Growing degree-days: an innovative method for predicting barn swallow nesting phenology and assessing linkages with invertebrate prey

Background/Question/Methods

Barn swallows, cattle, and various fly species have lived in close association for >2,000 years. Flies breed in manure and are harmful pests of cattle. Barn Swallows consume flies (~82% of nesting diet) and likely disturb fly activity. We are investigating whether swallows might be enhanced to offer an additional tool to reduce fly impacts. Climate change, however, may differentially affect flies that respond largely to temperature and swallows that migrate and respond to photoperiod. We are using Barn Swallow nesting records, from citizen science databases, and growing degree-days (GDD) to predict swallow nesting in relation to fly emergence. We expect nesting occurs when sufficient flies are present as a food resource. If GDD indicate high fly populations prior to swallow nesting, then asynchrony between swallows and flies would be suspected.

Results/Conclusions

Proportional hazards analysis indicates that GDD are a stronger indicator of when Barn Swallows nest than Julian days. This initial result allows more precise tracking of swallow nesting and comparisons with fly emergence over time.