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1. Individual specialisations in behaviour are predicted to arise where divergence benefits fitness. Such specialisations are more likely in heterogeneous environments where there is both greater ecological opportunity and competition-driven frequency dependent selection. 2. Such an effect could explain observed differences in rates of individual specialisation in habitat selection, as it offers individuals an opportunity to select for habitat types that maximise resource gain while minimising competition; however, this mechanism has not been tested before. 3. Here, we use habitat selection functions to quantify individual specialisations while foraging by black-legged kittiwakes Rissa tridactyla, a marine top predator, at 15 colonies around the United Kingdom and Ireland, along a gradient of environmental heterogeneity. 4. We find support for the hypothesis that individual specialisations in habitat selection while foraging are more prevalent in heterogeneous environments. This trend was significant across multiple dynamic habitat variables that change over short time-scales and did not arise through site fidelity, which highlights the importance of environmental processes in facilitating behavioural adaptation by predators. 5. Individual differences may drive evolutionary processes, and therefore these results suggest that there is broad scope for the degree of environmental heterogeneity to determine current and future population, species and community dynamics.

Environmental heterogeneity shapes the uneven distribution of resources available to foragers, and is ubiquitous in nature. Optimal foraging theory predicts that an animal's ability to exploit resource patches is key to foraging success. However, the potential fitness costs and benefits of foraging in a heterogeneous environment are difficult to measure empirically. Heterogeneity may provide higher-quality foraging opportunities, or alternatively could increase the cost of resource acquisition because of reduced patch density or increased competition. Here, we study the influence of physical environmental heterogeneity on behaviour and reproductive success of black-legged kittiwakes, Rissa tridactyla . From GPS tracking data at 15 colonies throughout their British and Irish range, we found that environments that were physically more heterogeneous were associated with longer trip duration, more time spent foraging while away from the colony, increased overlap of foraging areas between individuals and lower breeding success. These results suggest that there is greater competition between individuals for finite resources in more heterogeneous environments, which comes at a cost to reproduction. Resource hotspots are often considered beneficial, as individuals can learn to exploit them if sufficiently predictable. However, we demonstrate here that such fitness gains can be countered by greater competition in more heterogeneous environments.

Plastic pollution has become one of the largest environmental challenges we currently face. The United\ud Nations Environment Program (UNEP) has listed it as a critical problem, comparable to climate change,\ud demonstrating both the scale and degree of the environmental problem. Mortalities due to entanglement\ud in plastic fishing nets and bags have been reported for marine mammals, turtles and seabirds, and to date\ud over 690 marine species have been reported to ingest plastics. The body of literature documenting plastic\ud ingestion by marine megafauna (i.e. seabirds, turtles, fish and marine mammals) has grown rapidly over the\ud last decade, and it is expected to continue grow as researchers explore the ecological impacts of marine\ud pollution. Unfortunately, a cohesive approach by the scientific community to quantify plastic ingestion by\ud wildlife is lacking, which is now hindering spatial and temporal comparisons between and among species/\ud organisms. Here, we discuss and propose standardized techniques, approaches and metrics for reporting\ud debris ingestion that are applicable to most large marine vertebrates. As a case study, we examine how the\ud use of standardized methods to report ingested debris in Northern Fulmars (Fulmarus glacialis) has enabled\ud long term and spatial trends in plastic pollution to be studied. Lastly, we outline standardized metric\ud recommendations for reporting ingested plastics in marine megafauna, with the aim to harmonize the data\ud that are available to facilitate large-scale comparisons and meta-analyses of plastic accumulation in\ud a variety of taxa. If standardized methods are adopted, future plastic ingestion research will be better able\ud to inform questions related to the impacts of plastics across taxonomic, ecosystem and spatial scales.