In preparation for a field study on wild birds, we conducted an experiment on captive Great Cormorants Phalacrocorax carbo sinensis to evaluate methods of attaching GPS loggers. Loggers (65 g) were attached to two injured cormorants in a bird rehabilitation centre. One logger was attached to the first bird with TESA tape and another logger to the second bird with a teflon harness. The taped logger was attached on the lower back, and subsequently on the upper back and the tail. This logger was removed within 10 (lower back), 4 (upper back) and 15 (tail) days by the cormorant biting off and pulling out the feathers to which it was attached. The logger deployed with the teflon harness on the second bird stayed on for 21 days, after which we removed it undamaged.

With the increasing use of tracking devices for ecological research on birds, several different methods of attachment have been developed (e.g. Kenward 1985; Wilson et al. 1997; Tremblay et al. 2003; Mallory & Gilbert 2008). Two of the most commonly used techniques for seabirds are attachment by a harness (often made out of teflon) (e.g. Gyimesi et al. 2011), or with an adhesive tape (TESA is the most commonly used brand; e.g. Ropert-Coudert et al. 2006). Other methods to attach loggers to birds are the use of cable ties (e.g. Grémillet 1997), or a mix of tape and cable ties (e.g. Wanless et al. 1997), or a mix of tape and glue (e.g. Cook et al. 2010). Members of the Phalacrocoracidae, the family of cormorants, have also been deployed with different types of loggers in the past. Loggers deployed on species of shags and smaller species of cormorants have been attached with tape to the feathers on the back (Wanless et al. 1997; Cook et al. 2010) following the methodology described by Wilson et al. (1997). The methods used to deploy loggers on larger species such as Great Phalacrocorax carbo and Double-crested Cormorants P. auritus have been more diverse. Often devices were attached to the middle two or middle four tail feathers with a combination of tape and cable ties (Grémillet 1997; Grémillet et al. 1999) although attachment with tape on the back has also been used (Grémillet et al. 2006; Ropert-Coudert et al. 2006). In these studies tags stayed on the birds for periods ranging from 1–9 days, but often loggers were not lost after that period but were deliberately removed by the researchers to allow data downloading. Less common is the use of a backpack harness for cormorants but it has been successfully used in tracking Double-crested Cormorants in the USA and those devices stayed on the birds for more than one year (King et al. 2000; Guillaumet et al. 2011).

We planned to use GPS loggers to study foraging behaviour of Great Cormorants P. c. sinensis in the south-western part of The Netherlands during the breeding season of 2012. As it was intended for the loggers to be attached to the birds throughout the chick-rearing stage (> 12 weeks) and with a small- scale sampling scheme, we used specially made loggers that weighed 65 g. We trialled four logger attachment methods to study the potential consequences of logger deployment on this species, determine the ideal harness size for Great Cormorants, and to test the duration of deployment per method.

This study was done within the framework of the monitoring programme into the effects of the compensation measures designed for the construction of the seaward expansion of Rotterdam Harbour (‘Tweede Maasvlakte’). This programme (MEP-MV2) was initiated by the Dutch Ministry of Infrastructure and the Environment. The Bird Rehabilitation Centre ‘De Bonte Piet’ in Midwoud, and especially Marije de Heer, are greatly acknowledged for their hospitality during this project. We thank Martin Heubeck, Mark Newell and Ellie Owen for their input to improve the manuscript.

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