Knowledge of diet and dietary selectivity is vital, especially for the conservation of declining species. Accurately obtaining this information, however, is difficult, especially if the study species feeds on a wide range of food items within heterogeneous and inaccessible environments, such as the tree canopy. Hawfinches ( Coccothraustes coccothraustes ), like many woodland birds, are declining for reasons that are unclear. We investigated the possible role that dietary selection may have in these declines in the UK. Here, we used a combination of high-throughput sequencing of 261 hawfinch faecal samples assessed against tree occurrence data from quadrats sampled in three hawfinch population strongholds in the UK to test for evidence of selective foraging. This revealed that hawfinches show selective feeding and consume certain tree genera disproportionally to availability. Positive selection was shown for beech (Fagus), cherry (Prunus), hornbeam (Carpinus), maples (Acer) and oak (Quercus), while Hawfinch avoided ash (Fraxinus), birch (Betula), chestnut (Castanea), fir (Abies), hazel (Corylus), rowan (Sorbus) and lime (Tilia). This approach provided detailed information on hawfinch dietary choice and may be used to predict the effects of changing food resources on other declining passerine populations in the future.

From March-July 2017–2019, hawfinches were caught under licence using mist nets at artificial feeding stations within north Wales, the Wye Valley and the New Forest.

For prevalence of different tree species, we used data collected during other studies within the same woods from long-term ongoing hawfinch projects. Data from all three study areas were collected between 2013 to 2016 based on nest sites and random locations within the same woods. Further data were collected in 2017 from the north Wales study area. These data were based on foraging locations from GPS tracking of individual hawfinches and random locations in any woodland within the north Wales study area.

At all locations, a quadrat of 10m * 10m was marked out, with the given location at the SW corner of the quadrat. Within each quadrat, trees were identified, and circumference at breast height (CBH) of all trees and shrubs was recorded. As many closely related tree species are difficult to identify to species, with some genera frequently hybridizing, morphometrically similar species were recorded together (e.g. “oak” includes Quercus robur, Quercus petraea, and hybrids). Any trees which had a CBH less than 20cm diameter were not recorded to discount saplings, which are known not to be utilised as a food resource by hawfinch.

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