Hatching success in Lesser Black-backed Gulls Larus fuscus - an island case study of the effects of egg and nest site quality
* Correspondence author: email@example.com
1 British Trust for Ornithology, The Nunnery, Thetford, Norfolk IP24 2PU, UK;
2 Cardiff School of Biosciences, The Sir Martin Evans Building, Museum Avenue, Cardiff CF10 3AX, UK.
Two key components of a bird's reproductive success in any particular breeding attempt are hatching success and chick survival to fledging (Lack 1968). Although chick survival is often the more important factor determining reproductive success in seabirds that nest in the open (Nelson 1980; Camphuysen 2013), hatching success is also important (Paludan 1951; Schreiber 1970). The latter is influenced by intrinsic factors, including parental quality and condition, which in turn contribute to egg viability (e.g. Bolton 1991; Oro et al. 2014), together with extrinsic factors such as predation and climatic conditions (e.g. Jones et al. 2008; van de Pol et al. 2010). Many birds construct nests to protect their eggs, and thereby increase their reproductive success. In seabirds, nests can range from none at all e.g. the White Tern Gygis alba (Nelson 1980) to quite elaborate structures such as those made by kittiwakes Rissa spp. (Coulson 2011).
Nests of the Lesser Black-backed Gull Larus fuscus, like those of most gulls, tend to be simple, ranging from a scrape with little gathered nesting material, to a small bed of vegetation arranged in a shallow cup. These ground-nesting seabirds, which breed primarily in northwest Europe (Malling Olsen & Larsson 2004), have traditionally occupied colonies at coastal nesting sites on flat or moderately sloping ground, which may be towards the top of cliffs, or in open, sometimes low-lying areas. Nests are therefore often quite accessible, leaving eggs (and chicks) vulnerable to predation, for example by conspecifics and other gull species that often nest nearby, or by mammalian predators such as Red Foxes Vulpes vulpes (Davis & Dunn 1976; Camphuysen et al. 2010). Predation has been found to be responsible for up to 30% of Lesser Black-backed Gull egg losses at particular sites (Paludan 1951; Camphuysen 2013). This risk can be partially offset by the benefits of colonial defence and vigilance (e.g. Götmark & Andersson 1984; Beauchamp 2009), and by nesting close to vegetation that conceals eggs from potential predators (Haycock & Threlfall 1975; Hunt & Hunt 1975; Burger & Shisler 1978). Open nest sites also leave eggs exposed to cold and inclement weather conditions, and large numbers of eggs can fail to hatch because of this (Paludan 1951; Fox et al. 1978). However, pairs can again compensate by choosing to nest in an area with shelter, for instance tall vegetation around the nest (e.g. Kim & Monaghan 2005a). The extent to which gulls are able to counteract the disadvantages of their ground-nesting breeding behaviour is thought to be influenced by aspects of their phenotypic quality (Kim & Monaghan 2005b; Oro 2008).
Establishing and maintaining a breeding territory is energetically taxing for gulls. Competition can be intense, especially for first time breeders (Chabrzyk & Coulson 1976), whilst costly aggressive interactions, including fighting, calling and displacement activities such as grass pulling, are regularly seen between even established pairs (e.g. Tinbergen 1953; Butler & Janes-Bulter 1982; Pierotti & Annett 1994). These behaviours appear to be necessary throughout the season to prevent territorial encroachments, such as the theft of nesting materials, predation of eggs or chicks and extra-pair copulations (e.g. Burger & Beer 1975; Bukacińska & Bukaciński 1994). Poor quality birds not only struggle to secure and successfully breed at a nest site in habitats favoured by superior birds, but lack the surplus energy to produce and successfully incubate the large and fertile eggs and clutches necessary to match the reproductive success of high quality individuals. Measures of nesting habitat, egg size and hatching success can therefore indicate an individual's phenotypic quality.
Breeding Lesser Black-backed Gull numbers are currently declining at many 'traditional' rural coastal sites across this species' range, including in protected areas, while breeding populations in urban areas are increasing in number and range (e.g. Camphuysen et al. 2010; Balmer et al. 2013). A thorough understanding of this species' breeding ecology is necessary to facilitate effective conservation management (for a review, see Ross-Smith et al. 2014). This is especially vital given that Lesser Black-backed Gulls that breed on rooftops in urban areas (e.g. Raven & Coulson 1997; Rock 2005) are causing an increasing public nuisance, leading to calls for ever stricter controls.
In this study, we assessed Lesser Black-backed Gull hatching success, and explored the reasons underlying hatching failure, for approximately 350 pairs each year over two consecutive seasons, across a range of nesting habitats at an island breeding colony. This colony is free of mammalian predators, and at the time of this study, the breeding population was steadily increasing and there was no apparent food stress or other external pressures known to be detrimentally affecting breeding success (Ross-Smith et al. 2013). For each egg found, we measured a number of variables.We noted the size of the clutch it was part of, as eggs from small clutches are less likely to hatch than those from large clutches (Harris 1964; Brown 1967). We recorded laying date, as hatching success has been shown to vary throughout the breeding season (Brown 1967; Davis & Dunn 1976; García Borboroglu et al. 2008), along with laying order, as within-clutch variation in egg composition, with potential implications for egg outcome, has been demonstrated in a number of studies (e.g. Royle et al. 1999). We calculated egg volume, as large eggs are more likely to hatch than small eggs (Parsons 1970; Bolton 1991).We also measured the amount of vegetation around nests, because of its influence on hatching success, as discussed above, and we calculated how nests were distributed relative to others in the colony, as proximity to conspecific nests has been found to influence reproductive success in gulls (e.g. Ewald et al. 1980; Butler & Trivelpiece 1981). We discuss the relative importance of each factor in determining hatching success and place this information in a conservation context.
We thank Dewi Langlet, Jeff Davey and the Flat Holm Project for their assistance with this project. VR-S was funded by a Cardiff University Research Studentship. Work was carried out under licences OTH:SB:02:2007/2008 from the Countryside Council for Wales. We are grateful to Ruedi Nager and an anonymous referee, whose comments greatly improved this paper. Thank you also to Kees Camphuysen for encouraging the idea of revisiting and trying to publish this work so many years after it was carried out.
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