Volume 47, No. 1

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Evaluation of small unmanned aerial systems as a census tool for Aleutian Tern Onychoprion aleuticus colonies


1US Fish & Wildlife Service, Kenai National Wildlife Refuge, PO Box 2139, Soldotna, AK 99669, USA (
2US Fish & Wildlife Service, Alaska Maritime National Wildlife Refuge, 95 Sterling Highway, Homer, AK 99603, USA


MAGNESS, D.R., ESKELIN, T., LAKER, M. & RENNER, H.M. 2019. Evaluation of small unmanned aerial systems as a census tool for Aleutian Tern Onychoprion aleuticus colonies. Marine Ornithology 47: 11 - 16

Received 10 May 2018, accepted 24 August 2018

Date Published: 2019/04/15
Date Online: 2018/10/31
Key words: unmanned aerial system (UAS), tern nesting surveys, Aleutian Tern, Onychoprion aleuticus, disturbance


Aleutian Tern Onychoprion aleuticus numbers in Alaska appear to be in decline; however, colonies are difficult to count for the purposes of monitoring due to their co-occurrence with Arctic Terns Sterna paradisaea, low nesting densities, high variability in attendance, sensitivity to human disturbance, and remote terrain. We paired visual observations with unmanned aerial systems (UASs) to test the feasibility of using this technology to survey a small colony of nesting Arctic and Aleutian terns in southcentral Alaska. We used counts of terns in the air and nest attendance to test for disturbance from UAS flights. We conducted 11 UAS flights over seven days at altitudes ranging from 15-30 m and located 23 nests in the 4.1 ha (0.041 km2) colony site (5.6 nests ha-1) by systematically searching the orthomosaics. We were most likely to distinguish tern species in the 18-m and 15-m altitude photos; the white forehead was most visible in side angle rather than top-down. Nest attendance and the number of aerial birds were not influenced by the UAS in this colony, which experiences high Larus gull activity. Aleutian Terns arrived on 19 May and were attending nests by 02 June; nest attendance declined throughout June. We believe the best approximation of total nesting pairs will be achieved by counting nests 7-10 d after first initiation. In mixed species colonies, we recommend that UASs fly in a 'lawn mower' pattern with overlap to maximize side angle images and aim for an image resolution of < 4 mm (which was achieved by 15-m altitude flights with our camera setup). In single species colonies, 30 m is sufficient and can be achieved more efficiently.


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