Sighting Record for Utah

Common name:	Arctic Tern
Scientific name:	Sterna paradisaea
Date:	8-8-17
Time:	~420pm
Length of time observed:	20 minutes
Number:	1
Age:	adult
Sex:	unknown
Location:	Willard Bay State Park, air boat launch ramp
County:	Box Elder
Latilong:
Elevation:
Distance to bird:	20 to 200+ feet
Optical equipment:	Swarovski EL 10x42 binoculars
Weather:	mostly clear, some breeze
Light Conditions:	mostly good, sometimes the bird was backlit but sometimes ideal lighting
Description: Size of bird:	a small-ish tern, slightly smaller than Forster's
(Description:) Basic Shape:	longs wings & tail, small body
(Description:) Overall Pattern:	gray, white, & black
(Description:) Bill Type:	short and pointy
(Description:) Field Marks and Identifying Characteristics:	I first noticed a slightly smaller tern (than Forster's) with a noticeably dark belly, a full dark cap, and a relatively dark reddish bill as it flew past and I suspected an adult Common as that would be much more expected. Then I saw a very thin dark trailing edge to the outer primaries and remembered that being a fieldmark for Arctic so I kept watching as it came by every 30-60 seconds. Over the course of 10+ minutes I had enough views to discern a relatively small bill, rounded head, long tail feathers with dark on the outer web, and very contrasty plumage such that a narrow pale cheek separated the black cap from a dark gray belly. I called out to others to take a look and fortunately Sherry had a camera! (see photos)
Song or call & method of delivery:	I heard one soft call while in flight
Behavior:	Flying back and forth along water channel in loose company with ~10 Forster's Terns
Habitat:	Water channel surrounded by open habitats at edge of Great Salt Lake
Similar species and how were they eliminated:	Forster's and Common Tern. Forster's has a lighter orange and larger bill, much paler upperparts, and pale outer webs to tail feathers. Common is most similar to Arctic but is differentiated especially by having a larger bill and a wider area of black on the primaries. Leg length can be an additional feature (long in Common than Arctic) but - as this bird was in flight for the entirety of the observation - I wasn't able to assess this feature.
Previous experience with this & similar species:	I have seen at least 50 Arctic Terns - mostly in Alaska and on several pelagic trips but also several individuals in Idaho and Delaware.
References consulted:	Mostly Sibley but also the Collins Bird Guide
Description from:	From memory
Observer:	Jay Carlisle
Observer's address:	4202 N Whitehead Street
Observer's e-mail address:	**
Other observers who independently identified this bird:	Sherry Liguori (took photos) and several others that were a part of our field trip
Date prepared:	8-11-17
Additional material:	Sherry Liguori took several photos at the first sighting
Additional comments:	My eBird checklist with photos embedded is here: https://ebird.org/ebird/pnw/view/checklist/S38550447. Bryant Olsen and Matthew Pendleton re-found the bird on the following day and took many photos that are embedded in this eBird checklist: https://ebird.org/ebird/view/checklist/S38567850.