Attaching PathTrack GPS tags to Sooty Terns
In 2014 Christine and I undertook trials of different attachment methods of attaching GPS loggers to incubating adult Sooty Terns on Bird Island, Seychelles, initially using dummy loggers and subsequently attaching working loggers. Ten dummy tags were made to our specification (and kindly donated free-of-charge) by the manufacturer, Pathtrack (www.pathtrack.co.uk), to represent the size and approximate weight of their PathTrack Nanofix Geo+ solar-charged GPS tags. These dummy tags had two ventrally positioned tubes of internal diameter c. 2 mm, at the front and rear of the tags. In their working tags these are to support the harness material. The working tags additionally have small wire loops, two at the front and two at the back, which are terminals for the attachment of clips to connect to the programming board for data download. In PathTrack’s working tags these loops are c. 1 mm diameter but we asked for larger ones that could be used as additional attachment points for our trials using dummy tags. In PathTrack’s working tags the attachment tubes and the loops are integral parts of the epoxy structure of the tag and the wire loops are attached to the motherboard. In the dummy tags, however, the tubes and loops were glued to the epoxy tag body and this led to weaknesses that we discovered during the trials. The trials were compromised by these weaknesses but we nevertheless learned from the experience.
In addition to the PathTrack dummy tags, we made further dummies from 3 x 1 Lego bricks with holes drilled for the attachment of harnesses. In 2015 we made our own dummy loggers from 8 x 8 mm plastic rod, with attachment holes drilled appropriately, and tested these with the attachment method that we thought held the most promise following the 2014 trials.
These trials were not sufficiently robust for journal publication but our findings could still be useful to others considering using tags to track smaller (in comparison with albatrosses, boobies and frigatebirds) seabirds, and especially Sooty Terns.
2014 trials with dummy tags
In the initial trial three kinds of attachment were tried: body harness (tag on upper back with harness passing in front of wing, around breast and crossed under sternum, then passed round rear of wing and attached to posterior of tag); thigh harness (tag on lower back, harness passing down sides of body and around each thigh, then up sides of body for attachment to rear of tag); and tail attachment (tag fastened using adhesive tape (Tesa) around the base of the central pair of rectrices and to the sides and base of the tag, but not around the tag top, which would cover solar panels). This trial, with three birds tagged using each method, lasted eight days, at the end of which we caught the birds to examine the attachments and look for any negative effects to skin or plumage. Subsequently, we used the tenth dummy tag, together with a dummy tag that we had retrieved from the earlier trial, to trial wing harnesses (tag on upper back, harness looped beneath each wing); we also used two dummies made from Lego bricks in this trial.
The harness material for the body and thigh tags was 2 mm diameter slip elastic (www.prestonsinnovations.com), which biodegrades over 2-3 months so that it will eventually fall off a bird whose tag could not be retrieved. Three of the wing harnesses were of 0.9 mm nylon fishing line, while the fourth utilised 2 mm Teflon tubing.
The table below shows the results. All birds with tags attached by win loops failed to return after release, although one of them remained in the colony incubating for two days and was then replaced by its mate: the tagged bird was not seen again. One bird with a body harness also failed to return. A second bird with a body harness was at the nest site the day after marking with a part of the harness caught in the base of the bill, which could not be closed. The bird looked in poor condition, with unkempt plumage, suggesting that it had been unable to preen. Its mate had returned and was incubating.
The tag was removed and the bird released immediately. The next day it had left and two days later it returned looking healthy and it resumed incubation. The weaknesses of a tag attachment tube appeared to have contributed to the failure of this tag since one of the tubes had become detached from the body of the tag.
|No. failed to return||4||1||0||0|
|No. returned injured||0||1||0||0|
|Returned without tag||0||1||2||3|
|Returned uninjured with tag||0||0||1||0|
One bird with body harness and two with thigh harnesses returned without their tags but with no sign of injury, while all three birds fitted with tail attachment returned without their tags, all with the central pair of rectrices broken or missing.
Conclusion: For Sooty Terns, wing harnesses appear lethal and body harnesses may also be dangerous. We had no evidence of harm caused by thigh tags; the loss of two of these, but with birds returning in good condition, could have been due to poor judgement of the tension of the slip elastic or to weaknesses in the tag attachment tubes/loops. Tail attachment appeared unreliable due to breakage of the rectrices, possibly due to birds preening off the tags. The effect of feather damage/loss on the birds’ flight and foraging are not known.
We found the 2 mm slip elastic difficult to work with; it was difficult to tie and secure in position and superglue appeared to offer only a temporary hold. It was also difficult to judge the correct tension to apply and loss of the thigh harnesses and one of the body harnesses could have been due to the harnesses being insufficiently tight.
2014 trials with working GPS tags
In view of doubts over the use of harnesses we attached working GPS tags to the central pair of rectrices of incubating Sooty Terns. This proved successful for short-term deployment and seven tracks were obtained in 2014 (a paper describing this is in preparation). During their foraging absences the batteries were recharged, confirming that the solar cells received adequate sunlight and were not covered by feathers during flight. However, as in the dummy tag trial, tagged bids managed to remove the tags, normally involving breakage of the two central tail feathers but in one instance by loss of one feather and breakage of the other. The maximum duration of attachment was 6 days and all birds that returned more than 6 days after marking returned without their loggers. Soaned et al. (2015, Marine Ornithology 43: 235–239). However, all marked birds did return and, apart from the broken tail feathers, they all appeared healthy. Although there was variation between individuals there was no significant change in body mass between marking and return to the colony.
2015 trials with dummy tags using thigh harnesses
In 2015 we wanted to investigate thigh harnesses further to see if they offered opportunities for longer term deployment of GPS tags. We made dummy tags out of 8 mm x 8 mm plastic rod, cut into 25 mm length sections, painted black and drilled with 2 mm diameter holes at the front and rear of the tag to take the harness material. We trialled two harness materials, 2 mm Teflon tube and 1.25 mm diameter slip elastic (www.prestonsinnovations.com). The latter was much more flexible and manageable than the 2 mm diameter elastic used in 2014 and permitted much better control of the tension applied to the harnesses. The fitting of the tag required two people, one to hold the birds and the other to fit the tag.
The central point of c. 25 cm of the harness material was bent to form a double strand and this was passed through a loop made of fishing line at one end of the dummy tag. The two ends of the harness were inserted through the apex of the bend and pulled tight to secure the harness to the fishing line loop. The loose ends of the harness were then passed through a hole drilled at the other end of the dummy tag, the ends being pushed through in opposite directions. These ends were not tightened, so that the harness material formed loose loops on each side of the tag. During fitting, these loose loops were passed, one over each leg, with the tag held against the bird’s rump. Once in position, the loose ends of the material were gently pulled to tighten the harness until they passed neatly around the base of the inner thigh.
The ends of the harnesses were sealed using small aluminium ferrules (1.5 mm aluminium ferrule AF-15; www.s3i.co.uk ); these were cut in half to produce a crimp weighing 0.1 g. Once the ferrule had been fitted over the loose ends of the harness these were pulled until the fitter could lift the tag away from the bird’s back to enable the tip of the little finger to be inserted between tag and back. At this stage the ferrule was crimped using pliers to secure its position on the harness. Excess harness was trimmed off with scissors and the cut ends were sealed to the crimp with superglue. This was especially important with Teflon which, without this treatment, was highly prone to fraying. The weight of dummy logger, harness and crimp totalled c. 2.5 g.
Seventeen birds were fitted with dummy tags, eight using slip elastic and nine with Teflon tubing. Only one tag was lost – the first to be fitted using Teflon; the bird returned without its tag and continued incubation. All of the other tags survived while the birds undertook their incubation shifts and foraging trips. All of the birds were caught 10-30 days after fitting and the tags were removed and birds checked for feather or skin abrasion where tag and harness were in contact with the bird. The slip elastic of one bird had broken close to the crimp but the elastic was still firmly attached to the tag by its superglue. The tags and harnesses of all the other birds were in excellent condition and none of the birds showed any signs of skin or feather abrasion.
Attached by a thigh harness the tag was positioned c. 2 cm anterior to the uropygial gland and so is unlikely to interfere with the birds’ ability to preen. However, with working tags the antenna would lie above the gland, but not touching it. This might restrict the birds’ access for preening but observations are needed to determine this.
After marking the birds, their absences on their first foraging trips ranged from 1 to 11 days (average 4.67 ± 0.71 (s.e.) days). Changes in body mass between marking and return from the first foraging trip ranged from – 8 to + 54 g (average + 13.67 ± 4.46 g).
Thigh harnesses look promising for medium-term deployment of GPS loggers on Sooty Terns. In 2016 we want to use this attachment for GPS loggers to incubating birds, hoping to deploy them during incubation and recover them about a week after the eggs have hatched (in 2015 we demonstrated that this is possible using drop traps). This will allow us to determine the durability of working GPS tags, which are c. 0.7 g heavier than the dummies and, crucially, to check whether the solar panels recharge the batteries adequately, since at this stage we do not know whether working tags will be covered by back feathers when the birds are in flight. As we detected no difference between the durability of tags fitted with slip elastic and Teflon tube the former is preferred; in the event of our being unable to recapture a bird the tag will fall off after 2-3 months with the biodegradation of the elastic.
Our work on Bird Island was made possible by grants from the British High Commission (Seychelles), the Percy Sladen Memorial Fund, James Cadbury, Haslemere U3A and WildWings Bird Management. We are also grateful to Bird Island for continuing support for our seabird studies and to Gary Brodin and Ed Bryant at PathTrack for advice and practical help throughout.