On iOS the $2 DiveCrafter app is one of the more graphically elaborate apps, but though it's much nicer to look at than some of its peers it's also a little more basic, and doesn't offer fully comprehensive logging for your dive activity.
The diel distributions of dive activity north and south of 50°S were significantly different (p<0.001, χ212 = 245.1).
Missed detections of short duration dives should be more pronounced with the slow sampling tags, and since the distributions match, we do not believe that the diel distribution of dive activity is an artefact of sampling bias in this case.
Dive activity south of 50°S (n = 933 dives by two individual birds) also occurred throughout the day, but showed distinct peaks just after sunrise and just before sunset (white bars in Figure 4a).
The observed timing of sooty shearwater dive activity in Antarctic waters is also consistent with the proposition that the sooty shearwaters were foraging for myctophids, or other prey that migrate to near-surface waters during night-time hours.
Sooty shearwater dives recorded by fast sampling archival tags north of 50°S (n = 801 dives by seven individual birds) occurred from just before sunrise through to sunset, with broad peaks of dive activity centred on mid-morning and late afternoon (grey bars in Figure 4a).
If, for example, dive activity was in reality constant throughout the day, but the dive durations were shorter around midday and longer around dawn and dusk, then the distribution of detected dives would peak around dawn and dusk, as we found here.
However, we compared the diel distribution of dive activity from fast sampling tags (sampling rate 24s or 32s) to the distribution obtained from slow sampling tags (432s; n = 3301 dives total by 18 individual birds; results not shown), and found the distributions to be very similar.
The dives made by these penguins in the Polar Front zone were similar in pattern to those of the sooty shearwaters reported here, occurring from sunrise until mid-morning, and again from mid-afternoon until sunset, with little dive activity around midday (B. Wienecke, unpublished).
The observed distribution of dive activity in Polar Front waters (peaking just after sunrise and just before sunset, with no night-time diving) might represent a compromise between increased prey availability (proximity to surface) and decreased visual prey detectability during the hours of darkness.
