Spatio-temporal variation in diet among age and sex cohorts of a model generalist bird species, the Great Tit Parus major: new insights revealed by DNA metabarcoding

Dietary variation among cohorts can have a major impact on how populations
adapt to environmental variation. Although variation in diet between
cohorts and across habitats has been studied in many taxa, this is not
true for most birds, especially smaller generalist passerines whose
feeding habits are predominantly cryptic. 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. Most dietary species were rare but nevertheless
collectively made up 30% of the diet, as expected of a generalist. Winter
moth Operophtera brumata, a major focus in tit breeding phenology
research, was the most prevalent dietary item, but the next ten most
prevalent Lepidoterans were collectively four times more important. There
was considerable variation in dietary richness and composition among
seasons and years. In winter, natural plant and invertebrate species were
extensively represented in the diet, despite the constant availability of
supplemental food. Diet composition varied with woodland type: in conifer
woodlands, birds fed on species adapted to conifer plantations, as
expected, but they also fed on many species adapted to deciduous species.
In winter, birds in conifers used peanut feeders more than they did in
mixed woodlands, where beech was more prevalent in the diet. In winter,
first-year birds consumed more invertebrate species than adults,
presumably because they were less selective, and beech (Fagus) was almost
twice as prevalent in the first-year diet. Our results suggest
considerable spatio-temporal variation in diet and variation among
cohorts, and provide insight into the diet of a key model species in avian
ecology. Such variation is rarely considered, even though it is likely to
have important consequences for our understanding of how populations
respond to environmental change.