Help with a tern in mouth of River Plate

[John Chardine]

For several years in a row now I've sailed out of the River Plate from Buenos Aires, Argentina, south to the Falkland Islands in early November. On every occasion I see a few of these terns once we get out into the mouth of the river. The first image shows the back and sides of head; note the buffy coverts at the leading edge. The second image shows the red legs and largely black bill with a patch of red on the lower mandible. I am thinking maybe Roseate Tern but I am stumped. Thanks for any help.

[Ilija Dukovski]

Could it be S. hirundinacea (South american) tern? What was the size of these birds compared to Common tern?

[John Chardine]

Hi Ilija- Thanks. This is of course a possibility but I would like to narrow it down to species. Hard to estimate size in relation to Common.

[Ilija Dukovski]

South american tern has darker outer edges on the flight and possibly tail feathers,
as is shown on your images. Also the bill is on the large side. Here's an image of
hopefully correctly IDed bird:
http://www.pbase.com/ahlman/image/128116339
I hope this helps.

[John Chardine]

Seems like a very reasonable solution Ilija. Thank you.

[Wayne Richardson]

Hi John

This is a juv/1st winter, so it would be useful to know the date, as that may eliminate some species altogether.

Cheers: Wayne

[John Chardine]

Thanks Wayne. As mentioned in the OP, date is early November. BTW the place I always see them is out of sight of land. Maybe this is where the young SATEs go?

[Wayne Richardson]

Thanks Wayne. As mentioned in the OP, date is early November.

You are quite right, I must have still been half asleep!!!

However, the date will rule out Southern Hemisphere species, given tern's age & state of moult. I'll have a closer look & let you know what I think!!

Cheers: Wayne

[John Chardine]

You are quite right, I must have still been half asleep!!!

However, the date will rule out Southern Hemisphere species, given tern's age & state of moult. I'll have a closer look & let you know what I think!!

Cheers: Wayne

Thanks Wayne. So let's think this through. If the bird were a juv/1st winter southern hemisphere species, it would have to have fledged sometime in say February of 2012, assuming normal seasonality of breeding. Would not a bird that was about 9 months old and still in its first year look like this? Most terns show bright red legs as an adult so the bird maybe in its 2nd year or older.

[Paul Guris]

John, I asked a buddy of mine (name withheld since I forgot to ask him last night if it was OK to use it) who has lead multiple tours to South America about this bird and he thought it was most likely South American Tern. He said that plumage-wise they are pretty similar to Common Tern but are obviously larger birds. Hope that helps a bit.

[Wayne Richardson]

Hi John

Sorry for the delay in getting back to you! However, I have had time to research the matter more thoroughly & there is a breeding plumage tern is currently in Cornwall. Links from an ID discussion thread for that particular bird has given me access to images of Southern Hemisphere terns. Safer than potentially wrongly captioned terns 'googled' on the web.

On range alone, we can dismiss the following species; Forster's, Kerguelen & Roseate. This would leave 2 Northern (Arctic & Common) & 2 Southern (Antarctic & South American) Hemisphere species to consider. As stated in my initial post this is a juv/1st winter individual, given none of the primaries or secondaries have been replaced. This is consistent for a Northern Hemisphere tern in November. Common replace their inner primaries (only) from January-February, although sometimes earlier, i.e. in December. Arctic start replacing all primaries & secondaries during December-January & is completed by April-May. This is from a hatch date of either June or July for both northern species. I have not (yet) found any moult information for either of the southern species, but it must be safe to assume that by November both should be in 1st summer plumage. If this stance is accepted, given hatch dates December-January or even February, then their primaries will have already been partially (like Common) or fully (like Arctic) replaced.

If we now confine ourselves just to the 2 Northern species, then the patterned secondaries alone rules out Arctic. This would leave us with Common as the bird in your images after this elimination process.


Cheers: Wayne

PS: The Cornwall tern is a Common too, although quite rightly, it warranted close scrutiny!

[John Chardine]

Thank you Wayne. This in-depth analysis is more than I could have asked for. We know NA commons go into the Caribbean and farther south to ne. South America. This puts them a lot farther south on a regular basis.

[Wayne Richardson]

Thank you Wayne. This in-depth analysis is more than I could have asked for. We know NA commons go into the Caribbean and farther south to ne. South America. This puts them a lot farther south on a regular basis.

Hi John

An update!!

I wondered whether our Common Terns follow the same 'figure of 8' migration path that Arctics do in the Atlantic &, if so, could your images be of European birds. The simple answer is 'No', as most western Palearctic Common Terns winter off the coast of NW & W Africa. That said, British Common terns have been recovered as far away as Australia & inland along the Amazon.

However, perhaps of more interest to you is South American data that proves that there is a build-up of Common Terns numbers in the River Plate from October onwards. It also confirm that North American Common Terns regularly migrate up & down both South American coasts with non-breeders summering both sides of the Equator.

Cheers: Wayne

[John Chardine]

Many thanks for the update Wayne.