[cobirds] Re: Caution on Sterna Terns- Boulder Res

Christian et al,

I think there is an important re-evaluation going on of the numbers of Common Terns nesting in the interior.

Directly to the north of Colorado, in several areas of Montana, are decent numbers of breeding Common Tern. For over 20 years, I have been fortunate to live within a few miles of a location where annually 30-50 pair breed. Forster's also nest in the area, with Common using rocky islands and Forster's using old goose nests or building in the marshy spots. I'd say over the years, the proportion in population have stayed fairly even.

Face mask and crown molt, is excellent for all distances on distinguishing between the two species, as well as wingtip patterns. Much less so on any breast or underside coloration. I do not find that reliable across the seasons nor on many individuals. Arctic sure. Common- much less so.

Very best to all,

Tim

Timothy Barksdale

Choteau, MT and Mokane, MO

On Friday, August 28, 2020 at 4:09:18 PM UTC-5 paja...@hotmail.com wrote:

Birders,

I'm quarterbacking from my couch here, but just thought I'd let folks know that the widely reported adult Common Tern at Boulder Reservoir is more likely a Forster's Tern. A few things to consider:

-The bill is how I would describe as, "honking big."

-Assessing bill color on distant terns is often futile. One person's orange is another's red. Also, it's important that bill color on these species changes through the season. They are waxing towards dark at the moment. 

-Forster's Terns are white below, while Common/Arctic are gray with white chins/throats. The photos of the Boulder Res bird all show a pale white underside color. 

-Flight and perched photos show the primary color being silvery gray with narrow black tips. 

-On the photos available, it appears that the bird is molting its crown feathers. The shadow of the typical Forster's Tern basic plumage pattern (the dark eye mask) is apparent. 

Some of the photos. The adult bird is in question: 

https://ebird.org/checklist/S72849883 
https://ebird.org/checklist/S72879869 

Happy birding,

Christian Nunes

Boulder, CO

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