Summary 17 Dietary studies give vital insights into foraging behaviour, with implications for understanding 18changing environmental conditions and the anthropogenic impacts on natural resources. Traditional diet 19 sampling methods may be invasive or subject to biases, so developing non-invasive and unbiased 20 methods applicable to a diversity of species is essential. 21 2. We used digital photography to investigate the diet fed to chicks of a prey-carrying seabird, and 22 compared our approach (photo-sampling) to a traditional method (regurgitations) for the greater crested 23tern Thalasseus bergii. 24 3. Over three breeding seasons, we identified >24,000 prey items of at least 47 different species, more 25 than doubling the known diversity of prey taken by this population of terns. We present a method to 26 estimate the length of the main prey species (anchovy Engraulis encrasicolus) from photographs, with an 27 accuracy < 1 mm and precision ~0.5 mm. Compared to regurgitations at two colonies, photo-sampling 28 produced similar estimates of prey composition and size, at a faster species accumulation rate. The prey 29 compositions collected by two researchers photo-sampling concurrently were also similar. 30 4. Photo-sampling offers a non-invasive tool to accurately and efficiently investigate the diet 31 composition and prey size of prey-carrying birds. It reduces biases associated with observer-based 32 studies and is simple to use. This methodology provides a novel tool to aid conservation and 33 management decision-making in light of the growing need to assess environmental and anthropogenic 34 change in natural ecosystems. 35 36

Marine predators, such as seabirds, are useful indicators of marine ecosystem functioning. In particular, seabird diet may reflect variability in food-web composition due to natural or human-induced environmental change. Diet monitoring programmes, which sample diet non-invasively, are valuable aids to conservation and management decision-making. We investigated the diet of an increasing population of greater crested terns Thalasseus bergii in the Western Cape, South Africa, during three successive breeding seasons (2013 to 2015), when populations of other seabirds feeding on small pelagic schooling fish in the region were decreasing. Breeding greater crested terns carry prey in their bills, so we used an intensive photo-sampling method to record their diet with little disturbance. We identified 24,607 prey items from at least 47 different families, with 34 new prey species recorded. Fish dominated the diet, constituting 94% of prey by number, followed by cephalopods (3%), crustaceans (2%) and insects (1%). The terns mainly targeted surface-schooling Clupeiformes, with anchovy Engraulis encrasicolus the most abundant prey in all three breeding seasons (65% overall). Prey composition differed significantly between breeding stages and years, with anchovy most abundant at the start of the breeding season, becoming less frequent as the season progressed. The proportion of anchovy in the diet also was influenced by environmental factors; anchovy occurred more frequently with increasing wind speeds and was scarce on foggy days, presumably because terns rely in part on social facilitation to locate anchovy schools. The application of this intensive and non-invasive photo-sampling method revealed an important degree of foraging plasticity for this seabird within a context of locally reduced food availability, suggesting that, unlike species that specialise on a few high-quality prey, opportunistic seabirds may be better able to cope with reductions in the abundance of their preferred prey.

Zoonotic thermophilic Campylobacter and nontyphoidal Salmonella enterica are a major cause of foodborne human gastroenteritis worldwide. There is little information about reservoirs of these zoonotic agents in Africa. Thus, chicks of kelp gulls (Larus dominicanus, n = 129) and greater crested terns (Thalasseus bergii, n = 100) were studied at five colonies on the Western Cape coast (South Africa) during summer 2013/2014. Campylobacter spp. occurrence was 14.0% (CI : 9.9-19.3), with C. jejuni the most frequently isolated species, whilst that of Salmonella was 27.5% (CI : 21.9-33.9) overall, with a higher prevalence in gulls (43.0%, CI : 34.8-52.4) than terns (7.0%, CI : 3.1-14.4). Among the 16 different S. enterica serovars found, Anatum, Enteritidis and Hadar were the most frequent. The same or highly similar pulsed-field gel electrophoresis genotype was found in some Salmonella isolates from seabirds and humans presenting with salmonellosis in Cape Town hospitals. Both Campylobacter and Salmonella isolates exhibited antimicrobial resistance to several agents, including critically important antimicrobials (quinolones, tetracyclines and β-lactams) and multidrug resistance in Salmonella serovars from kelp gulls. Our results highlight the importance of seabirds as reservoirs of Campylobacter and Salmonella resistant strains and their role in the maintenance and transmission of these bacteria in the environment, with implications for public health.

Mixed-species assemblages are common in nature, providing mutual benefits to associating species including anti-predator advantages or resource facilitation. However, associating with other species may also impose costs through kleptoparasitism (food theft). Identification of these costs, and how they vary when different species breed alongside one another, is essential to understand the payoffs of mixed-species assemblages. We explore the costs of kleptoparasitism for greater crested terns Thalasseus bergii provisioning offspring at a single-species colony, where individuals suffer kleptoparasitism from conspecifics, and at a mixed colony where terns breed alongside Hartlaub's gulls Chroicocephalus hartlaubii and are vulnerable to both intra-and interspecific kleptoparasitism.Gull presence likely contributes to increases in both kleptoparasitic attacks and the proportion of prey lost or stolen during provisioning, relative to the single-species colony. Provisioning adults suffered additional energetic costs in response to gull kleptoparasitism, requiring more attempts to deliver prey, taking longer to do so, and swallowing more prey (to the detriment of their offspring).Gulls also appear to increase the duration of tern vulnerability to kleptoparasitism, because they continued to steal food from adults and chicks after precocial chicks left the nest, when intraspecific kleptoparasitism is negligible. Terns breeding in a mixed colony therefore suffer direct and indirect costs through decreased provisioning and increased provisioning effort, which may ultimately impact reproductive success, resulting in colony decline where kleptoparasitism is frequent. This study illustrates how forming a mixed-species seabird breeding assemblage has costs as well as benefits, potentially fluctuating between a parasitic and a mutualistic relationship.

Seabirds use several methods to transport food to their chicks; most species carry food in their stomachs or crops, but some terns and auks carry prey in their bills. Terns usually only carry one prey item at a time, limiting the rate at which they can provision their chicks, and restricting their effective foraging range. However, some terns do occasionally carry multiple prey, which should offer a selective advantage, but there are very few studies investigating the factors influencing the occurrence of multi-prey loading. We investigated the occurrence of multi-prey loads in provisioning Greater Crested Terns (Swift Tern) Thalasseus bergii bergii breeding on Robben Island, South Africa. Of 24,173 loads photographed, 1.3% comprised multiple prey items. Up to 11 fish were carried at once, but most multiprey loads contained two Anchovies Engraulis encrasicolus, the most common prey item for this population of terns. Mixed species prey were recorded for the first time in a tern. Multi-prey loads occurred more frequently during mid-and lateprovisioning, presumably because large chicks can cope with multiple prey, and have higher energetic requirements than small chicks. Mean standard length of Anchovies in multi-prey prey loads was less than Anchovies in single loads, possibly suggesting terns compensate for smaller prey sizes by bringing multiple prey back to their chick. The orientation of multiple Anchovies in a tern's bill tended to be the same, suggesting that they were captured from polarized fish schools. At least some multi-prey loads were caught in a single dive.