Foraging ecology, movements and abundance of an avian predator of mesopelagic prey: new and deeper insights using novel techniques and ideas

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Temporal and age-related dietary variations in a large population of yellow-legged gulls Larus michahellis : Implications for management and conservation

Publication . Alonso, Hany Rafael de Drummond Ludovice Garcia; Almeida, Ana Pereira; Granadeiro, José Pedro; Catry, Paulo

There was an extraordinary increase in the numbers of European gulls during the twentieth century which has been linked to higher availability of food derived from human activities. At Berlenga island (Portugal), the population of yellow-legged gulls Larus michahellis increased from 2600 individuals to a peak of 44,698 gulls (1974–1994), after which control measures have been put in place. Despite the management effort, little is known about the feeding ecology of this population. To investigate temporal and age-related variations in the diet of yellow-legged gulls at Berlenga, 1668 adult pellets and 145 chick regurgitates were collected and analysed between 2009 and 2012. Contradicting the generally accepted idea that these birds depend mainly on human-related food, adult gulls relied substantially on a locally abundant natural prey, the Henslow’s swimming crab Polybius henslowii. Nevertheless, large amounts of refuse and fish were consumed in periods of apparent lower availability of swimming crabs. Despite the large temporal shifts in diet and feeding areas (change from marine to terrestrial prey), adult gulls consistently provisioned their chicks with a fish-based diet and chick condition remained constant. These results not only highlight the great resilience of this population to changes in food availability but also indicate that food from different human activities remain highly accessible. With the implementation of recent EU legislation regarding the reduction of fishery discards, and the increase of urban populations in the mainland, the monitoring and appropriate management of gull populations will be decisive for the healthy conservation of coastal systems used by these gulls.

2015Journal article

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Sex and migratory strategy influence corticosterone levels in winter-grown feathers, with positive breeding effects in a migratory pelagic seabird

Publication . Pérez, Cristóbal; Granadeiro, José Pedro; Dias, Maria P.; Catry, Paulo

To overcome unpredictable stressful transitory events, animals trigger an allostatic response involving the hypothalamic-pituitary-adrenal cortex. This hormonal response, which involves the release of glucocorticoids which in turn mediate between the main physiological mechanisms that regulate the energetic demands and resource allocation trade-off with behavioural responses to environmental perturbations and may ultimately lead to variation in fitness. We have used the Cory's shearwater Calonectris borealis, a sexually dimorphic pelagic seabird with a partial migratory strategy, as a model bird species to analyse a number of traits related to the stress response. We investigated whether the activation of a stressful response, mediated by corticosterone, during the wintering period (1) correlated with the previous breeding success, (2) was affected by the migratory behaviour of male birds and (3) had consequences in the fitness of the birds. Corticosterone levels in feathers grown overwinter were analysed in 61 adult birds during three consecutive migratory periods (2009-2012) and in 14 immature birds in the wintering period 2010-2011. Moreover, the levels of corticosterone were analysed in experimental birds which were freed from their reproductive duties and compared with control birds which raised fledglings to the end of the breeding period. The results show that the levels of corticosterone were sex dependent, differed between years and were affected by the migratory strategy performed by the birds. The activation of the stressful response over the wintering period generated residual carry-over effects that positively affected the reproductive output in the subsequent breeding stage, a phenomenon previously undescribed in a long-lived pelagic seabird. Our study provides evidence that the analysis of corticosterone from feathers is a useful tool to evaluate carry-over effects in birds far away from breeding sites, opening new possibilities for future studies in this field.

2016Journal article

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Niche segregation between immature and adult seabirds: Does progressive maturation play a role?

Publication . Campioni, Letizia; Granadeiro, José Pedro; Catry, Paulo

In long-lived species with slow maturation, prebreeders often represent a large percentage of the individuals alive at any moment, but their ecology is still understudied. Recent studies have found prebreeding seabirds to differ in their isotopic (and trophic) niche from adult breeders attending the same nesting colonies. These differences have been hypothesized to be linked to the less-developed foraging performance of younger and less-experienced immatures or perhaps to their inferior competitive abilities. Such differences from adults would wane as individuals mature (“the progressive ontogenetic shift hypothesis”) and could underpin the prolonged breeding deferral until adulthood displayed by those species. This study documents a marked difference in the nitrogen and carbon isotopic ratios measured in the whole blood of immatures and breeders in 2 pelagic seabird species (Cory’s shearwaters, Calonectris borealis, and black-browed albatrosses, Thalassarche melanophris) nesting in contrasting environments. However, blood isotopic values did not present a relationship with prebreeder age, suggesting no gradual ontogenetic shift from an immature toward an adult isotopic niche. Furthermore, isotopic signatures of sabbatical adults could not be separated from those of immatures attending the same colonies, but were clearly segregated from adult breeders. These results suggest that isotopic differentiation between immatures and breeders is mainly linked to a factor unrelated to previous experience and hence probably unrelated to a hypothetical gradual improvement of foraging competence or competitive abilities. Any ecological differentiation between breeders and nonbreeders is more likely related to the severity of the central-place foraging constraints and to the energetic requirements of reproduction (“the reproductive constraint hypothesis”).

2016Journal article

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The diet of a nocturnal pelagic predator, the Bulwer’s petrel, across the lunar cycle

Publication . Waap, Silke; Symondson, William O. C.; Granadeiro, José Pedro; Alonso, Hany Rafael de Drummond Ludovice Garcia; Gonçalves, Catarina Serra; Dias, Maria P.; Catry, Paulo

The lunar cycle is believed to strongly influence the vertical distribution of many oceanic taxa, with implications for the foraging behaviour of nocturnal marine predators. Most studies to date testing lunar effects on foraging have focused on predator activity at-sea, with some birds and marine mammals demonstrating contrasting behavioural patterns, depending on the lunar-phase. However, to date no study has focused on how the lunar cycle might actually affect predator-prey interactions in the upper layers of the ocean. Here, we tested whether the diet of the predominantly nocturnal pelagic predator, the Bulwer's petrel (Bulweria bulwerii) would change throughout the lunar cycle, using molecular analysis to augment detection and taxonomic resolution of prey collected from stomach-contents. We found no evidence of dietary shifts in species composition or diversity, with Bulwer's petrel always consuming a wide range of mesopelagic species. Other co-variables potentially affecting light availability at-sea, such as percentage of cloud cover, did not confound our results. Moreover, many of the species found are thought not to reach the sea-surface. Our findings reveal that nocturnal predators are probably more specialized than previously assumed, irrespective of ambient-light, but also reveal deficiencies in our current understanding of species vertical distribution and predation-dynamics at-sea.

2017Journal article

Open access

Distribution and at-sea activity of a nocturnal seabird, the Bulwer's petrel Bulweria bulwerii, during the incubation period

Publication . Dias, Maria P.; Romero, Joana; Granadeiro, José Pedro; Catry, Teresa; Pollet, Ingrid L.; Catry, Paulo

Bulwer'spetrelsarenocturnalseabirdsthatmostlypreyonmesopelagicfauna.Asaerialforagersand shallowdivers,theirfeedingopportunitiesarelimitedbynear-surfaceavailabilityoftheirprey,whichis highlyvariablebothtemporally(reflectingdiurnalandlunarcycles)andspatially.Herewestudiedhow Bulwer'spetrelscopewiththeseconstraintsbyanalysingtheirat-seadistributionandactivityduringthe incubationperiod.Wetrackedthemovementsof20birdsfromSelvagemGrande(NEAtlantic)duringa completelunarcycle,andrecorded30foragingtripsthatlasted11daysonaverage.Birdswereboth distributedaroundthecolonyandinwatersclosetotheAzoreanarchipelago(mid-Atlantic)located 1700kmaway,andweresignificantlymoreactiveatnight(especiallyjustaftersunsetandbeforesunrise), whenmesopelagicfaunaisalsoclosertotheseasurfaceduetotheirdielverticalmigrations.Bulwer's petrelsspentsignificantlymoretime flyingduringmoonlight,althoughtheeffectofthemoonwasrela- tivelyweak(ca.10–15%differencebetweenmoonlitanddarkperiodsofthenight),andnotobviouswhen birdswereforaginginmid-Atlanticwaters,whichwerealsotargetedmoreoftenduringfull-moon.These resultsrevealkeyadaptationsoftheBulwer'spetreltothehighlydynamicecologyofitsmesopelagicprey.

2016Journal article

Restricted access