Fitness Consequences of Timing of Migration and Breeding in Cormorants

Peer reviewe

Unmanned Aircraft Systems as a Powerful Tool to Detect Fine-Scale Spatial Positioning and Interactions between Waterbirds at High-Tide Roosts

The surveillance of behavioral interactions between individuals in bird populations is important to understand social dynamics and explain distribution patterns caused by competition for food and space. For waterbirds, little is known about interactions between individuals at high-tide roosts. In the present study, we used surveying with unmanned aircraft systems (UASs) to provide enhanced information on previously hidden aspects of the highly dynamic communities of roosting waterbirds in the non-breeding season. Fine-scale density estimations, derived from aerial photos obtained with UASs, were used as a measure to explain intra- and inter-species interactions for 10 selected waterbird species on a major roost site in the Danish Wadden Sea. Uniquely defined density distributions were detected, which, to some degree, were dependent on species and species size, with smaller waders exhibiting densely packed flocks (e.g., dunlin Calidris alpina and golden plover Pluvialis apricaria), whereas larger species, such as ducks and geese (Anatidae) exhibited lower densities. Multi-species flocks were observed to occur frequently (31.9%) and generally resulted in lower densities than single-species flocks for each of the species involved. Furthermore, it has been demonstrated that UAS aerial photos can be used both to assess positions for roosting waterbirds and to classify habitats (i.e., mudflats, vegetated areas, waterline, and flooded areas) during high-tide. This facilitated the collection of precise data for temporal habitat choices for individual species when using the studied roost site. Our study highlights UAS surveys as an effective tool to gather hitherto unobtainable data for individual occurrences of roosting waterbirds on a spatiotemporal scale

A test of positive association for detecting heterogeneity in capture for capture-recapture data

The Cormack–Jolly–Seber (CJS) model assumes that all marked animals have equal recapture probabilities at each sampling occasion, but heterogeneity in capture often occurs and should be taken into account to avoid biases in parameter estimates. Although diagnostic tests are generally used to detect trap-dependence or transience and assess the overall fit of the model, heterogeneity in capture is not routinely tested for. In order to detect and identify this phenomenon in a CJS framework, we propose a test of positive association between previous and future encounters using Goodman–Kruskal’s gamma. This test is based solely on the raw capture histories and makes no assumption on model structure. The development of the test is motivated by a dataset of Sandwich terns (Thalasseus sandvicensis), and we use the test to formally show that they exhibit heterogeneity in capture. We use simulation to assess the performance of the test in the detection of heterogeneity in capture, compared to existing and corrected diagnostic goodness-of-fit tests, Leslie’s test of equal catchability and Carothers’ extension of the Leslie test. The test of positive association is easy to use and produces good results, demonstrating high power to detect heterogeneity in capture. We recommend using this new test prior to model fitting as the outcome will guide the model-building process and help draw more accurate biological conclusions. Supplementary materials accompanying this paper appear online

Recovery data for Bayesian CMR model

Contains information on year and location (area) of ringing and recovery for 4511 cormorant

Net-entrapment of great cormorants Phalacrocorax carbo sinensis in relation to individual age and population size

Population control of great cormorants Phalacrocorax carbo sinensis has been suggested as a tool to mitigate fish losses by cormorants foraging in pound nets, gill nets and fykes. The objective of our study was to quantify the difference between first-year birds and older birds in the risk of drowning in fishing gear, and to explore the influence of population size on the proportion of individuals drowning. An index of the proportion of first-year birds and older birds that drowned was obtained for cormorants ringed in the Danish Vorsø colony using resightings of colour-ringed individuals and ring recoveries of individuals found dead. First-year birds were approximately 10 times more likely to drown than older birds. We used ring recoveries of cormorants ringed in Denmark and found dead in Denmark, North Germany and South Sweden (i.e. in the main postbreeding area) to reveal changes over a 25-year period in the proportion drowned among those recovered. Among first-year birds the proportion drowned declined from 66% in 1978-1984 to 24% in 2000-2002, and among older birds the proportion declined from 46% in 1978-1984 to 26% in 1999-2002. During 1978- 2000, breeding numbers in Denmark increased from 1,400 to 42,500 nests, and the proportion of cormorants drowned among those recovered was significantly negatively correlated with population size. This suggests that the proportion of the population foraging in nets declined as the population increased. The damage caused by cormorants foraging in pound nets is, therefore, unlikely to decrease in proportion to reductions in population size reached through population control