Observer aging and long‐term avian survey data quality

Abstract

Long-term wildlife monitoring involves collecting time series data, often using the same observers over multiple years. Aging-related changes to these observers may be an important, under-recognized source of error that can bias management decisions. In this study, we used data from two large, independent bird surveys, the Atlas of the Breeding Birds of Ontario (“OBBA”) and the North American Breeding Bird Survey (“BBS”), to test for age-related observer effects in long-term time series of avian presence and abundance. We then considered the effect of such aging phenomena on current population trend estimates. We found significantly fewer detections among older versus younger observers for 13 of 43 OBBA species, and declines in detection as an observer ages for 4 of 6 vocalization groups comprising 59 of 64 BBS species. Consistent with hearing loss influencing this pattern, we also found evidence for increasingly severe detection declines with increasing call frequency among nine high-pitched bird species (OBBA); however, there were also detection declines at other frequencies, suggesting important additional effects of aging, independent of hearing loss. We lastly found subtle, significant relationships between some species' published population trend estimates and (1) their corresponding vocalization frequency (n ≥ 22 species) and (2) their estimated declines in detectability among older observers (n = 9 high-frequency, monotone species), suggesting that observer aging can negatively bias long-term monitoring data for some species in part through hearing loss effects. We recommend that survey designers and modelers account for observer age where possible.

Temperature‐related geographical shifts among passerines: contrasting processes along poleward and equatorward range margins

“…Similarly, observer effects associated with age‐related hearing loss also lower count estimates for certain species and may additionally contribute to nondetection (Farmer et al. ). First‐year observer and observer effects are not expected to exert a directional bias in range margin location through time, although they could potentially have a minor effect on abundance‐weighted estimates for the few species in our study that are affected.…”

Monitoring boreal avian populations: how can we estimate trends and trajectories from noisy data?

“…Observation error arises from multiple sources and, similar to population processes, may have both systematic and stochastic components. Systematic biases can arise from factors like variation in detection among and within species amid different habitats (Schieck 1997, Pacifici et al 2008, directional changes in observer abilities over time such as declining hearing (Farmer et al 2014), seasonal variation in detectability or spatial variation in observer skill (Johnston et al 2018), improved confidence and training (Kendall et al 1996, Sauer et al 1994, Tingley and Beissinger 2013, changes in microphone sensitivity for ARUs (Turgeon et al 2017), or sampling protocols failing to track the advancement of the breeding season because of climate change (McClure et al 2011). The systematic and stochastic components can be modeled to reduce their impacts on population inferences when surveys are appropriately designed (Matsuoka et al 2014) and relevant covariates are recorded (Sauer et al 1994, Pacifici et al 2008, Farmer et al 2014, Johnston et al 2018).…”

Monitoring black grouse Tetrao tetrix in Isère, northern French Alps: cofactors, population trends and potential biases

“…double counting) or sub-estimations of abundance (e.g. variable experience and age of observers, Hancock et al, 1999;Farmer et al, 2014; variable male lek count attendance in black grouse, Baines, 1996;acoustic limitations, Simons et al 2007; low or variable probability of detection in tetraonids, Zimmerman et al, 2007, Fremgen et al, 2016. Several observers are often required for fieldwork and current protocols already tend to minimize or avoid double counts (see Method section).…”