Tuesday, 25 August 2009

Ageing immature wood owls

Received the specimen depicted c/o a friend from Garleton Hills on Sunday (retrieved from a spot where I'd seen one seated on the road in Feb, perhaps same?). The variable lengths on the inner flight feathers immediately confirm this is not a bird of the year (a juvenile would have a set of equal length feathers) but since moult of adult wood owls is generally quite complete (cf. Barn Owl) it might initially be expected that nothing more could be deduced on its age. However, more careful analysis suggests it could be a 2nd-summer - all the primaries are apparently new (P8, i.e. 3rd feather in from outer P10, is either missing or hidden on left wing) - though they can all be replaced in 1st post-breeding moult the innermost and outermost feather are usually retained, suggesting the bird is more than a year old; but there is clearly a retained juvenile secondary on each side in images immediately above, the browner feather with more irregular subterminal bar which is probably S2, with S1 and S4 still growing, rather behind on the left wing compared to the right; S3 on both wings fully grown. Secondaries aren't completely replaced in the 1st summer and the centre at S2 may not become active (per BWP), so this is expected on a 1st-summer and presumably may also occur on 2nd-summer but not beyond, hence the tentative conclusion on age. However, there are remaining uncertainties - if S3 is replaced this ought to have followed S2 so either a feather is missing (uncounted) or the sequence can be more irregular than suggested!

Certainly, things can be rather complex and it's interesting to compare with this 2nd-winter bird which still has juvenile primaries retained at P2-P4. Hopefully the post mortem analysis will generate extra information in due course - will update this page with any further details of age/sex.

Also, note the similarity with the Long-eared Owl shown below, which was found to be a 2nd-winter female via post mortem analysis - also showing (3) retained juvenile secondaries. Based on plumage details I had thought this bird was a male, for example the underwing covert patch was modest, as per lower pics (measured the tips at c. 2cm - and BWP states (for female) "greater number of greater primary under wing-coverts with black tips, these tips usually broader than in male (up to 3–3.5 cm rather than up to 1–2)" - but this was wrong, the post mortem finding: "immature ovary normal, uterus pregravida". Lots to learn!

Henk Jan Koning had commented on age of Tawny mentioned above so I emailed him regarding the bird depicted here. He kindly provided a very detailed reply (and gave permission to post here, though emphasising this was a tentative analysis and he would have preferred to examine the bird in the hand). Full details as per Comments box and annotated images below, the moult patterns can be seen in this linked spreadsheet. An example of the alternating moult pattern in successive years is shown in the next picture. A final comment on this bird is it seems to have moulted rather few primaries last year, and even fewer secondaries, suggesting it was in poor condition - a situation consistent with last year's relatively poor breeding performance by local owls (probably reflecting a low in vole numbers).

Post-script - in spring 2010 Henk kindly forwarded this comparison shot of juvenile and 1st-summer Tawny Owls - the differences are striking; NB - these owls are part of Henk's study and are alive, see eye top left!

1 comment:

  1. Comments received from Henk van Konig via email:

    Dear Stephen,

    For aging the bird we look at color differences. Unfortunately pictures aren't the ideal situation for detecting such differences as you cannot turn the wings into the light. Also not every feather is (sufficiently) visable to be judged. However on yours some feathers show clear differences in coloration. In the attachments you'll find your pictures with the feathers marked as I think the may have been molted. J=juvenile N=new 1=done in the former (first) year. The bird is clearly molting as can be seen from the missing feathers and as it should be doing this time of year.

    When looking at the wings one sees clearly that there are some very old feathers, I think that these are juvenile because of there appearence. The 1st secondary of the right wing shows an irragular end-band; tippical for juvenile plumage. It's less clear in the other old secondary of this wing and I'm having my doubts on that feather wheather it's juvenile or done last year. This is similar in the left wing. Also in the left wing the outermost two primaries can be seen; these are also vey worn and the second (from outside) also shows an irreguler last band.

    Furthermore one can clearly see some new feathers (some are still growing) from this year's molt (marked with N). The have un-worn edges and the dark parts are darker than those on the other feathers. Additionally the banding is regular and these are clear new adult feathers.

    This leaves us some other feathers. Most of them are primaries as the rest of the secondaries are not shown on the pictures. These feathers are slightly worn (lighter than the new ones), but show correct (adult) banding. Therefore I think that this is an adult bird that died during the molt between it's second and third year. Being busy changing it's last juvenile feathers into adult ones.

    When looking at molt I found that most tawny owls start to molt in there second year at primary 5 (from inside) outwards and change most of the outermost 6 primaries (5-8 in the left wing here) and only few (2 or 3) secondaries. In the following molt they usually change primaries 1-4 (from inside, as can be seen also in this bird), most of the secondaries that waren't done in the first molt (this owl is clearly busy molting them) and some of primaries that waren't done the first year or even some of the primaries extra that were already molted in its first molt. When finding an older bird than this (or this bird after it's molt) you'll usually find no more juvenile feathers and than one can only conclude that it's older than 2 years. In the attachmensts you'll find some graphs based on the molt patterns that we have examined. For each age (2 and 4 in graph 1; 3 and 5 in graph 2; 2 and 3 in graph 3) the percentage of the owls that molted that feather that year was calculated. You can see a clear difference in these patterns especially in graph 3. And one can see that patterns are rather similar between even and uneven years.

    I hope that I answered your question good enough, forgive me if my English isn't entirely correct. And if you you have any further questions: just ask them.

    yours sincerely, Henk.