter and Coulson 1987; Cadiou et al. 1994; Spear et al. 1995; Coulson and Coulson 2008; Oro et al. 2013). Therefore, we had to assume that adults were breeding, so by default the first age of reproduction was the fourth year of life, which is the age when the species reaches its sexual maturity (Olsen and Larson 2004). This assumption, however, is rather irrelevant for the conclusions made in this study, since many gulls were sighted several years after their 4th year of life, and the flow among colonies was extremely low.
What we were able to confirm is that immatures were rare within the colonies and this is due to the fact that these birds do not have the need to breed, hence they remain in other areas where they can feed and rest without getting in conflict with adults. The observation of these immature birds within the colonies may be associated to prospecting processes (Dittmann et al. 2005). Even though our data set comprised mostly adult birds, since immatures avoid entering the colonies, therefore, they are much more unlikely to be seen, we still had data to estimate their movement among colonies. During prospection, seabirds visit several sites in order to evaluate the quality and suitability of each colony for future breeding (Dittmann et al. 2005). It is interesting to note here that in Gipuzkoa most immatures were found in their hatching colonies, i.e., the philopatry was high even for these immature birds, suggesting that the prospection was small or very fast, until most birds decided to settle in their origin colony. This suggests that, if prospection exists, our birds may decide at relatively early age recruiting in their natal colony; otherwise, the proportion of resightings at their natal sites would be remarkably lower.
Ultimate causes underlying such a high philopatry in these colonies still remain to be determined, so we cannot do more than advancing some plausible explanations: (1) The familiarity with hatching site may have played a role in this phenomenon (Greenwood and Harvey 1982), but not the colony size (as found by Oro and Pradel 2000), since local recruitment was not higher in Ulia, by far the largest colony within the region; (2) Differences in feeding patterns between the colonies can also influence these high rates of fidelity to natal sites (Enners et al. 2018). In spite of their proximity, our colonies are well known to depend on different feeding resources (Zorrozua et al. 2020), and maybe the juveniles from a given colony would tend to specialize on feeding on those resources exploited by their parents which, consequently, would favour to settle in their hatching sites; (3) The density of the colony has been reported to have a negative effect on recruitment: in areas with high rates of mortality in adults (Duncan 1978), the recruitment for immature individuals is possible in greater number. However, this does not seem to be the case for our colonies. Adults’ annual survival was assessed to be almost 90%, which might be considered a ‘normal’ value for a Larus gull (Gaston 2004). After applying this estimate in a simple population model, we found that our population was stable or even experiencing a slight, moderate increase (Additional file 1: Appendix S1). In this scenario, it could also be stated that the colonies may have a higher carrying capacity, which would still allow a high philopatry (Duncan 1978); (4) High recruitment rates would also be possible in contexts of very high availability of food subsidies, both due to the existence within the region of a number of fish harbours with high activity (Arizaga et al. 2011; Zorrozua et al. 2020), as well as some still open-air landfills (Egunez et al. 2017; Arizaga et al. 2018). However, the landfills still remaining in the region are expected to be closed in a very short-term period, which is expected to have direct consequences on the dynamics and trophic ecology of this population (Steigerwald et al. 2015; Zorrozua et al. 2019). A sudden food shortage should result in a decreasing reproductive output or survival (Oro et al. 1995), and may also increase dispersal (Arizaga et al. 2014), and it’s likely to generate population declines relatively fast (Galarza 2015). As a consequence, local recruitment rates would be lower because natal dispersal would be expected to increase (Oro and Pradel 2000). This incoming new scenario will offer us an excellent opportunity to test for the effect of landfill closure on local recruitment rates.
Globally, our results are within the upper limit of other species where very high rates of local recruitment have been also detected, e.g., in colonies of Audouin’s gull (Oro and Pradel 2000). By contrast, other smaller gulls, such as the Black-legged Kittiwake (Rissa trydactyla), were shown to have much smaller local recruitment rates, ranging between 35 and < 10% (Porter and Coulson 1987; Coulson and Coulson 2008; McKnight et al. 2019). Estimates for the Herring Gull (Larus argentatus) have also reported considerably low local recruitment rates (< 40%) (Chabrzyk and Coulson 1976; Parsons and Duncan 1978), though in this case the cull of breeding birds could have promoted philopatry rates values abnormally high (Bosch et al. 2000, 2019). The effect of external factors like culling programs on recruitment is important, and the values from some colonies may not be extrapolated to other colonies since this would lead false premises in demographic models (Brooks and Lebreton 2001). Even though the use of mean values is a common practice when building population models, we highlight here the need to be very cautious, and to estimate values obtained from the surveyed colonies always that this is possible.
Local apparent survival estimation values did not vary substantially as compared to previous works (Juez et al. 2015), providing a relatively low value for the first year of life (where the first weeks after fledgling are the most critical ones; Genovart et al. 2017). The real survival value for these first-year birds after fledging could indeed be slightly higher, since all these birds were ringed when they were chicks, so pre-fledging mortality should be considered. It is also true, in addition, that pre-fledging survival varies with the chick age, being lower for those birds ringed at an age closer to fledging time (Delgado and Arizaga 2017). Overall, if we assume that that daily mean survival rate from hatching to fledging is ca. 0.98, and that the chicks were ringed when they were 20 days, their survival from ringing to fledging is roughly 0.668 (Delgado and Arizaga 2017). Thus, survival from fledging to next year could be about 0.40. As we assessed apparent local survival rates, low values in first-year birds might be also interpreted as an artefact associated to dispersal outside our three-colony system. This is possible, and it is probably the case for some individuals, but our population is resident, and the majority of birds remain close to their natal colonies even in their first year of life (Arizaga et al. 2010). Therefore, it can be stated that local survival rates must be rather close to true survival.
The survival estimation of adults felt within the range found for other large gulls (Chabrzyk and Coulson 1976; Pons and Migot 1995). Our models did not detect differences among colonies hence suggesting that, within such a small geographic range, the factors driving the survival of our population may operate at spatial scales larger than a very few kilometres around each colony.
Encounter probabilities were, overall, relatively low (0.15–0.26 in adult birds), showing that finding ringed individuals in the colonies was relatively difficult. A higher sampling effort should help to enhance these results, though, as deducted from the relatively low confidence intervals obtained for the other two parameter estimates, we consider that this higher effort may not have a statistical effect on recruitment or survival. Encounter probabilities were found to be still much lower in immature gulls. Causes underlying this result must focus on the fact that immature specimens tend to remain outside the colonies, often exploiting different foraging areas (Pettex et al. 2019). The colony-dependent variation in these encounter rates is attributed by us to the topography and the accessibility.