Electronic Supplementary Materials (ESM)
The jellyfish buffet: Jellyfish enhance seabird foraging opportunities by concentrating prey
Nobuhiko N. Sato, Nobuo Kokubun, Takashi Yamamoto, Yutaka Watanuki,
Alexander S. Kitaysky, Akinori Takahashi
Method detail
(a) Data loggers
We attached video and time-depth-temperature-acceleration loggers on birds’ backs. The combined weight of
loggers deployed were 2.0 ~ 2.4 % of birds’ body mass. This is lower than the recommended limit for a maximum
logger mass, above which birds’ behaviour might be affected [1]. Although our logger birds had a mean dive depth
(78.2 m) and duration (150 s) typical for this species [2], and previous studies of thick-billed murres on St. George
Islands reported no strong logger deployment effects on birds’ foraging patterns [3], we acknowledge that there is still
a possibility that the deployment of video loggers could have affected the foraging behaviour of birds in our study. A
video logger (Box shaped device, 20×50×10 mm, 14.5 g in air, including batteries; DVL200, Little Leonardo, Japan)
can record video footage for 2.5 hours, and has a delayed timer function to start recording. This logger does not have
any light sources. The time-depth-temperature-acceleration logger (Cylindrical shaped device, 12×45 mm, 9 g in air,
including batteries; ORI400-D3GT, Little Leonardo, Japan or box shaped device, 12×31×11 mm, 5.5 g in air,
including batteries; Axy-Depth, TechnoSmArt, Italy) recorded 3 axis accelerations at 20 or 25 Hz, and depth and
temperature at 1 or 0.2 Hz. Depth data recorded at 0.2 Hz (by Axy-Depth) were linearly-interpolated to 1 Hz.
The video footage consisted of 10 hours in total, and covered 97 dives obtained from four birds (dive depths
<1.0m were excluded from analysis). We also analysed depth, temperature and acceleration records, obtained with
video footage (for 36 dives from two birds). Data from the other two birds were not available for analysis due to
technical problems with loggers. We used Quick Time Player ver. 7.7.5 (Apple, USA) to analyse the video footage,
and Igor Pro ver. 6.0 (Wave Metrics, USA) to analyse depth, temperature and acceleration data. We made surface
adjustments of depth data [4]. We computed vertical speed every second during dives. We also computed dynamic and
static accelerations along the surging axis by using 1.9 Hz of low-pass filters [2]. Using the high frequency
components of the fluctuations of heaving acceleration, we computed wing beat frequency every second during dives.
(b) Energetic calculations on the importance of feeding events during the ascent phase of dives
The following energetic calculations were made to assess the importance of feeding events during the ascent
phase of dives for the overall energy budget of thick-billed murres. Based on video logger observations, we assumed
that birds fed on age-0 walleye pollock (Theragra chalcogramma, the most abundant juvenile fish in the study area)
during the ascent phase. A comparison of the fish size to the bird’s beak size in video footage suggests that all fish had
a total body length (TL) less than 50 mm. Mean energy density of the Bering Sea age-0 walleye pollock (TL < 50mm)
is ~3.5 kJ g-1 (wet mass) [5,6], and mean wet mass is ~1.5 g [7]. We calculated the mean energy density of a prey item
as follows:
𝐸𝑛𝑒𝑟𝑔𝑦 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 (𝑘𝐽 𝑓𝑖𝑠ℎ−1 ) = 𝐸𝑛𝑒𝑟𝑔𝑦 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 (𝑘𝐽 𝑔−1 ) × 𝑊𝑒𝑡 𝑚𝑎𝑠𝑠 (𝑔 𝑓𝑖𝑠ℎ−1 )
The mean number of dives per day was 70.1 in 2014, based on a larger dataset of time-depth records (N = 13 birds,
Sato et al. unpublished data). The mean number of prey captured during the ascent phase of a dive was 2 fish dive-1 (N
= 97 dives including non-feeding dives, from four birds, based on video logger observations). We calculated the mean
number of prey (ascent phase) per day as follows:
𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑝𝑟𝑒𝑦 𝑎𝑠𝑐𝑒𝑛𝑡 𝑝ℎ𝑎𝑠𝑒 (𝑛𝑜. 𝑑𝑎𝑦 −1 ) = 𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑝𝑟𝑒𝑦 𝑑𝑖𝑣𝑒 (𝑛𝑜. 𝑑𝑖𝑣𝑒 −1 ) × 𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑑𝑖𝑣𝑒𝑠 (𝑛𝑜. 𝑑𝑎𝑦 −1 )
Assuming that the assimilation efficiency is 0.75 [8], total energy intake during the ascent phase (kJ day-1) was
calculated as follows:
𝑇𝑜𝑡𝑎𝑙 𝑒𝑛𝑒𝑟𝑔𝑦 𝑖𝑛𝑡𝑎𝑘𝑒 𝑎𝑠𝑐𝑒𝑛𝑡 𝑝ℎ𝑎𝑠𝑒 (𝑘𝐽 𝑑𝑎𝑦 −1 )
= 𝐸𝑛𝑒𝑟𝑔𝑦𝑑𝑒𝑛𝑠𝑖𝑡𝑦 (𝑘𝐽 𝑓𝑖𝑠ℎ−1 ) × 𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑝𝑟𝑒𝑦 𝑎𝑠𝑐𝑒𝑛𝑡 𝑝ℎ𝑎𝑠𝑒 (𝑛𝑜. 𝑑𝑎𝑦 −1 )
× 𝐴𝑠𝑠𝑖𝑚𝑖𝑙𝑎𝑡𝑖𝑜𝑛 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦
A previous study on St. George Island measured daily energy expenditures of breeding thick-billed murres as 1,678.5
± 461.6 (mean ± s.d.) kJ day-1 per bird [9], which is similar to other estimates made by using the doubly labelled water
method in our study species [10]. Therefore, the estimated daily energy intake during ascent phase (552.0 kJ day-1)
constitutes 25.8 ~ 45.4 % of the daily energy expenditures of a chick-rearing thick-billed murre.
Supplementary movies
Movie S1. A foraging thick-billed murre encountered jellyfish during the ascent phase of a dive.
Movie S2. A foraging thick-billed murre fed on juvenile fish associated with the tentacles of jellyfish. The date and
time in the display is wrong due to technical problems with this video footage. The correct start time of the footage is
09:42:14, 4th August 2014.
Movie S3. A foraging thick-billed murre fed on fish not associated with jellyfish (solitary fish) during ascending.
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