Marine top predators are expected to adjust their foraging behaviour at multiple time scales concomitantly with changes in forage fish availability. Rhinoceros auklets Cerorhinca monocerata rearing chicks at Teuri Island, Japan Sea, fed on anchovy Engraulis japonicus in 2012 and 2013 (anchovy regime) but switched to sand lance Ammodytes spp in 2019 and 2020 (sand lance regime). Here, we studied their at-sea behaviour using the GPS locations of 33 birds and the depth-acceleration records of 26 birds, and compared their foraging behaviour between these prey regimes. At the trip scale, auklets used offshore waters (>50 m sea depth) and coastal waters in the anchovy regime but used mainland coastal waters (<50 m sea depth) in the sand lance regime. In the sand lance regime, the birds also conducted more overnight 2- to 4-day trips in 2020 and spent more time flying during 1-day trips as they fed in further areas compared to the anchovy regime. At the dive scale, auklets frequently dove to both <5 m and 20 – 30 m depths in the anchovy regime but mainly to <5 m depth in the sand lance regime. Within each dive, auklets showed a greater number of fast/strong wing stroke events in the anchovy regime than in the sand lance regime. These changes in auklet behaviour reflected the different habitats, depth distribution, and swim speed of the targeted prey species. Our study shows the behavioural flexibility of a wing-propelled flying-diving seabird in response to the inter-annual shifts in the dominant forage fish community. It also indicates the ecological constraints on the mechanisms determining nest productivity in this day-foraging/night-provisioning seabird.

We deployed the depth-acceleration data-logger (ORI400-D3GT, 9.0 g, 12 mm diameter×51 mm, Little Leonardo, Japan) on the back of  Rhinoceros Auklets rearing chicks at Teuri Island. The sampling interval was 1 second for depth and  0.02 s (50 Hz) or 0.05 s (20 Hz) for 3-axis acceleration; sway (X, left-right), surge (Y, tail-head), and heave (Z, dorso-ventral). We retrieved depth-acceleration loggers from 8 of 8 birds in 2012, 8 of 8 birds in 2013, 10 of 11 birds in 2019, and 8 of 8 birds in 2020. Because of malfunction or battery shortage in some devices, we finally collected depth-acceleration data from 26 birds. Here are the Igor files. You can see data and calculation processes using Igor software.

We defined dives as depth greater than 1m considering the accuracy of the depth sensor (0.5m), “dive duration” as the time between the previous time the bird first attained 1 m and the following time it last attained 1m, “dive depth” as the maximum depth during each dive, and “surface time” as the time between the end of dives to the start of the next dives. We defined “descent” phase as the time from the start of the dives to the time when birds attained 80% of dive depth, “ascent” phase as the time between the last time when birds attained 80% of maximum dive depth and the end of dive, and “bottom” phase as time from the end of descent phase to the start of ascent phase as in Kuroki et al. (2003). Rhinoceros Auklets often make “rotations” where they switch from ascent to descent and from descent to ascent with a depth change rate of greater than 0.5 m s^-1 (Kuroki et al. 2003). We typed dives with dive depth < 5 m as “shallow-type”, those > 5 m, with no rotation and proportional bottom time smaller than 50% of dive duration as “V-type”, and those deeper than 5 m, with no rotation and the proportion of bottom time greater than 50% as “U-type” referring to Lescroël and Bost (2005), Pütz and Cherel (2005), and Halsey et al. (2007). Dives > 5 m and with these rotations were typed as “W-type”. 

During the descent phase, breath hold divers are expected to regulate body angle and depth change rate to maximize the proportion of foraging time at foraging depths by minimizing the transit time but also may search for prey (Wilson et al. 1996). To characterize the behaviour during descent, we measured body angle, descent rate (depth change per second), and wing stroke frequency (number of wing strokes per second, see next section) using depth and acceleration data. The body acceleration reflects both slow changes in the body angle and the dynamic/fast movement of the body (Yoda et al. 2001). Low frequency component of the acceleration was separated by low pass filer using Ethographer (Sakamoto et al. 2009) on IGOR Pro ver. 8.03 (WaveMetrics, Lake Oswego, USA). As the attachment position of acceleration data-logger varied among individuals, we set the critical frequency of low pass filter for each bird. The mean values of the critical frequency were 2.2 ± 0.6 Hz for tail-head (surge, Y) and 1.8 ± 0.8 Hz for dorso-ventral (heave, Z) accelerations. The fast component was derived by subtracting the low frequency component. 

Body angle (θ) was calculated from the low frequency component of surge (YL, Fig. 2 a, b) as arcsinYL and incorporating attachment angle of each bird that was the body angle when the birds were sitting on the water (Watanuki et al. 2003). We compared descent body angle, descent rate, and swim speed at the 1 s-window closest to 5 m and 10 m depths between prey regimes. The body angle of >90° was assumed to be 90°. 1-s windows with the body angle of < 0° (7.7% and 7.2% of all windows for 5 m and 10 m depths respectively) were excluded from the analyses as these could be measurement errors. As the angle of trajectory was almost the same as the body angle when Rhinoceros Auklets are descending (Watanuki et al. 2006), swim speed can be estimated as r/sin θ where r was the descent rate (m s-1). Speed values faster than 2.5 m s^-1 were unreliable (Watanuki and Sato 2008) and excluded from the analyses (3.5% and 3.6% of windows with speed data for 5 m and 10 m depths, respectively). Dynamic component of the heave (dorso-ventral, Z) acceleration reflects up-down body movement in association with wing stroke. The number of local minimum of the dynamic component of heave acceleration (Fig. 2 c inserted) in each 1-s window was defined as “wing stroke frequency”. The maximum amplitude of dynamic component of heave in each 1-s window was measured using IGOR (Fig. 2c). Rhinoceros Auklets stroked wings by 2-3 Hz while they were cruising in the descent phase (Watanuki and Sato 2008, see Fig. 2c).  Wing-propelled seabirds often stroke wings fast during dives and these are assumed to relate to the prey chase (Ropert-Coudert et al. 2006). We categorize 1-s windows with stroke frequencies greater than 4 Hz (mean + 2 * SD of the frequency in 1-s window excluding those with no stroke, Supplementary Material 3a) and the maximum stroke amplitude greater than 1.04 g (mean + 2 * SD of the maximum amplitude in 1-s window excluding those with 0 amplitude) as “fast/strong stroke windows”, then we defined a series of the fast/strong stroke windows as “fast/strong stroke event” (Fig. 2d, e). The fast/strong wing stroke events at the start of descent could be acceleration phase to approach against buoyancy to the prey aggregation.  In effect, we observed fast/strong stroke events in the descent phase mostly at the start of dives (<2 m depth) (Supplementary Material 4a). In Rhinoceros Auklets, the quick change of the depth (one of the indicators of prey chase) was observed mainly in the bottom and ascent phases (Kuroki et al. 2003) and the attack scars on the prey fish brought back to the chicks are mostly observed underside of the fish (Burger et al. 1993); indicating that they possibly chase individual prey in bottom/ascent phases. The occurrence of fast/strong wing strokes during the descent (Supplementary Material 4a) and the bottom/ascent phases (Supplementary Material 4bc), therefore, were analyzed separately. Maximum amplitude in 1-s window can be biased to be lower for acceleration sampled at longer intervals. To check this potential bias, we put sampling frequency (high, 0.02 s interval or low, 0.05 s interval) as potential explanatory factor when we analyzed the effects of regime on the duration and frequency of fast/strong stroke events.

 IGOR Pro ver. 8.03 (WaveMetrics, Lake Oswego, USA) for the .pxp files. Txt files are provided for accessibility.