Bird diet reveals matrix foraging and drives quantitative seed dispersal effectiveness in recovering Neotropical forests

Fruit-eating birds play a vital role in seed dispersal across fragmented and regenerating landscapes, shaping forest recovery. Simultaneously, human disturbance influences early successional forest communities by favoring habitat-generalist birds and facilitating the spread of exotic plants. To evaluate quantitative seed dispersal effectiveness (SDE) and identify bird-consumed plants in these novel communities, we collected bird droppings at ten early successional sites in semi-deciduous Atlantic Forest in São Paulo, Brazil. Monthly mist netting over one year (2763 net hours) yielded fecal samples from captured birds. Droppings were inspected for seeds to assess the quantitative component of SDE among bird species. To broaden our diet analysis beyond seeds, we also conducted DNA metabarcoding of 74 droppings from the most effective seed dispersers using a universal plant mini-barcode. Only 25% of droppings from frugivores contained seeds. We identified Chiroxiphia galeata (Pipridae) as the most quantitatively effective seed disperser, followed by Turdus leucomelas (Turdidae) and Ramphocelus carbo (Thraupidae). Quantitative SDE was associated with species' degree of frugivory but not body mass. Using both seeds and DNA metabarcoding, 84 plant taxa were detected across 42 families, including exotic, weedy, and potentially invasive species. Native Casearia (Salicaceae) and Miconia (Melastomataceae) were the most frequently consumed plants. Despite limited habitat quality and low fruit availability, a forest-dependent species (C. galeata) proved to be the most effective disperser. In contrast, the diets of generalist species reflected resource use in the matrix (e.g., pastures, residential areas), suggesting these birds may facilitate both forest connectivity and the spread of exotic plants. Our study highlights the complementary roles of generalist and forest-dependent bird species, as well as the potential consequences of matrix foraging on the future community composition in recovering neotropical forests.