Faecal metabarcoding reveals pervasive long-distance impacts of garden bird feeding

Supplementary feeding of wildlife is widespread, being undertaken by more
than half of households in many countries. However, the impact that these
supplemental resources have is unclear, with impacts assumed to be
restricted to urban ecosystems. We reveal the pervasiveness of
supplementary foodstuffs in the diet of a wild bird using metabarcoding of
blue tit (Cyanistes caeruleus) faeces collected in early spring from a
220km transect in Scotland with a large urbanisation gradient.
Supplementary foodstuffs were present in the majority of samples, with
peanut (Arachis hypogaea) the single commonest (either natural or
supplementary) dietary item. Consumption rates exhibited a distance decay
from human habitation but remained high at several hundred metres from the
nearest household and continued to our study limit of 1.4km distant.
Supplementary food consumption was associated with a near quadrupling of
blue tit breeding density and a five-day advancement of breeding
phenology. We show that woodland bird species using supplementary food
have increasing UK population trends, while species that don’t, and/or are
outcompeted by blue tits, are likely to be declining. We suggest that the
impacts of supplementary feeding are larger and more spatially extensive
than currently appreciated and could be disrupting population and
ecosystem dynamics.