Opening a can of worms: Can the availability of soil invertebrates be indicated by birds?

•Large-scale monitoring of invertebrates, especially soil invertebrates, is lacking.•UK school children collected data on soil invertebrates and bird counts.•Earthworm abundance correlated with counts of earthworm-reliant birds.•Thrush counts (genus Turdus) could be used indicate of earthworm abundance. Recently detected invertebrate population declines are likely to have far-reaching impacts for ecosystem function. However, very little large-scale monitoring of invertebrates, especially soil invertebrates, has taken place. To address this gap, we established a school-based citizen science project to collect data on soil invertebrate abundance and bird counts across the UK. We examined the association between earthworms (which comprised 93% of the total soil invertebrate biomass in the surface soil) and 15 bird species on school playing fields over three years. We also tested whether simple bird counts of species that rely on earthworms could be used to indicate the earthworm abundance in soil. We found a clear link between earthworm abundance and counts of robins (Erithacus rubecula), blackbirds (Turdus merula) and other thrushes (Turdus pilaris, Turdus viscivorus, Turdus philomelos and Turdus iliacus), all of which rely on earthworms as an important component of their diet. None of these relationships varied with season. There were no correlations found between earthworm abundance and bird counts for species that do not rely on earthworms. We found that the total thrush count (including blackbirds) could be used as a simple indicator of earthworm abundance, explaining up to 20% of the variation in earthworm counts. In spite of the uncertainties associated with the data, these results raise the possibility that existing, robust, long-term bird monitoring data could, with further research, be used to infer changes in soil invertebrates, such as earthworms, and therefore aspects of soil health. We conclude by identifying a series of criteria that would need to be fulfilled in order for such inference to be robust.