The following item is an edited version of a short paper in the current edition
of Science. [SCIENCE VOL 295 15 FEBRUARY 2002 1259]. I have a pdf version which
I can forward on request.
GPS Tracking of Foraging Albatrosses
Henri Weimerskirch,1,2* Francesco Bonadonna,1 Fre´de´ric Bailleul,1 Ge´raldine
Mabille,1 Giacomo Dell?Omo,3 Hans-Peter Lipp3
Developments in satellite telemetry have recently allowed considerable progress
in the study of long-range movements of large animals in the wild (1), but the
study of the detailed patterns of their foraging behavior on a small to medium
scale is not possible because of the imprecision of satellite telemetry systems
(2). We used a miniaturized Global Position System (GPS) that recorded
geographic position at 1-s intervals (3) to examine the exact flight pattern and
foraging behavior of free-ranging wandering albatrosses (Diomedea exulans).
We deployed GPS loggers on breeding birds (3) either starting a long foraging
trip in ocenic waters during the incubation period or searching for food close
to the colony during the chick brooding period (Fig. 1, A and B, respectively).
The distribution of ground speeds measured between 924,712 GPS locations was
bimodal, with speeds varying from 0 to 9 km hour indicating that birds were
sitting on the water (59.5% of foraging time) and speeds ranging between 18 and
135 km hour when birds are in flight. When in flight, birds frequently attained
(8.2% of time) ground speeds higher than 85 km hour, the maximum travel speed
predicted for wandering
albatrosses (4, 5). Small-scale flight paths show typical zigzag patterns with
continuous changes in flight speed according to the position of the bird with
respect to wind (Fig. 1C). Because they rely extensively on wind conditions to
reduce flight costs (4?6), wandering albatrosses have to adjust their searching
behavior according to wind conditions, but at the same time they must adjust
their foraging movements to increase the probability of encountering prey.
The zigzagging small-scale movements added to the larger scale changes in
overall direction affect overall the sinuosity of the track. The straightness
index of the path, as measured by the ratio of straight-line distance between
the initial and final positions of two consecutive landings relative to the
actual path (7), was on average 0.512 (range 0.72 to 0.280, with 1.0 being a
straight-line course). The ratio was not affected by wind direction with respect
to overall route direction because birds always have a zigzagging flight when
they move with head, cross, or tail winds. Predators foraging in a heterogeneous
environment are expected to adjust their search pattern (e.g., the straightness
of their route, the flight speed, and/or turning rate) to increase the
probability of encountering prey (8), but this prediction is generally
impossible to test on marine animals. We tested whether birds modified the
straightness of their movements according to the season or the marine habitat
visited. The straightness index of the track was lower during the brooding
period (0.41 6 0.1), when birds are searching for food close to the colonies
(9), compared with the incubation period (0.588 6 0.09; Kruskal-Wallis, U 5 4.0,
P 5 0.028), when birds are only moving away from the shelf area during the first
day of foraging. The difference was only due to the higher sinuosity of tracks
during brooding over the shelf break (0.294 6 0.084), where birds are known to
catch most prey (9), compared with when birds were over the shelf itself (0.693
6 0.182; Wilcoxon paired test, Z 5 2.42, P 5 0.015) or over oceanic waters
(0.648 6 0.09). Thus, birds increase the sinuosity of their flight only over a
specific area, the edge of the peri-insular shelf. When foraging, birds landed
regularly on the sea surface (Fig. 1, A and B), on average every 1.8 6 0.9
hours, and drifted when sitting on the water. The overall direction of the drift
was partly due to wind direction, but marine currents
also played an important role. Several birds spent long periods drifting over
the shelf break region, and the trajectory of the drift over this area was not
straight as might be expected if birds were transported by a unidirectional
currents or by wind as occurs in oceanic waters. In contrast, the drift tracks
showed a smooth looping movement, indicating the presence of medium-scale
turbulence such as small gyres (Fig. 1D). These looping movements only occurred
over the Crozet shelf break. In these gyres, probably associated with upwelling
movements often present on shelf breaks, prey are probably pushed to the surface
and become concentrated and accessible to surface feeding in a restricted and
predictable area.
<snip>
Supplementary Material
Methods: The study was carried out in January to April 2001, on the Crozet
Islands. We used a fully self-contained GPS-MS1 receiver with an onboard
non-volatile memory that stores up to 100,000 positions. Details of the GPS are
given by I. Steiner et al. [Physiol. Behavior 71, 1-8 (2000)]. The GPS used has
a circular error probability of 4 m for horizontal position. Accuracy for
altitude was lower and was not used, especially because albatrosses rarely fly
over the sea at altitudes higher than 20 m. The GPS devices, weighing 105 g
(1-1.3% of birds mass) including batteries and waterproof packaging, were taped
to back feathers on the bird leaving the nest
after a change over by its partner. Eight GPS units were deployed during the
incubation period (average trip duration 8.26 ± 1.88 days) and nine units were
deployed during brooding (trip duration 2.99 ± 1.42 days). Wind speed and wind
directions were derived from meteorological models that estimated twice daily
the wind strength and direction from NOAA/NESDIS, based on near real-time data
collected by NASA/JPL's SeaWinds Scatterometer aboard the QuikSCAT satellite.
Wind direction was mainly from the west. In order to acquire the precise
movements of birds compared to obtaining movements over longer periods with
lower resolution, the loggers were programmed to run in continuous mode by
measuring one fix every second for a period of 27.7 hours from the time the GPS
was started before the memory was full.
<snip>
Additional legend to Fig. 1. In (A), the movement of a male was recorded for
20.2 hours at sea with the bird spending 68.8% of its time in flight and
covering a total distance of 1014 km (i.e., 996 km in flight and the rest
drifting). The average flight speed was 71.6 km hour-1 and the overall
straightness ratio of the track between two landings was 0.63. In (B), a
female's movement was recorded for 23.4 hours at sea during which she spent
38.9% in flight, flew a total distance of 625 km (i.e., 580 km in flight and the
rest drifting on the water), traveled at an average flight speed of 63.7 km
hour-1, and had a straightness ratio of 0.36. (C) Smaller scale part of a flight
bout with cross-head winds; ground speeds progressively decreased when birds
soar against the wind or with side winds, while speeds increased abruptly after
birds has oriented from a head to tail wind. (D) Drifting movement on the sea
surface after landing over the shelf edge.
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