Monday, 23 January 2012

SEO ageing and sexing

The White Sands quarry SEO's have attracted a stream of admirers and photographers over recent weeks (I believe 10-12 carloads present on some occasions!). Not been there myself but with so many high quality photos it really ought to be possible to age and sex these birds, even to pin down which is which. Having also recently collected an SEO casualty at Blindwells relevant features can also be illustrated, as confirmed by post mortem details from vet.

So in these shots, the flight feathers (above) provide the first clues being very white below, and inner webs of secondaries almost completely white (albeit overexposed) - these are consistent with a male (female has bars on inner webs, with underwing coverts buff); the tail (below) is sufficient to determine both sex and age, first of all confirming sex in the very few faint bars on inner web of outer feather (female had 3-5 more distinct bars across both webs); tip of central pair of tail feathers gives the age, shown well here with broad dark streak along shaft and obvious dark markings on pale edge panels, juveniles have a tapering narrow dark streak down centre with very limited markings on sides (similar to the outer tail feathers shown here). For reference, the vet made this an "imm m", based on "immature testes black" - so given the plumage features with juvenile tail replaced almost certainly a second-winter.

Applying these features to White Sands birds and others is a challenge as some clues are concealed or hard to see in flight or when perched. The diving shot by Ronald Richardson (halfway down Dec images, dated 18/12/11) shows the upper tail and seems consistent with the juv pattern on central feather - need higher res image to confirm. The flight shots by Mike Thrower and Ron McCombe on BirdGuides thread don't really show any of the above features but are distinguishable in terms of flank streaking and basal colour of underparts, the narrow streaks and pale wash on Thrower images suggest its a male, vice versa on McCombe images is more suggestive of a female. Easier to see, the facial disk of females should also be obviously darker, being "more extensively and deeper buff" on a female. Careful comparison of plumage details of Richardson and Thrower shots suggest they're the same bird, so I (tentatively) suggest this may be a juv male.

Below is another image c/o Abbie, taken 11/1/12, which shows one of four birds present that day; it seems to be a different individual than both those linked above, the very dark facial disk may indicate that it's a female and what is visible of the tail tip is entirely consistent with a juvenile. Reported to be hunting on its own, away from other 3 owls. As ever would be nice to confirm but interesting if at least 2 of the 4 birds are juvs, information elsewhere on the internet (one link here) mentions a bumper breeding season for owls and harriers in Scandinavia last summer, and certainly there was an above average number of SEO reports locally last autumn.

Further to above, if it was known "which was which" other interesting questions could then be addressed, in the mid-air tussling shown in various images who is fighting with who, and who is dominant? e.g. the bird on the left here may be female?

An unrelated aspect which makes me curious is the daily pattern of these birds - if, as reported, they only show sometimes for 15 mins an hour before dusk, then where are they for the rest of the time?! BWP mentions nocturnal hunting being commonplace but of unknown duration/importance. Use of night vision may assist in discovering more but I guess is going to need some very dedicated observer!

Final PS - nice blog post on conditions for good owl food here.

[Refs: Birds of the Western Palearctic, Javier Blasco's Identification Atlas of Birds of Aragon]

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