Diets play a key role in understanding trophic interactions. Knowing the actual structure of food webs contributes greatly to our understanding of biodiversity and ecosystem functioning. The research of prey preferences of different predators requires knowledge not only of the prey consumed, but also of what is available. In this study, we applied DNA metabarcoding to analyze the diet of 4 bird species (willow tits Poecile montanus, Siberian tits Poecile cinctus, great tits Parus major and blue tits Cyanistes caeruleus) by using the feces of nestlings. The availability of their assumed prey (Lepidoptera) was determined from feces of larvae (frass) collected from the main foraging habitat, birch (Betula spp.) canopy. We identified 53 prey species from the nestling feces, of which 11 (21%) were also detected from the frass samples (eight lepidopterans). Approximately 80% of identified prey species in the nestling feces represented lepidopterans, which is in line with the earlier studies on the parids' diet. A subsequent laboratory experiment showed a threshold for fecal sample size and the barcoding success, suggesting that the smallest frass samples do not contain enough larval DNA to be detected by high‐throughput sequencing. To summarize, we apply metabarcoding for the first time in a combined approach to identify available prey (through frass) and consumed prey (via nestling feces), expanding the scope and precision for future dietary studies on insectivorous birds.

Migratory birds need considerable energy reserves to fuel long-distance flights to their breeding grounds in spring. To attain sufficient energy deposits before departure, birds require high daily intake rates, which can be reached by utilizing high-quality food. During such periods of high energy demand, animals often track changes in the nutritious value of their food, for example by switching to a more profitable habitat or diet. Pre-migratory Barnacle Geese Branta leucopsis staging along the Wadden Sea coast are known to switch from pastures to salt marshes during spring. Previous studies have suggested that Barnacle Geese switch habitat to track changes in protein levels, which drop in pastures as spring progresses, and to avoid high levels of disturbance. Here we made use of detailed information on pre-migratory habitat use of individual Barnacle Geese tracked by GPS/accelerometer tags to assess which factors may drive a habitat switch. We analysed habitat use and time budgets of individual birds and combined this with data on food quality in two habitats to analyse differences in food intake. We found large individual variation in pre-migratory habitat use, both in the extent of salt marsh use, as well as in the timing of a switch from pastures to salt marshes. In salt marshes, geese spent more time grazing and made fewer flight movements, potentially as they experienced lower levels of disturbance compared to geese in pastures. By increasing grazing time and reducing flight movements, geese in salt marshes may compensate for reduced food quality. Our results show that Barnacle Geese trade-off high intake rates and high costs in pastures with low intake rates and low costs in salt marshes.

Climate warming advances the optimal timing of breeding for many animals. For migrants to start breeding earlier, a concurrent advancement of migration is required, including premigratory fueling of energy reserves. We investigate whether barnacle geese are time constrained during premigratory fueling and whether there is potential to advance or shorten the fueling period to allow an earlier migratory departure. We equipped barnacle geese with GPS trackers and accelerometers to remotely record birds’ behavior, from which we calculated time budgets. We examined how time spent foraging was affected by the available time (during daylight and moonlit nights) and thermoregulation costs. We used an energetic model to assess onset and rates of fueling and whether geese can further advance fueling by extending foraging time. We show that, during winter, when facing higher thermoregulation costs, geese consistently foraged at night, especially during moonlit nights, in order to balance their energy budgets. In spring, birds made use of the increasing day length and gained body stores by foraging longer during the day, but birds stopped foraging extensively during the night. Our model indicates that, by continuing nighttime foraging throughout spring, geese may have some leeway to advance and increase fueling rate, potentially reaching departure body mass 4 days earlier. In light of rapid climatic changes on the breeding grounds, whether this advancement can be realized and whether it will be sufficient to prevent phenological mismatches remains to be determined.

The development of DNA-based methods in recent decades has opened the door to numerous new lines of research in the biological sciences. While their speed and accuracy are clearly beneficial, the sensitivity of these methods has the adverse effect of increased susceptibility to false positives resulting from contamination in field or lab. Here, we present findings from a metabarcoding study on the diet of and food availability for several insectivorous birds, in which multiple lepidopteran species not known to occur locally were discovered. After describing the pattern of occurrences of these non-local species in the samples, we discuss various potential origins of these sequences. First, we assess that the taxonomic assignments appear reliable, and local occurrences of many of the species can be plausibly ruled out. Then, we look into the possibilities of natural environmental contamination, judging it to be unlikely, albeit impossible to fully falsify. Finally, while the pattern of occurrences did not suggest lab contamination, we find overlap with material handled in the same lab, which was undoubtedly not coincidental. Even so, not all exact sequences were accounted for in these locally conducted studies, nor was it clear if these and other sequences could remain detectable years later. Although the full explanation for the observations of non-local species remains inconclusive, these findings highlight the importance of critical examination of metabarcoding results, and showcase how species-level taxonomic assignments utilizing comprehensive reference libraries may be a tool in detecting potential contamination events, and false positives in general.