Studies have shown that an individual Barn Owl may eat one or more rodents per night; a nesting pair and their young can eat more than 1,000 rodents per year. Locally superabundant rodent species in the weight class of several grams per individual usually make up most of the prey. Such animals probably make up at least three-quarters of the biomass eaten by each and every "T. alba", except in some island populations. In Ireland, the accidental introduction of the Bank vole in the 1950s has led to a major shift in the Barn Owl's diet: where their ranges overlap the vole is now by far the largest prey item. The diet is supplemented with local small vertebrate and large invertebrate life.