Can accelerometry be used to distinguish between flight types in soaring birds?
© Williams et al. 2015
Received: 28 January 2015
Accepted: 28 January 2015
Published: 8 October 2015
Accelerometry has been used to identify behaviours through the quantification of body posture and motion for a range of species moving in different media. This technique has not been applied to flight behaviours to the same degree, having only been used to distinguish flapping from soaring flight, even though identifying the type of soaring flight could provide important insights into the factors underlying movement paths in soaring birds. This may be due to the complexities of interpreting acceleration data, as movement in the aerial environment may be influenced by phenomena such as centripetal acceleration (pulling-g). This study used high-resolution movement data on the flight of free-living Andean condors (Vultur gryphus) and a captive Eurasian griffon vulture (Gyps fulvus) to examine the influence of gravitational, dynamic and centripetal acceleration in different flight types. Flight behaviour was categorised as thermal soaring, slope soaring, gliding and flapping, using changes in altitude and heading from magnetometry data. We examined the ability of the k-nearest neighbour (KNN) algorithm to distinguish between these behaviours using acceleration data alone.
Values of the vectorial static body acceleration (VeSBA) suggest that these birds experience relatively little centripetal acceleration in flight, though this varies between flight types. Centripetal acceleration appears to be of most influence during thermal soaring; consequently, it is not possible to derive bank angle from smoothed values of lateral acceleration. In contrast, the smoothed acceleration values in the dorso-ventral axis provide insight into body pitch, which varied linearly with airspeed. Classification of passive flight types via KNN was limited, with low accuracy and precision for soaring and gliding.
The importance of soaring was evident in the high proportion of time each bird spent in this flight mode (52.17–84.00 %). Accelerometry alone was limited in its ability to distinguish between passive flight types, though smoothed values in the dorso-ventral axis did vary with airspeed. Other sensors, in particular the magnetometer, provided powerful methods of identifying flight behaviour and these data may be better suited for automated behavioural identification. This should provide further insight into the type and strength of updraughts available to soaring birds.
KeywordsSoaring flight Acceleration Magnetometry Pulling-g Centripetal acceleration Daily Diary KNN
Accelerometers have been proven to be a powerful tool in the study of animal movement and behaviour , and their use has proliferated in both terrestrial and aquatic environments (e.g. [2–6]). Researchers have used accelerometers to identify behaviour (e.g. [3, 7, 8]) and estimate activity levels (and their associated energetic costs) [9–11] by quantifying patterns of body posture and motion. This has provided insight into a range of ecological issues, from foraging success in Antarctic fur seals (Arctocephalus gazella)  to levels of infection in diseased cockroaches (Blaberus craniifer) . In the aerial environment, however, investigation of behaviour in flight has been mostly limited to the identification of a single behaviour; flapping flight. Flapping flight is readily classifiable as each upstroke and downstroke are associated with peaks in dynamic acceleration [14, 15], and the absence of these peaks has been used to identify passive flight .
Nonetheless, passive flight is the dominant flight type for many soaring birds, which rely on updraughts to maintain or gain height to move [14, 17]. The updraughts exploited by soaring birds can broadly be categorised as thermal updraughts and slope soaring using orographic lift (see below), with the former being driven by uneven heating of the substrate, and the latter by wind being deflected upwards by sloping terrain [18, 19]. The distribution and strength of these updraughts vary in space and time , which is likely to have profound consequences for the flight paths selected by these animals as well as their time and energy budgets . Therefore, the ability to further classify soaring flight types according to the source of lift, as well as periods of gliding between them, would contribute greatly to our understanding of the factors driving space use for these birds .
GPS data have been used to identify the type of updraughts used by soaring birds along their flight path (e.g. ), namely through the calculation of ground speed, and/or the identification of circling behaviour indicative of thermal soaring or by sequential locations lying along a windward slope  (indicative of the use of orographic lift). However, high frequency data can be difficult to acquire for many species due to the power requirements of GPS sensors and associated battery weights. GPS locations are more commonly recorded at a frequency of several fixes per hour. At this frequency, insight into the updraughts exploited by soaring birds can only be gained where locations are combined with high-resolution meteorological data, including the local wind field for the estimation of slope lift (e.g. [19, 21, 23], cf. ), and acceleration data  (which enable researchers to distinguish between powered climbs and use of updraughts). Given the number of workers currently using accelerometers, their reduced cost and extended recording duration for a given sampling frequency (e.g.  albatross, bi-axial accelerometer, 16–32 Hz, 23–68 h,  raptors, tri-axial accelerometer, 100 Hz, 3–19 min per run), it would be valuable if the resulting data could be used to investigate the type of updraught that birds exploit when soaring.
There are several reasons why soaring and gliding flight types may be distinguishable using acceleration data. We predict that the dynamic motion measured by an animal-attached accelerometer may increase with turbulence experienced by the bird  according to the characteristics of the air mass the animal is moving through , cf.  as well as the bird’s airspeed. Birds modulate their airspeed in relation to the availability of rising air, reducing their airspeed in updraughts and increasing their airspeed as they glide between them . This may also be associated with postural changes if, in an analogous manner to gliders, airspeed is affected by changes in the forward pitch as well as modulating wing area by altering the shape of the wings [30, 31].
Changes in forward pitch angle as well as body roll can be calculated from the gravitational (the smoothed) component of acceleration for animals that are largely stationary or undertaking linear locomotion [9, 27]. However, this relatively simplistic interpretation of acceleration data may not be appropriate for flying animals. Pilots, for instance, use a combination of sensors, including magnetometers and gyroscopes to quantify their orientation during flight rather than relying on accelerometers alone (e.g. ). This is because the gravitational component of acceleration, as estimated by smoothing the raw data, may be affected by directional changes in velocity. Birds typically experience high centripetal accelerations when pulling out of stoops (also called pulling-g), with the highest value ever recorded being nine times greater than gravitational acceleration, as measured in the stooping aerial displays of Anna’s hummingbirds, Calypte anna . Birds and pilots can also enter free-fall, where each axis, and therefore the sum, of acceleration axes would equal 0 g, irrespective of the orientation of the bird or accelerometer during this phase.
The aim of this study was to characterise how acceleration data vary between active (flapping) and passive flight types (thermal soaring, slope soaring, and gliding). First, we define flight types by patterns in acceleration, magnetometry and altitude change and second, assess the power of an automated classification method (k-nearest neighbour algorithm) to identify these behaviours using acceleration data alone. Finally, we assess the patterns in dynamic, gravitational and centripetal acceleration that contribute to the recorded signal for the different flight types. We also report on preliminary results from a differential pressure sensor system to quantify variation in airspeed with body posture according to flight type.
We attached Daily Diary (DD) devices to two species of soaring bird, the Andean condor (Vultur gryphus) and the Eurasian griffon vulture (Gyps fulvus). These devices incorporate tri-axial accelerometers and magnetometers, and a barometric pressure sensors , which together provide high-resolution data on animal movement and behaviour. Both species are obligate soaring birds although they differ in body mass, with the Andean condor being the heaviest terrestrial soaring bird (ranging from some 9–16 kg; wingspan ca. 3 m [34, 35]) and the Eurasian griffon being substantially smaller (6.2–11.3 kg; wingspan ca. 2.5 m for captive birds ).
Five adult Andean condors were fitted with DD near Bariloche, Argentina, and each unit recorded at 40 (n = 3) or 20 Hz (n = 2). DDs were attached to the lower back of the Andean condors by taping a bio-degradable base plate to the feathers with Tesa Tape. The unit housing the DD was then slotted onto the plate and held in place with fishing line. Stability of the devices was ensured by attaching the plate to numerous rows of back feathers to restrict movement of the plate relative to the animal’s own movement. Moreover, the 3D printing method of constructing the housing ensured that components slotted together with minimal space for free movement. DD Units were programmed to release from the plate and drop from the bird in the night (to ensure they dropped during roosting) after 7–9 days, after which time they were recovered from the roosting sites (see  for full details). Units weighed between 70 and 90 g, representing less than 1 % of mean adult body mass; therefore falling below the recommended 3 % body mass. Procedures for tagging free-living animals were approved by the Swansea University Ethics Committee.
A captive Eurasian griffon vulture was fitted with a DD unit during three flights at the raptor centre at Rocher des Aigles, Rocamadour, France. This site offers very good thermal soaring conditions for free-flying raptors, being located on top of a 120-m-deep canyon (see  for details). In addition to the usual parameters, this Daily Diary also measured airspeed, using a differential pressure sensor to quantify the difference between the static and dynamic pressure, with the latter being recorded through a forward-facing tube (Pitot tube, which was made of brass and had a bore diameter of 3 mm) (Fig. 1) . The unit of airflow measured in volts (V) directly from the differential pressure sensor is given as relative airspeed. Prior to deployment on the vulture, this system was tested in a wind tunnel, which confirmed that the sensor output correlated well with airspeed (ES, unpublished data). To accurately measure airflow, the Pitot tube should be in line with the direction of airflow over the back of the bird (Fig. 1). This is achieved in ballistic flight (soaring and gliding), but during steep descents prior to landing, the tube may not be in line with the airflow and hence these periods were removed for analysis regarding the vulture. The DD was set to record at 20 Hz. The device was fitted to the back of the bird with a leg loop harness of Teflon ribbon, and removed after a single day [14, 37]. Attachment of loggers to captive vultures was conducted under a formal aggreement between the animal rearing facility at Rocher des Aigles and the CNRS with the permit of O. Duriez for the attachment of logging devices (under a licence from the Research Centre for Bird Studies, Natural History Museum, Paris). Care was taken in harness design to minimise discomfort and loggers were removed promptly after flights.
Identification of flight types
Definitions of flight types identified visually from time series plots
A series of flapping bouts (>3) in constant flapping flight seen mainly in the heave channel
Increasing altitude; no flapping; circling behaviour seen in x–y magnetometry channels
Increasing or maintaining altitude; no flapping; no clear circling pattern in x–y magnetometry channels; rather irregular changes in direction
Decreasing altitude; no flapping; often associated with relatively little change in direction in magnetometry channels
Following these definitions, the data were classified by visual inspection of time series plots in the custom made software DDMT. Data were only labelled as one of the four flight types where the behaviour was observed for at least 30 s, or in the case of flapping flight, by at least three bouts of flapping (e.g. Fig. 2a includes eight flapping bouts). The raw data were cut by flight type and merged into single data files per flight type for each of the five Andean condors and the griffon vulture.
Patterns in dynamic and smoothed acceleration by flight type were examined using the data of the free-ranging Andean condors, as well as the use of the k-nearest neighbour classification method. The flight of the griffon vulture was short in duration, and although its airspace was not restricted, its flight was localised around the raptor centre. We therefore did not consider the data of the griffon vulture alongside that of the Andean condors, but instead used the griffon vulture data to quantify the relationship between body pitch and airspeed.
Classification by acceleration
Machine-learning algorithms [16, 39, 40], clustering techniques  and normalised correlation  have been used to identify behaviour from acceleration data in a range of species. Here, we used the k-nearest neighbour (KNN) cluster analysis to investigate the ability of such automated classification methods to distinguish between the four flight types with raw acceleration data alone. This classification method was chosen due to the previous success of the k-means clustering method distinguishing between flapping and non-flapping flight , which has been found to be of equal success to other more complex methods in the study of soaring birds . The KNN algorithm classifies raw acceleration data according to the identity of its k-nearest neighbours in three-dimensional space, where the nearest neighbours are from a separate training data set of known classification (cf. ). Having identified four flight types using data from all sensors we were in the position to train the clustering algorithm of the KNN rather than use a k-means cluster analysis method that blindly searches for a predetermined number of clusters (cf. [43, 44]).
KNN was applied to each of the five Andean condors separately, where training sets consisted of 12,000 data points (5 min for datasets of 40 Hz and 10 min for 20 Hz). Test sets of the same length were taken from the flight type specific data files for each individual (from the centre data point of the file). The analysis was performed as described in Bidder et al. , using a value of 21 for k (the number of the nearest neighbours in the algorithm) and their reported mean minimum majority thresholds (proportion of k-nearest points of the same classification to be deemed a positive classification) of 0.7 for accuracy, 0.9 for precision and 0.5 for recall. Classifications that surpassed the given thresholds were defined as positives, where if correct according to the test set as true positives (TP) or where incorrect as false positives (FP). Classifications that fell below the threshold were negative and again either true negatives (TN) or false negatives (FN) according to whether they were classified incorrectly or correctly, respectively. Classification was evaluated by calculating the performance metrics of (1) Accuracy, the proportion of data points correctly assigned [Accuracy = (TN + TP)/(TN + TP + FN + FP)], (2) Precision, the proportion of positive results that were correctly assigned [Precision = TP/(TP + FN)] and (3) Recall, the proportion of data assigned to behaviours that were classified as positive correctly [Recall = TP/(TP + FN)].
Patterns in body posture and motion
To investigate patterns of body posture, raw acceleration values were smoothed with adjacent averaging over 3 s to estimate the gravitational acceleration in the three axes [27, 45, 46]. To control for individual differences in device alignment, smoothed acceleration data in each of the three axes were centred on the individual’s in-flight mean. Smoothed acceleration values therefore ranged from −1 to 1, where negative values indicate increasingly upward pointing pitch from the individual mean and positive values an increasingly downward pointing pitch from the individual mean. Patterns of body motion were examined by calculating the dynamic acceleration component, defined as the acceleration due to changes in speed. This was calculated by subtracting the smoothed signal from the raw acceleration values in each axis.
VeSBA, as the sum of the gravitational acceleration, should equal 1 g for any animal that is a stationary state or constant state of motion, i.e. linear. The deviation of VeSBA from 1 g (VeSBA—1) is therefore a measure of the centripetal acceleration (and an indication that the animal is pulling-g) .
We examined the frequency distributions of VeDBA and VeSBA for the Andean condor by flight type and describe the VeSBA deviance of the different flight types from 1 g. The distribution of smoothed sway (centred on individual means, described above) was also assessed with frequency distributions by flight type.
We performed a linear mixed model (LMM) to quantify the predictive effects of smoothed surge (g) in interaction with flight type (as defined by the visual inspection of the combined sensors) on altitude change (m/s). Individual was included as a random factor. A random 0.01 % sample of the Andean condor data (n = 785) was taken to remove autocorrelation. Considering the enormity of data collected at 40 Hz, the resultant relationship between smoothed surge and altitude change was plotted with a kernel density frequency distribution with probability density function at levels 0.9 and 0.5.
The relationship between the rate of altitude change and smoothed surge in the griffon vulture was quantified with a Pearson’s correlation test, again with a randomly subsampled data set to remove autocorrelation (0.2 %, n = 111). The data set of the griffon vulture was restricted to ballistic flight (gliding, slope soaring and thermal soaring); hence, landing and flapping flight were removed. Using the same subsample of data, we performed a linear model with the response variable of relative airspeed (V) to quantify the effect of smoothed surge and flight type as explanatory variables in interaction.
The significance of terms in the two linear models was calculated with an ANOVA, and the term specific statistic and its associated p value are given. Model fit was assessed with an adjusted R 2, or in the case of the LMM the marginal (variance explained by the fixed effects alone) and the conditional R 2 values (the variance explained by the fixed and random effects together) . Statistical treatments were performed in R3.1.1 and figures produced in Origin 9.1 (OriginLab).
Individuals and their flights
Occurrence of the different flight types for the five tagged Andean condors (Vultur gryphus) and the Eurasian griffon vulture (Gyps fulvus) as a proportion of the total classified flight time, and the percentage of soaring whereby thermal updraughts are the source of lift rather than orographic slope updraughts
ID (total classified time)
% Thermal soaring
1 (25.68 h)
2 (20.28 h)
3 (6.50 h)
4 (17.43 h)
5 (8.58 h)
1 (0.80 h)
Flight type classification by KNN
Average accuracy, precision and recall values (mean ± SD) of KNN where k = 21 and n = 12,000 obtained for each of the five Andean condors (Vultur gryphus)
0.70 ± 0.05
0.51 ± 0.12
0.68 ± 0.07
0.54 ± 0.13
0.92 ± 0.07
0.32 ± 0.33
0.39 ± 0.46
0.29 ± 0.36
0.79 ± 0.11
0.56 ± 0.10
0.57 ± 0.28
0.57 ± 0.13
Dynamic, centripetal and gravitational acceleration
VeSBA values tended to deviate little from 1 g during flight, though distributions differed between flight types (Fig. 3b). The distribution of VeSBA values in slope soaring peaked at a mean of 1.10 ± 0.11 g SD. Flapping and gliding averaged close to 1 g (1.03 ± 0.19 g and 1.03 ± 0.18 g, respectively), though there was slight bimodality in the distribution of both. The deviation from 1 g was greatest for thermal soaring with a mean VeSBA of 1.17 ± 0.12 g.
All flight types exhibited a normal distribution in smoothed sway values. The centred sway showed little deviation from 0 g for all but one individual (see Additional file 1: Figure S1), and there was no evidence for bimodality that might have indicated banking in thermal soaring.
Smooth surge and Airspeed
Flight types were clearly identifiable when acceleration, magnetometry and barometric pressure signals were considered together. This combination of sensors allowed for the accurate classification of passive flight types, providing insight into the type of updraught used by the birds (cf. ) and the movement strategies that they adopt. Previously, soaring flight type had only been alluded to with the use of GPS-derived speed (e.g. ) and high-precision GPS locations recording at high frequency (e.g. ), or by assessing meteorological data associated with GPS locations (e.g. [18, 21]). Though use of GPS allows for some behaviours to be identified, coupling the use of accelerometers and magnetometers provides fine-resolution movement data and for an extended recording duration. As apparent in previous studies on free-living raptors, flapping flight contributes to only a small fraction of the flight time budget [14, 16]. Here, we have shown that the majority of flight is passive, and that this is more-or-less equally split between soaring and gliding behaviour. The relative use of thermal and orographic updraughts varied between individual condors, although as only five birds were tagged and at different periods over the summer months, further analysis is required to disentangle the relative influences of route selection and meteorological conditions. This type of approach is likely to provide insight into the role of thermal and orographic updraughts in the movement ecology of soaring species , particularly when information on flight type is combined with positional information at fine scales.
Smoothed acceleration in the sway axis did not vary between flight types, even though birds are predicted to adopt a bank angle of around 24 degrees during thermal soaring . Unlike calculation of body roll when the animal stationary or moving in a linear fashion, the distribution of VeSBA in thermal soaring suggests that a centripetal force in turning may mask the effect of the bank angle on the smoothed sway values. To gain altitude within a thermal the bird must maintain a balanced turn about a centre point, slowing its speed to maximise height gain . This is maintained with the constant change in the direction of the velocity, creating a centripetal force when circling; the magnitude of which increases with a decreasing turn radius . As a consequence, the smoothed acceleration values in the lateral axis cannot be used to derive the bank angle. The phenomenon of pulling-g due to centripetal acceleration is not confined to the aerial environment and has the potential to influence acceleration values recorded on any animal that experiences dramatic changes in velocity. Other examples may include brachiating gibbons or cheetahs cornering as they chase their prey .
While birds may experience pulling-g in other axes of acceleration, when pulling out of stoops  for instance, or other manoeuvres that may occur during social interactions, these occur relatively infrequently, at least for soaring birds. Smoothed surge data should therefore be able to provide insight into body pitch for straight passive flight. Indeed, our results demonstrated that gliding and flapping flight generally showed little deviation from 1 g. A bimodal distribution in flapping flight may suggest some deviation, though this could also reflect the difficulties of separating gravitational and dynamic components (cf. ) (a potential issue for all flight types).
Our results suggest that changes in speed during gliding, largely achieved by reducing the wing area, are accompanied by measurable changes in body pitch. This follows from the strong relationship between smoothed surge and altitude change, a parameter that also tends to differ between flight types. Considering parameters according to altitude change may actually offer a more suitable variable in the analysis of space use (cf. ) than the grouping of behaviour into discrete flight types. Furthermore, data collected from the individual griffon vulture showed that smoothed surge was related to airspeed. The changes in the relative airspeed were consistent with theoretical predictions of changes in flight speed; showing a reduction in airspeed as the bird moved into updraughts and an increase as birds glided between them . To the best of our knowledge, this represents the first description of airspeed, as quantified using an on-board differential pressure sensor, in relation to the posture of the bird (in terms of smoothed surge) (but see ). Overall, the differential pressure system shows promise and could be further refined, e.g. by experimentation with the positioning of the device, the height of the tube in relation to the bird’s back and the diameter of the Pitot tube.
We predicted that VeDBA may differ between soaring and gliding, on the basis that these behaviours represent movement through different air masses and at different airspeeds. However, VeDBA values were in fact similar and the increased airspeed seen during gliding did not appear to manifest itself as an increase in experienced turbulence relative to other flight types (cf. ). The lack of variance in VeDBA may reflect the range in air masses experienced within one behavioural category and factors such as gustiness, which vary in both space, and time are difficult to quantify. It is unlikely to represent noise from our method of device attachment, as units were stable when they were on-board the birds.
The ability of the KNN algorithm to classify passive flight behaviour from acceleration was limited, though it was able to distinguish flapping from non-flapping flight accurately. The increased dynamic movement and posture adopted during flapping separates this flight type in tri-axial acceleration space . While we did find evidence for variation in the smoothed surge signal between passive flight types, these differences were not sufficient to form discrete clusters for high accuracy classification. This is in contrast to other studies such as that by Shamoun-Baranes et al. , which distinguished between specific behaviours from smoothed tri-axial acceleration values using supervised classification trees. As KNN has been shown to perform similarly to a number of automated classification algorithms, at least for the identification of active flapping and passive non-flapping flight  it would seem that, even at the high frequency of 40 and 20 Hz, passive flight types cannot be distinguished between from their acceleration values alone with such algorithms.
We suggest that the use of magnetometer data may be more powerful than accelerometer data to distinguish between different flight behaviours. Machine-learning approaches and other algorithms are currently being used to automate the identification of behaviour types from acceleration data [7, 16, 43, 44, 51], and the application of automated classification to magnetometry data is new (cf. ). Nonetheless, we believe this approach holds promise for the identification of many behavioural patterns. The independence of the magnetometer signal from dynamic movement may be a particular asset when it comes to the investigation of flight behaviour.
For soaring birds, the classification of their dominant flight types relies on the consideration of multiple sensors. The combination of magnetometry, barometric pressure, airspeed and accelerometry, provided a detailed insight into animal movement for the accurate identification and quantification of behaviour, from which the space use of soaring birds and their reliance on updraughts can be investigated. We highlight some of the circumstances under which it is not possible to use the smoothed acceleration signal to derive precise estimates of body orientation (here during thermal soaring) though we do find evidence that the smoothed surge values should vary with body pitch affecting airspeed in flight. The effects of centripetal acceleration should be considered in the interpretation of the dynamic and gravitational acceleration, especially concerning animals that experience changes in velocity during certain behaviours.
global positioning system
a single magnetometry channel (in the case of figures this refers to the lateral x-axis)
vectorial dynamic body acceleration
vectorial static body acceleration
This study was conceived by HJW and ELCS. HJW carried out data analyses and wrote the manuscript with input from ELCS. OD and ELCS tagged the Eurasian griffon vultures. SAL and ELCS collected the Andean condor data. All authors commented on final manuscript. All authors read and approved the final manuscript.
We thank the field team in Bariloche, Argentina for their assistance with the condor capture, tag attachment and recovery and at Rocher des Aigles, Rocamadour, France, for tag deployment during their daily flight of the griffon vulture. Andean condor data collection was funded by the National Geographic (grant GEFNE89-13 awarded to ELCS), CONICET-PIP-0095, ANPCyT-PICT-0725 (2014), and FBBVA (project BIOCON-08-126). Permissions to tag condors were given by Fauna de Río Negro, the Argentine National Park Administration and the owners and managers of local farms. The authors would also like to thank Mark Holton and Phil Hopkins for DD tag and housing design, and Rory Wilson, Ed Grundy and James Walker for assistance with DD analysis. HJW is funded by a Swansea University PhD studentship, and we would like to acknowledge ELCS, Andrew King and Adrian Luckman for their supervision.
Compliance with ethical guidelines
Competing interests The authors declare that they have no competing interests.
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