Dietary differentiation of two co‐occurring common bat species ( <i>Eptesicus nilssonii</i> and <i>Pipistrellus pygmaeus</i> )

Sympatric bat species can co‐exist and avoid interspecific competition via niche differentiation e.g. diet. Detecting dietary differences can be achieved by comparing dietary niches of sympatric and allopatric populations. If dietary overlap is higher in sympatry versus allopatry, co‐occurrence may be altering the dietary niche of the species. Our study region offers an excellent opportunity to investigate this because two species, Eptesicus nilssonii and Pipistrellus pygmaeus, occur sympatrically across southern Sweden and E. nilssonii occurs allopatrically in the north. We analysed droppings from 19 roosts during two months: six P. pygmaeus , seven E. nilssonii located in the known distribution of P. pygmaeus (sympatric) and six roosts located outside the known range of P. pygmaeus (allopatric). We used DNA metabarcoding to assess dietary overlap and dietary niche breadth in both species. We found a high dietary overlap between E. nilssonii and P. pygmaeus at all taxonomic levels of prey. The dominant prey orders in the three bat populations were Ephemeroptera (mayflies) and one Diptera species (cranefly); five species from these orders accounted for &gt; 70% of all reads. When using quantitative relative read abundance data at the species level interspecific overlap was higher than intraspecific overlap. We find a consistent but not significant pattern that E. nilssonii populations co‐occurring with P. pygmaeus have the narrowest dietary niche breadth. This could indicate that the co‐occurrence with P. pygmaeus is having an impact on prey selection for sympatric E. nilssonii populations. However, confirming competition between these species would require sampling of the insect population near to the colonies. Both bat species' diets are also heavily dominated by only five species. These are all currently common insect species and most likely indicates opportunistic foraging on abundant insect populations.