Dietary variation among individuals and cohorts can have a major impact on how populations adapt to environmental variation. Although variation in diet between cohorts and across habitats has been well studied in many taxa that feed on few or easily observed food items, the same is not true for most birds, especially smaller generalist passerines whose feeding habits are predominantly cryptic. For these and similar species, DNA metabarcoding can be used to good effect but remains underutilised. Here we used DNA metabarcoding with next-generation sequencing to assess spatio-temporal dietary variation among age and sex cohorts of the great tit Parus major, a model species in avian ecology. We identified animal prey throughout the year, and plants in winter, across different habitat fragments, achieving sample completeness of >90% for most analyses. There was considerable variation in dietary richness and composition among seasons and years, and diet composition varied between coniferous and mixed-deciduous habitats. Invertebrate prey were more species rich in mixed-deciduous habitats than in coniferous, primarily in winter, but when plants were also included there were no differences between habitat types in species richness. “Rare” species dominated the diet accounting for 30% of prey items across all samples. In winter, first year birds consumed more invertebrate species than adults. Beech (Fagus) was also almost twice as prevalent in the diets of first years (96% vs 52% in adults). Most differences between ages and sexes seemed driven primarily by differences in rare species, which we speculate is caused by divergent foraging modes and microhabitat use. We discuss the likely mechanisms for the patterns observed, all of which suggest considerable spatio-temporal variation in diet among cohorts. Our results also highlight that understanding the role of food in driving phenological phenomena requires consideration of dietary variation over space, time and between consumers.

Animal-borne telemetry devices provide essential insights into the life-history strategies of far-ranging species and allow us to understand how they interact with their environment. Many species in the seabird family Alcidae undergo a synchronous moult of all primary flight feathers during the non-breeding season, making them flightless and more susceptible to environmental stressors, including severe storms and prey shortages. However, the timing and location of moult remains largely unknown, with most information coming from studies on birds killed by storms or shot at sea. Using light-level geolocators with saltwater immersion loggers, we develop a method for determining flightless periods in the context of the annual cycle. Four Atlantic puffins (Fratercula arctica) were equipped with geolocator/immersion loggers on each leg to attempt to overcome issues of leg-tucking in plumage while sitting on the water, which confounds the interpretation of logger data. Light level and saltwater immersion time-series data were combined to correct for this issue. This approach was adapted and applied to 40 puffins equipped with the standard practice deployments of geolocators on one leg only. Flightless periods consistent with moult were identified in the dual-equipped birds, whereas moult identification in single-equipped birds was less definitive and should be treated with caution. Within the dual-equipped sample, we present evidence for two flightless moult periods per non-breeding season in two puffins that undertook more extensive migrations (> 2000km), and were flightless for up to 76 days in a single non-breeding season. A biannual flight feather moult is highly unusual among non-passerine birds, and may be unique to birds that undergo catastrophic moult, i.e. become flightless when moulting. Though our conclusions are based on a small sample, we have established a freely available methodological framework for future investigation of the moult patterns of this and other seabird species.