Accuracy of Depth Recorders

Abstract

Depth recorders are among the most useful tools available for ornithologists interested in waterbird foraging behavior. Despite their widespread use in the literature, there is little information available about their precision and accuracy, including, for the case of TDRs, device drift. We examined the uncertainty associated with two types of depth-recorders deployed on Thick-billed Murres Uria lomvia in the Canadian Arctic in 2007 for up to 48 hours. The maximum depth obtained by capillary tube maximum-depth gauges (MDGs), a cheap and simple depth-recorder, was highly correlated (R2 = 0.87) with maximum depth obtained by electronic time-depth recorders (TDRs) attached to the same bird (n = 29) up to depths of 100 m. Deeper than 100 m or in deployments of 144 hours, MDGs were unreliable. We suggest that the maximum depth for Thick-billed Murres in the Canadian Arctic is about 150 m, rather than the 210 meters previously reported using MDGs recorders, and that caution should be used when quoting maximal maximum depths for species diving deeper than 100 m using this method. We also attached two Lotek TDRs to the same bird (n = 18) and examined the similarity of the two recorders. The average difference increased from about 0.5 m near the surface to about 1.0 m below 60 m, with extreme differences of up to 4 m obtained. Furthermore, TDRs submerged to known depth were accurate within ± 2 m. The effect of these variations on measurements of maximum and average depth and duration was about 0.6–1.3 m (depth) or s (duration), which is similar to the manufacturer's accuracy specifications (±1%). Finally, we examined the drift (offset from zero at the surface) within the TDRs. Drift varied from -2.5 to 2 m, with 9 out of 36 recorders showing no drift, and no change amongst years for individual recorders. Drift was lowest (most negative) at the colony, higher during flight and highest (most positive) on the water surface, despite very small differences in altitude (<50 m). We suggest that drift may be a useful tool for quantifying at-sea behavior, especially in conjunction with temperature logs. We conclude that MDGs are reliable up to 100 m and within 48 hours, and that TDRs are precise within ±2%, but that more research needs to be completed on device accuracy and precision.