Wing tags severely impair movement in African Cape Vultures

Background The use of tracking technologies is key for the study of animal movement and pivotal to ecological and conservation research. However, the potential effects of devices attached to animals are sometimes neglected. The impact of tagging not only rises welfare concerns, but can also bias the data collected, causing misinterpretation of the observed behaviour which invalidates the comparability of information across individuals and populations. Patagial (wing) tags have been extensively used as a marking method for visual resightings in endangered vulture species, but their effect on the aerodynamics of the birds and their flight behaviour is yet to be investigated. Using GPS backpack mounted devices, we compared the flight performance of 27 captive and wild Cape Vultures (Gyps coprotheres), marked with either patagial tags or coloured leg bands. Results Individuals equipped with patagial tags were less likely to fly, travelled shorter distances and flew slower compared to individuals equipped with leg bands. These effects were also observed in one individual that recovered its flight performance after replacing its patagial tag by a leg band. Conclusions Although we did not measure the effects of patagial tags on body condition or survival, our study strongly suggests that they have severe adverse effects on vultures’ flight behaviour and emphasises the importance of investigating the effects that tagging methods can have on the behaviour and conservation of the study species, as well as on the quality of the scientific results.

If the weight and drag of a tag on the wing were problematic per se, the way in which anatomic  In this study, we compare the impact of patagial tags vs leg bands on the flight performance 94 of an endangered soaring bird species, the Cape Vulture (Gyps coprotheres). Specifically, we 95 investigate the effects of tag attachment in wild and captive bred individuals on flight probability, 96 proportion of time spent flying, cumulative distance travelled and flight speed.

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During the study, one captive vulture originally released with a patagial tag was found grounded, 117 whereupon its patagial tag was replaced with a leg band (see Methods). After the tag replacement,

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The following three models were all run only on days in which at least one flight segment was

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In captive bred individuals, the difference between distance travelled when wearing leg bands and 174 patagial tags was smaller than in wild individuals (about 7 km) and statistically non-significant.

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When comparing within the same tag attachment type, we did not find any significant difference 176 between wild and captive bred individuals. The random structure of the model showed small 177 among-individual variation, with 95 % of the individuals travelling between 50 and 72 km/day.

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The variability attributable to individual variation (intercept SD = 0.37) was lower relative to the 179 effect associated to the patagial attachment on wild birds (table 3).

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The model showed no significant differences between wild and captive bred individuals, however the effect sizes suggest that captive individuals wearing leg bands travelled at the fastest speeds, 189 about 7.2 ms −1 , while captive individuals with patagial tags at the lowest, about 5.9 ms −1 (pata- Although statistically not significant, we believe it to be relevant that among all groups, captive 205 individuals wearing patagial tags also spent the least proportion of time flying. Finally and as 206 a consequence of the above, our models also showed that wild birds equipped with patagial tags 207 travelled significantly shorter distances compared to those wearing leg bands.

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Age and experience are known to affect ranging behaviour and distance travelled in vultures, 209 with younger individuals moving more than older ones, who already established their breeding 210 territories [21,22]. In this study, due to our small sample size, it was not possible to account for 211 age in the models. Our dataset included 21 individuals among which were fledglings, juveniles and 212 subadults and only six adults. However, the adult individuals were equally distributed between 213 the two attachment types, thus we are confident that omitting age from the models did not bias 214 our findings.

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Shorter daily distance travelled means smaller area covered every day by the birds to forage.

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Vultures are scavengers and as such cover large areas to feed on ephemeral food resources [23].

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At individual and population level, a restricted flight potential and therefore a restricted area 218 available to forage might not be a problem if food resources are abundant, but they surely reduce 219 an individual's ability to react to changes in food availability and environmental conditions. A 220 restricted area covered daily by these birds can also lead to ecosystem-level effects. Scavengers 221 play an important role in the ecosystem thanks to the services they provide, such as preventing 222 the spread of infectious diseases, recycling organic material into nutrients and stabilising food 223 webs [24]. Therefore a restricted flight potential and reduction in the area covered by these birds 224 caused by improper tag attachment can have far-reaching consequences at the ecosystem level.  in numbers dramatically [29]. In such species, the use of the least invasive tagging method can 256 have a significant impact on their population stability and extinction risk.  When assessing which device to use, it is therefore necessary to understand its impacts on the 267 specific study species or on phylogenetically related or functionally similar species. Discrepancies 268 between studies may also be generated by differences in the temporal and special scales at which 269 data were collected, or by differences in the environmental conditions the animals experienced [6].  Captive birds were born and raised in captivity at VulPro's rehabilitation centre and released 305 before the age of one. All wild birds were found grounded due to minor injuries (e.g. heat   Tables  Table 1: GAMM with occurrence of flight included as dependent variable, interaction between tag attachment and group as fixed term and individual identity as random intercept. Number of days since deployment, number of locations and day of the year were included as smooth terms. The model was fitted with the restricted maximum likelihood using the binomial family and a logit link function.

Coeff.
Est Adjusted R 2 0.30 NS p≥0.05; * p < 0.05; ** p < 0.01; *** p < 0.001 Table 2: GAMM with daily proportion of time spent flying included as dependent variable, interaction between tag attachment and group as fixed term and individual identity as random intercept. Number of days since deployment, number of locations and day of the year were included as smooth terms. The model was fitted with the restricted maximum likelihood using the binomial family and a logit link function.

Coeff.
Est Adjusted R 2 0.93 NS p≥0.05; * p < 0.05; ** p < 0.01; *** p < 0.001 Table 3: GAMM with the square root of the daily cumulative distance included as dependent variable, interaction between tag attachment and group as fixed term and individual identity as random intercept. Number of days since deployment, number of locations and day of the year were included as smooth terms. The model was fitted with the restricted maximum likelihood using the gaussian family.

Coeff.
Est Adjusted R 2 0.92 NS p≥0.05; * p < 0.05; ** p < 0.01; *** p < 0.001 Table 4: GAMM with the square root of the daily median flight speed included as dependent variable, interaction between tag attachment and group as fixed term and individual identity as random intercept. Number of days since deployment, number of locations and day of the year were included as smooth terms. The model was fitted with the restricted maximum likelihood using the gaussian family.