Diet and grit characteristics in young Eurasian Bullfinches (Pyrrhula pyrrhula) inhabiting Iberian hedgerows

For the first time, the diet of young Iberian Bullfinches (Pyrrhula pyrrhula iberiae) is studied, specifically in a hedgerow habitat in northwestern Spain, through stomach (younger nestlings up to 8 d of age, which died without researcher intervention) and faecal sac (older nestlings) analysis, and secondarily direct observation (nestlings and dependent juveniles). Also, for the first time, grit use by bullfinch nestlings is described in some detail. Bullfinches fed their young with a mixture of seeds and invertebrates, with greater quantitative importance of the former. The identity of the seeds varied considerably between spring and summer, and animal fraction gradually decreased from May to July for older nestlings, in both cases presumably because of the seasonal changes in food availability. Caterpillars and spiders were the most important arthropod prey in the diet. Apparently, the young were not fed non-arthropod invertebrates. The relative importance of invertebrates, which are very rich in proteins, was greater for younger nestlings than for older ones. Difficult to digest hard-bodied prey, such as beetles, were not present in the stomachs of the youngest nestlings. The frequency of occurrence and amount of grit in stomachs increased with nestling age, along with the need to grind food. There were no remarkable differences in number of units, size, or number of colour types of gastroliths between months. The high floristic diversity in the study area, which has great overall conservation value, provides a wide range of resources for bullfinches, including plenty of food for their young.