Site‐dependent regulation of breeding success: Evidence for the buffer effect in the common guillemot, a colonially breeding seabird

Abstract

Density‐dependent regulation can offer resilience to wild populations experiencing fluctuations in environmental conditions because, at lower population sizes, the average quality of habitats or resources is predicted to increase. Site‐dependent regulation is a mechanism whereby individuals breed at the highest quality, most successful, sites, leaving poorer quality, less successful sites vacant. As population size increases, higher quality sites become limiting but when populations decline, lower quality sites are vacated first, offering resilience. This process is known as the ‘buffer effect’. However, few studies have tested whether such regulation operates in populations experiencing changes in size and trend. We used data from a population of common guillemots Uria aalge, a colonially breeding seabird, to investigate the relationship between site occupancy probability, site quality and population size and trend. These data were collected at five sub‐colonies spanning a 38‐year period (1981–2018) comprising phases of population increase, decrease and recovery. We first tested whether site quality and population size in sub‐colonies explained which sites were occupied for breeding, and if this was robust to changes in sub‐colony trend. We then investigated whether disproportionate use of higher quality sites drove average site quality and breeding success across sub‐colony sizes and trends. Finally, we tested whether individuals consistently occupied higher quality sites during periods of decline and recovery. Higher quality sites were disproportionality used when sub‐colony size was smaller, resulting in higher average site quality and breeding success at lower population sizes. This relationship was unaffected by changes in sub‐colony trend. However, contrary to the predictions of the buffer effect, new sites were established at a similar rate to historically occupied sites during sub‐colony decline and recovery despite being of lower quality. Our results provide support for the buffer effect conferring resilience to populations, such that average breeding success was consistently higher at lower population size during all phases of population change. However, this process was tempered by the continued establishment of new, lower quality, sites which could act to slow population recovery after periods when colony size was low.

Opposing effects of spatiotemporal variation in resources and temporal variation in climate on density dependent population growth in seabirds

“…Increased climatic variation would exacerbate the effects of density on processes affecting population growth, such as resource limitation of suitable nesting sites (forcing individuals to use poorer quality nest sites, potentially more exposed to weather) and food (via climate‐mediated impacts on prey), and by increasing the energetic requirements of seabirds, potentially aggravating competition between individuals. Population growth rates of UK seabirds have been reported to be limited by terrestrial and marine factors including the number of good quality breeding sites in Black‐legged Kittiwakes and Common Guillemots (Bennett et al, 2022 ; Coulson, 1983 ; Kokko et al, 2004 ; Porter & Coulson, 1987 ), by density dependent depletion of prey in Northern Gannets (Davies et al, 2013 ; Lewis et al, 2001 ), by resource limitation affecting recruitment in Common Guillemots (Crespin et al, 2006 ), and by territory formation under high population densities in Herring Gulls Larus argentatus (Coulson et al, 1982 ; Raven & Coulson, 1997 ). More widely, density dependence in seabirds beyond UK waters is affected by a number of drivers, in relation to food resources for three seabird species in the northern Humboldt Current System off the coast of Peru (Barbraud et al, 2018 ), in Antarctic species (e.g., Pacoureau et al, 2019 ), and in a gull species in the western Mediterranean (Genovart et al, 2018 ), and in relation to limited breeding sites in Antarctic seabird species (e.g., Southwell & Emmerson, 2020 ).…”

“…Solid lines are posterior means with 95% credible intervals (dotted lines). and Common Guillemots (Bennett et al, 2022;Coulson, 1983;Kokko et al, 2004;Porter & Coulson, 1987), by density dependent depletion of prey in Northern Gannets (Davies et al, 2013;Lewis et al, 2001), by resource limitation affecting recruitment in Common Guillemots (Crespin et al, 2006), and by territory formation under high population densities in Herring Gulls Larus argentatus (Coulson et al, 1982;Raven & Coulson, 1997). More widely, density dependence in seabirds beyond UK waters is affected by a number of drivers, in relation to food resources for three seabird species in the northern Humboldt Current System off the coast of Peru (Barbraud et al, 2018), in Antarctic species (e.g., Pacoureau et al, 2019), and in a gull species in the western Mediterranean (Genovart et al, 2018), and in relation to limited breeding sites in Antarctic seabird species (e.g., Southwell & Emmerson, 2020).…”

Earlier and more frequent occupation of breeding sites during the non‐breeding season increases breeding success in a colonial seabird

“…Sites of higher quality are preferentially occupied during the breeding season in a density-dependent manner and have a higher likelihood of a successful breeding outcome. This has been observed in two separate analyses of our study population (Bennett et al, 2022;Kokko et al, 2004). Hence, for subcolony 1, we used the average breeding success of a site based on data collected from 1981 to 2016 as a measure of site quality.…”

“…Lastly, we used structural regression modeling via a path analysis to determine whether occupancy directly affected breeding success, or whether any effects were indirect and sequential such that site quality affected return date, then occupancy frequency, lay date, and ultimately breeding success. The relationships between site quality, lay date, and breeding success for guillemots are well established in the literature; breeding commences earlier at higher‐quality sites, and these sites have higher breeding success (Bennett et al, 2022 ; Kokko et al, 2004 ). However, the relationships between site quality, non‐breeding occupancy, and lay date and breeding success are not well characterized.…”

Increased parental effort fails to buffer the cascading effects of warmer seas on common guillemot demographic rates

“…Density‐dependent effects on breeding performance are important in this population (Ashbrook et al, 2010; Bennett et al, 2022). Accordingly, we tested for the effects of population size (breeding pairs) in the current year.…”