Using wingbeat frequency to estimate mass gain

Abstract Energy intake is a fundamental currency in ecology that is critical to reproductive success, survival and lifetime fitness. Measuring foraging success in wild animals via biologgers has been a long‐standing challenge but is essential to understanding the mechanisms underlying population dynamics and species distributions. Flying animals gain mass during foraging, and they must counteract the associated increased gravitational force by creating additional lift. Pennycuick proposed that wingbeat frequency ( w ) should vary with the square root of body mass ( m ), w ∝ , when other variables influencing wingbeat frequency are held constant. We present a state–space model that estimates continuous changes in body mass by modelling this relationship with wingbeat frequency. Using simulations, we demonstrated the performance of the model in predicting body mass and estimating the parameters associated with the covariates affecting mass gain. We also used simulations to assess the sensitivity of our method to parameter misspecification and the increase in accuracy gained from including the known mass at recapture. To show the usefulness of this method, we applied it to 55 biologging tracks from thick‐billed murres ( Uria lomvia ) collected during the incubation period. The state–space model identified dive characteristics (maximum depth and complexity) that positively influenced mass gain while foraging. We used the continuous mass predictions to explore factors influencing foraging trip success and identify areas around the colony that are associated with higher mass gains. As estimates of energy intake allow for testing of long‐standing hypotheses in foraging ecology, our method provides a new tool to help answer ecological questions with any animal that engages in flapping flight.