Factors affecting burrow occupancy and bank persistence for Bank Swallows breeding in aggregate (sand and gravel) pits and natural habitats

Abstract

As the availability of natural nesting habitat for Bank Swallows (Riparia riparia) continues to decline, operational aggregate (sand and gravel) pits have begun to provide alternative nesting habitat. With best management practices being developed and revised for the aggregate industry, an understanding of site factors affecting burrow and bank occupancy by Bank Swallows is needed. We compared patterns of burrow occupancy of breeding Bank Swallows in lakeshore and aggregate habitats and documented other potential factors that could influence occupancy levels using data collected over a two-year period. We also used a larger sample of data from other studies collected over longer time periods to compare the persistence of colonies in lakeshore, riverbank, and aggregate habitats. In both habitats, rates of burrow occupancy were impacted by colony size and bank-face height. At lakeshore sites, burrow occupancy decreased in low burrows with increasing colony size whereas, in medium/high burrows, there was no relationship between colony size and burrow occupancy. In aggregate pits, burrow occupancy in low burrows increased rapidly with colony size, whereas in medium/high burrows, occupancy increased only slightly as colony size increased. Colonies of Bank Swallows in lakeshore bluffs were the most persistent, whereas those in aggregate pits had the lowest year-to-year persistence, likely due to annual removal of targeted resources. Colonies in aggregate pits were more likely to persist from one year to the next if the previous year’s colony was large, and faces > 200 m². Although colonies at natural sites were the most persistent over time, the aggregate pit industry can play an active role in the conservation of this threatened species by encouraging pit operators to maintain large, tall, vertical faces on stable, unused banks to increase the probability of Bank Swallows recolonizing these sites in future years.