Case report: a movement-based method using GPS data to estimate timing of parturition in bighorn sheep

Abstract Background Traditional methods for determining birth timing and subsequent habitat information of ungulates can be costly, time-consuming, or can potentially negatively affect parturient females. Identifying ungulate birth sites and timing of births is possible using movement data from global positioning systems (GPS) collars. We investigated whether we could estimate parturition in Rocky Mountain bighorn sheep ( Ovis canadensis canadensis ) using the analysis of parturition indicators (API) (i.e., velocity, turning angle, and seasonal range size) approach. These types of movement-based methods can be used to accurately detect parturition. Methods We determined birthdates of young Rocky Mountain bighorn sheep through consistent, visual monitoring of collared females on Antelope Island, Utah, USA, from 2020 to 2022. We then used step length, velocity, turning angle, and seasonal range size collected from GPS collars to estimate 19 birthing events from 13 females for which we observed parturition within 72 h. Results Parturition of 13 female bighorn sheep was identified by a step length of < 95 m, a velocity of < 0.05 km/h, a turning angle of > 1.8 radians, and a seasonal range of < 8 ha. We then applied those metrics to verify 34 birthdates of all female bighorn sheep. Step length and the API method predicted a combined cumulation of 19% of births on the observed birthdate, 24% within one day, 58% within two days, 72% within three days, and 100% within four days. Conclusions Our results indicate that the API method in combination with step length can estimate birthdates in bighorn sheep, which will help when delineating and conserving birthing habitat for these ungulates.