Abstract: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 se… Show more
“…Chick observed DFI increased from early provisioning (19.9 g·d −1 , 17.2–23.0, n = 126) to late provisioning (45.1 g·d −1 , 34.6–58.7, n = 629). Figure 3 Posterior distributions for mean anchovy mass (g) in the diet of greater crested terns estimated using photo-sampling 34 across three breeding seasons (2013–2015) at Robben Island during early and late provisioning. Black tick-marks show means and grey tick-marks 95% Bayesian credible intervals.…”
Section: Resultsmentioning
confidence: 99%
“… Posterior distributions for mean anchovy mass (g) in the diet of greater crested terns estimated using photo-sampling 34 across three breeding seasons (2013–2015) at Robben Island during early and late provisioning. Black tick-marks show means and grey tick-marks 95% Bayesian credible intervals.…”
Section: Resultsmentioning
confidence: 99%
“…In contrast, greater crested terns breeding in the Benguela ecosystem could buffer these limitations due to their flexible diet, which includes ca. 50 different prey species 34 and their low fidelity to breeding sites, which are believed to be chosen depending on the local availability of prey immediately preceding the breeding season, rather than by philopatry 32 . In addition, the recent major decrease of migrant tern populations to the Benguela ecosystem (e.g.…”
Section: Discussionmentioning
confidence: 99%
“…Prey carried by greater crested terns returning to the breeding colony to feed chicks were recorded as part of a program monitoring tern diet 34 . Prey were photographed using a non-invasive photo-sampling technique, allowing for an accurate determination of fish species and standard length 64 For anchovy, we converted estimated fish lengths to mass using a yearly species-specific regression (see Supporting Information S1 and Table S3 ).…”
Section: Methodsmentioning
confidence: 99%
“…Perhaps surprisingly, numbers of greater crested terns Thalasseus bergii , which rely on the same resources and breed in the same region, have tripled over the last few decades; the reasons for these contrasting fortunes remain equivocal 32 , 33 . Considerable foraging plasticity 34 and their ability to move breeding sites 35 could have helped greater crested terns maintain high annual survivorship in the face of ecosystem-wide changes 36 . In addition, it is possible that their small body size (~390 g), single egg clutch, and short breeding period (68 days) reduce the greater crested tern’s overall energy requirements compared to other sympatric breeding seabirds.…”
There is a growing desire to integrate the food requirements of predators living in marine ecosystems impacted by humans into sustainable fisheries management. We used non-invasive video-recording, photography and focal observations to build time-energy budget models and to directly estimate the fish mass delivered to chicks by adult greater crested terns Thalasseus bergii breeding in the Benguela ecosystem. Mean modelled adult daily food intake increased from 140.9 g·d−1 of anchovy Engraulis capensis during incubation to 171.7 g·d−1 and 189.2 g·d−1 when provisioning small and large chicks, respectively. Modelled prey intake expected to be returned to chicks was 58.3 g·d−1 (95% credible intervals: 44.9–75.8 g·d−1) over the entire growth period. Based on our observations, chicks were fed 19.9 g·d−1 (17.2–23.0 g·d−1) to 45.1 g·d−1 (34.6–58.7 g·d−1) of anchovy during early and late provisioning, respectively. Greater crested terns have lower energetic requirements at the individual (range: 15–34%) and population level (range: 1–7%) than the other Benguela endemic seabirds that feed on forage fish. These modest requirements – based on a small body size and low flight costs – coupled with foraging plasticity have allowed greater crested terns to cope with changing prey availability, unlike the other seabirds species using the same exploited prey base.
“…Chick observed DFI increased from early provisioning (19.9 g·d −1 , 17.2–23.0, n = 126) to late provisioning (45.1 g·d −1 , 34.6–58.7, n = 629). Figure 3 Posterior distributions for mean anchovy mass (g) in the diet of greater crested terns estimated using photo-sampling 34 across three breeding seasons (2013–2015) at Robben Island during early and late provisioning. Black tick-marks show means and grey tick-marks 95% Bayesian credible intervals.…”
Section: Resultsmentioning
confidence: 99%
“… Posterior distributions for mean anchovy mass (g) in the diet of greater crested terns estimated using photo-sampling 34 across three breeding seasons (2013–2015) at Robben Island during early and late provisioning. Black tick-marks show means and grey tick-marks 95% Bayesian credible intervals.…”
Section: Resultsmentioning
confidence: 99%
“…In contrast, greater crested terns breeding in the Benguela ecosystem could buffer these limitations due to their flexible diet, which includes ca. 50 different prey species 34 and their low fidelity to breeding sites, which are believed to be chosen depending on the local availability of prey immediately preceding the breeding season, rather than by philopatry 32 . In addition, the recent major decrease of migrant tern populations to the Benguela ecosystem (e.g.…”
Section: Discussionmentioning
confidence: 99%
“…Prey carried by greater crested terns returning to the breeding colony to feed chicks were recorded as part of a program monitoring tern diet 34 . Prey were photographed using a non-invasive photo-sampling technique, allowing for an accurate determination of fish species and standard length 64 For anchovy, we converted estimated fish lengths to mass using a yearly species-specific regression (see Supporting Information S1 and Table S3 ).…”
Section: Methodsmentioning
confidence: 99%
“…Perhaps surprisingly, numbers of greater crested terns Thalasseus bergii , which rely on the same resources and breed in the same region, have tripled over the last few decades; the reasons for these contrasting fortunes remain equivocal 32 , 33 . Considerable foraging plasticity 34 and their ability to move breeding sites 35 could have helped greater crested terns maintain high annual survivorship in the face of ecosystem-wide changes 36 . In addition, it is possible that their small body size (~390 g), single egg clutch, and short breeding period (68 days) reduce the greater crested tern’s overall energy requirements compared to other sympatric breeding seabirds.…”
There is a growing desire to integrate the food requirements of predators living in marine ecosystems impacted by humans into sustainable fisheries management. We used non-invasive video-recording, photography and focal observations to build time-energy budget models and to directly estimate the fish mass delivered to chicks by adult greater crested terns Thalasseus bergii breeding in the Benguela ecosystem. Mean modelled adult daily food intake increased from 140.9 g·d−1 of anchovy Engraulis capensis during incubation to 171.7 g·d−1 and 189.2 g·d−1 when provisioning small and large chicks, respectively. Modelled prey intake expected to be returned to chicks was 58.3 g·d−1 (95% credible intervals: 44.9–75.8 g·d−1) over the entire growth period. Based on our observations, chicks were fed 19.9 g·d−1 (17.2–23.0 g·d−1) to 45.1 g·d−1 (34.6–58.7 g·d−1) of anchovy during early and late provisioning, respectively. Greater crested terns have lower energetic requirements at the individual (range: 15–34%) and population level (range: 1–7%) than the other Benguela endemic seabirds that feed on forage fish. These modest requirements – based on a small body size and low flight costs – coupled with foraging plasticity have allowed greater crested terns to cope with changing prey availability, unlike the other seabirds species using the same exploited prey base.
The Arctic has experienced greatly decreased sea ice and increased ocean temperatures in recent decades but there is a paucity of biological time‐series data allowing assessment of resulting temporal variation in the region's marine ecosystems. Seabirds, as highly mobile and highly visible, upper trophic‐level predators, can be valuable monitors of modifications in marine ecosystems, especially for regions lacking commercial fisheries or regular oceanographic sampling. Since 1975, we have studied annually an Arctic Alaskan colony of Mandt's black guillemot (Cepphus grylle mandtii), an ice‐obligate diving seabird, specializing on Arctic cod (Boreogadus saida), the primary forage fish of the ice‐associated cryopelagic ecosystem. Using multi‐state capture–mark–recapture models, matrix population models, and perturbation analysis, we quantified the environmental and demographic drivers of population change from 1980 to 2019 for the individually marked population. The colony increased rapidly, from <20 to >200 breeding pairs from 1975 to 1990 in response to increased availability of nesting cavities, before experiencing intermittent declines to <50 pairs in 2021. Immigration and apparent survival were the primary demographic parameters affecting population growth with sea ice extent in late summer and fall the primary environmental driver. The initial growth occurred during a period of primarily negative winter Arctic Oscillations (WAO) and extensive summer sea ice. The decline began when an extremely positive WAO in 1989/1990 initiated changes in atmospheric and oceanographic circulation causing major reductions in summer sea ice throughout the region. The three‐decade decline in the population saw plateaus or minor growth with increasing frequency of negative WAOs and increasing declines following two previously identified “tipping points” in sea ice loss. Breeding success at the study colony declined with decreased availability of Arctic cod due to sea ice loss and increasing sea surface temperature and is presumed to have occurred at the source colonies for immigrants where similar oceanographic changes were occurring. Quasi‐extinction of the colony (reduction to <25 pairs) is predicted within the next two decades. The sensitivity of Mandt's black guillemot to multi‐decadal changes in the Arctic's cryopelagic ecosystem makes it an excellent sentinel species for the region with its recent collapse having dire implications for the Arctic Ocean's constituent species.
Monitoring seabird diet can provide insights into marine ecosystems that are logistically difficult or costly to observe with traditional fisheries survey methods. Using digital photography, we described the diet of greater crested terns (Thalasseus bergii) breeding on Montague Island (36°15′ S, 150°13′ E), a colony located in an oceanographically dynamic region of southeast Australia. We investigated how the type and size of prey brought back to the colony changed in relation to both breeding stage and variation in local environmental conditions. 2469 prey items were identified to species or family level over 35 consecutive days of photo-sampling in 2018. Australian anchovy (Engraulis australis), a surface-schooling clupeid fish, was the most abundant prey returned to the colony during all breeding stages (84.5%). The proportion of anchovy increased from 77.0% when birds were provisioning their adult partners during incubation, to 92.4% when they were provisioning chicks, suggesting selective foraging behaviour on this energy-rich species to facilitate rapid chick growth. Anchovy size was significantly larger during incubation (91.1 ± 14.9 mm), smaller during early chick provisioning (71.8 ± 11.0 mm), and increased slightly during mid provisioning (79.6 ± 11.9 mm), indicating adaptive prey selection that is matched to the physical requirements of different breeding stages. The proportion of anchovy prey was also influenced by sea surface temperature (SST), with anchovy becoming more dominant with increasing local SSTs, up to ~ 17.5 °C. This study gives new insight into the types of prey that are seasonally available to predators in this region. The strong specialisation by greater crested terns for one energy-rich species of schooling fish (anchovy) suggests that variation in the size and composition of their prey may serve as an indicator of change in the pelagic ecosystem off southeast Australia.
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