'Ageing & Sexing a Snowy Owl: is it possible?' by Dave Suddaby.
On the 9th September 2006 I was fortunate to find a Snowy Owl (Bubo scandiacus) on Tarmon Hill, at the southern end of the Mullet Peninsula, Co. Mayo. Unbeknown then, this began an unprecedented opportunity to study this magnificent bird over time as it has returned to the same area of the hill in each of the following three autumns.
At the time of finding, the bird was aged as ‘at least a second-winter’ and sexed as a ‘female’. The reasoning for this being ‘clean’, broad, dark brown barring on the tertials, the contrast between old and new primaries, the presence of two or three ‘broken’ tail bars and the general impression of the bird as ‘small headed / big bodied’. As such the record was documented in the 2006 Irish Rare Bird Report as a ‘second calendar-year female’ (Milne & McAdams 2008). In fact this is incorrect, it should have been documented as a ‘third calendar-year female’.
Very little has been published on the ageing and sexing of Snowy Owls; the main reference is Josephson (1980) which is based on the examination of museum specimens. Other than that, Pyle (1997) gives features based on feather shape and colour characters for use with birds in the hand whilst BWP (Cramp 1985) summarises features and offers useful clues. Josephson (1980) suggests that recognising age categories other than either first-winter or adult isn’t possible and McGowan (2001) doubts whether a bird could be recognised as ‘second-year female’ unless trapped; whilst Hough (1992) suggests that it is possible to age second-winter birds in the field on plumage characteristics. So how can we be confident that this was a ‘third calendar-year female’?
Josephson (1980) aptly sums up the difficulties with ageing and sexing by stating ‘it is difficult to age Snowy Owls unless sex is known and vice versa’; that is the conundrum to decipher. Although adult males are unlikely to be confused with first-year females, adult females can show considerable overlap in plumage characteristics with first-year males, therefore it is important to correctly determine the sex of the bird. The key features with sexing a bird relate to an assessment of size, the extent of the barring on the upperparts and underparts (along with the white ‘bib’) and the presence, extent and number of tail bars.
Females are known to be larger than males; the wing and tail lengths being significantly longer, however this is obviously difficult to assess in the field. Females are up to 20% larger and tend to look big bodied with a small head compared to more evenly proportioned males. Of course impressions of shape change depending on the bird’s behaviour but in relaxed mode the Tarmon bird was noticeably ‘small headed and big bodied’ hence suggestive of it being a female. Other indications of this relate to the extent of barring on the body; this is significantly denser and thicker on the underparts of females compared to males, regardless of age. Coupled with this, the barring on the upperparts of females is dense and wide. These plumage characters are evident in the Tarmon bird. Another feature often quoted is the extent of the white ‘bib’, although this is variable depending on the bird’s position, it does overlap considerably between first-year males / females and adult females and therefore is not a reliable feature. Similarly, the darkness of the nape patch is not considered to be a reliable feature.
The one ‘measurable’ feature to discern the sex of a Snowy Owl is the extent and number of any tail bars that are present. Females show at least three (first-year females can show up to five) complete or near complete bars which tend to be broad, whilst males show three or less rather narrow and mostly broken bars (or none as in some adult males). When assessing this, the key feathers are the central tail feathers (t1). On the Tarmon bird, there are two near complete bars and a third incomplete bar, the barring is broad and evenly spaced. Crucially, the Tarmon bird shows three bars on the central tail feathers and the barring of one extends onto the outermost tail feathers. In addition, the shape of the central tail feathers is broad and slightly rounded (as apposed to narrow and ‘bluntly pointed’). These features clarify the sex of the Tarmon bird as a female.
Ageing of Snowy Owls on plumage characteristics generally revolves around the presence of any brownish suffusion / mottling evident on the closed wing which often obscures any barring. This is generally only evident in first-year birds. Following the moult in their second year the barring becomes much more clearly defined. The extent of barring overlaps considerably between the ages and sexes (although adult males are unlikely to be confused with first-year birds) and hence it has generally been considered that ageing beyond either first-winter or adult isn’t possible. However, a feature for ageing beyond first-year (and up to at least third calendar-year) is to assess any moult sequence within the primaries, particularly with a bird seen between June and October (the moult period). Snowy Owls, from their second year, undergo an incomplete primary moult each summer / autumn. This generally follows predictable patterns and, as long as different generation feathers can be distinguished i.e. old from new, the pattern can be used for precise ageing up to third and possibly fourth calendar-year (Pyle 1997). During the first year, all the flight feathers are retained through to the following summer (second calendar-year). At this stage, all body feathers, tertials, some secondaries and usually primary (p) 7 (descending from outer) and perhaps occasionally p6 and p8 are replaced. In the following years, primary replacement generally follows the pattern of p8, p9, p6, and possibly p5 and p1 (third calendar-year) with the last of the primaries being replaced in the fourth calendar-year or perhaps into the fifth calendar-year (Cramp 1985). After their third calendar-year the differences between old and new primaries is slight.
With the aid of high-quality photographs an assessment of the moult sequence can be made of the Tarmon bird. There is a clear marked contrast between old and new primaries (less so within the secondaries but it is evident), and these relate to p1, p5, p6, p9 and p10 being replaced - although not in that order. This sequence and the marked moult contrast clearly indicate that this bird is in its third calendar-year. This was further confirmed when the bird returned to moult in the following autumns. In 2007, the primary moult sequence was p2, p3, and p4 (fourth calendar-year) with completion in 2008 when the moult sequence was p6, p9, and p10 (fifth calendar-year). The difference between old and new feathers has become less marked each autumn. In 2009, the moult sequence started again with p1 - p2 and p7 - p8 being replaced; this fits with the serial moult sequence for adults centred on p7 and p1 with two to six primaries being replaced each season and the full moult completed over three seasons (Cramp 1985).
So in 2006 the Tarmon bird can be confidently aged and sexed as a third calendar-year female. We can also be confident that this is the same bird that has returned each autumn (although only by studying photographs of the open wing - otherwise this would be an assumption based on the bird’s choice of location and habitat) by the sequence and shape of individual bars present on the primaries which have been consistent throughout. This therefore has presented an unprecedented opportunity to look for any changes in the plumage in relation to a known-age individual. The age at which the definitive adult plumage is attained is unknown and as Witherby et al (1952) stated ‘there is no proof that birds get whiter after their second winter’.
Up to the 2009 autumn the plumage of the Tarmon female has remained consistent in the extent and density of barring on the body. Indeed, females only show a slight difference in underpart barring with age (Josephson 1980). However it is evident that, now in her sixth calendar-year, this female has become distinctly whiter as a consequence of less barring to the underparts and head - inviting potential confusion with a male. Although overall the bird appears whiter, the upperparts are still heavily barred and the tail still retains the pattern as in her third calendar-year. On this evidence it would appear that definitive adult plumage is not attained until at least into the sixth calendar-year. It will be intriguing to see this bird again in the coming autumns.
Click on the thumbnail image for more details.
Many thanks to Dave McAdams, Steve Blain, Colin Bradshaw, Derek Charles, Michael Davis and Pat Lonergan for improving this note with their photographs. Also to all the other photographers who made their photos available on the various web sites.
References & Useful Reading
Bannerman, D.A. & Bannerman, W.M. 1966. Birds of the Atlantic Islands. A History of the Birds of the Azores. Oliver and Boyd. Edinburgh and London.
Cramp, S. (ed.) 1985. Handbook of the Birds of Europe, the Middle East and North Africa, The Birds of the Western Palearctic, Volume 4 - Terns to Woodpeckers. Oxford University Press.
Hillis, J.P. 2004. First Annual Report of the Irish Rare Breeding Birds Panel, 2002. Irish Birds 7(3): 375-384 (378).
Hough, J. 1992. Snowy Owl plumages. Birding World 5(3): 96-97.
Hume, R. & Boyer, T. 1991. Owls of the World. Dragon's World, Limpsfield.
Josephson, B. 1980. Aging and sexing Snowy Owls. Journal of Field Ornithology 51: 149-160.
Kennedy, P.G., Ruttledge, R.F. & Scroope, C.F. 1954. Birds of Ireland. Oliver & Boyd, Edinburgh and London.
McGowan, K.J. 2001. Snowy Owls (Nyctea scandiaca): some recent visitors and thoughts on aging and sexing [online]. Available from: http://www.birds.cornell.edu/crows/snowy.htm [Accessed December 2009].
Mikkola, H. 1983. Owls of Europe. T & AD Poyser, Calton.
Milne, P. & McAdams, D.G. 2008. Irish Rare Bird Report 2006. Irish Birds 8(3): 395-416.
Pyle, P. 1997. Identification guide to North American birds - part 1. Slate Creek Press, Bolinas CA.
Robinson, M. & Becker, C.D. 1986. Snowy Owls on Fetlar. British Birds 79(5): 228-242.
Sangster, G., Collinson, J.M., Knox, A.G., Parkin, D.T. & Svensson, L. 2004. Taxonomic recommendations for British birds: second report. Ibis 146: 153-157.
Witherby, H.F., Jourdain, F.C.R., Ticehurst, N.F. & Tucker, B.W. 1952. The Handbook of British Birds, Volume 2. H.F. & G. Witherby Ltd., London.