CAC X ARG from southernmost Sweden

[David Erterius]

Hi all,

Have posted some photos with comments on Netfugl of a putative 1st w hybrid cac x arg from Simrishamn harbour in southernmost Sweden on the other day, Jan 20th:

http://www.netfugl.dk/pictures.php?id=s ... e_id=45927

http://www.netfugl.dk/pictures.php?id=s ... e_id=45928

http://www.netfugl.dk/pictures.php?id=s ... e_id=45929

http://www.netfugl.dk/pictures.php?id=s ... e_id=45930


On the same day, we also had another putative 1st w hybrid, even though it's maybe "not as good as" the other one:

http://www.artportalen.se/birds/gallery ... d=24317456


Comments would be appreciated!


// Best Wishes,

David Erterius, Sweden

[lou bertalan]

hi david,

i used chris'/brian's PCA (principle component analysis) to score this bird and the sum was 21 (20-22).
according to this your gull is well within caspian variability: 12-25, while hybrids have a score 22-32 and pure argentatus with a score of 29-37.
this supports my subjective view: most of the critical traits you mentioned (dark marked body, streaked head with mask, retarded scapular moult, barring in greater coverts) are well within caspian variation. however, the pattern of inner primaries is not used in this score analysis and this is the most conspicous and unusal feature shown by your bird: pale outer webs like in herring gull, unlike the vast majority of cachinnans. i think i have seen similar dubious birds somewhere (pics), but this is surely the most outstanding trait in your gull and of course might indicate a hybrid. but i would not be so sure. in my opinion, if there is no ring, even with the PCA scoring scoring system we won't be 100% sure of apparent hybrids/odd looking caspians. many very odd caspians have been photographed in their core breeding ranges, so i'd expect a very small minority even to show such a pale window. still, with the closeness of poland/lithuania/belo-russia/russia surely this might bear some argentatus genes in it...

PCA for david erterius’ presumed cach x tatus:

extent of scapular moult: 1
greater covert pattern: 1
ventral bulge: 0/1
primary projection: 1
greater covert moult: 5
median covert moult: 5 (4?)
tertial moult: 3
darkness of head and body: 2-3
first generation tertial pattern: 1
scapular pattern (2nd gen.): 2
_______________________________
sum: 21



best, and thanks for sharing,

lou

[David Erterius]

Thanks for your prompt and informative reply, Lou. Is it ok with you to copy and paste it to the photo discussions on the swedish report database "Svalan"?

[lou bertalan]

Thanks for your prompt and informative reply, Lou. Is it ok with you to copy and paste it to the photo discussions on the swedish report database "Svalan"?

sure is.

cheers, lou

[David Erterius]

I agree that except for the inner primaries, it's hard to say that this appearance doesn't fall within the variation of cachinnans. However, as usual when dealing with field identification of large gulls, in my personal view one must see to the general appearance of individuals like this. All characters together make me believe that this can't be a pure Caspian. And especially (as mentioned before) the pale outer webs on P1-P4 clearly supports the hybrid thesis.