This description of ageing Great Grey Owls is based on the article: Suopajärvi, P. & Suopajärvi, M. 1994. Lapinpöllön iän määrittäminen. (English summary: Ageing of Great Grey Owls) Linnut 29: 2/1994 27-30.

Some background

In the post-juvenile moult Great Grey Owls do not moult their primaries, secondaries, tertials, primary coverts or tail feathers. To distinguish these feathers from corresponding adult feathers forms the basis of ageing.

Figure 1. Numbering of primaries (P) and secondaries (S). Feathers are numbered so that 1st primary is beside 1st secondary.

Moult pattern

In handbooks the moult of Eurasian Great Grey Owls is poorly described [1,2,3]. According to our data, Finish Great Grey Owls have their post-breeding moult in June-September - at the same time with the North American nominal race (S. n. nebulosa) [4].

The museum skins showed first signs of the moult in the beginning of June (6 June one secondary missing from both wings) and on the other hand at the end of September new flight feathers were almost full-grown.

Capturing live birds during breeding time indicates that moult of nesting birds does not start before July; males and females have been captured before mid of July and only twice we have met a moulting bird. It is also exceptional to find feathers from the ground near the nest.

We have not had opportunity to study how birds moult if they do not breed.

The following description of the moult of Great Grey Owls is based on moult patterns of sub-adult birds.

Post-juvenile moult

In the post-juvenile moult Great Grey Owls do not moult their primaries, secondaries, tertials, primary coverts or tail feathers. To distinguish these feathers from corresponding adult feathers forms the basis of ageing.

Moults of adult birds

In the second calendar year spring one-year-old owl has a complete moult which covers usually all tail feathers and part of the primaries, secondaries and primary coverts. Secondaries are moult quite similarly with Eagle Owl (B. b. bubo) [5]. Moult starts from the innermost secondaries and continues towards the wing tip. However, before this moult wave reaches the secondary number five, a new wave starts from there and it travels inwards. The third moulting cycle starts from the second secondary, but usually this does not happen until the next summer. Primary moult starts from the fifth primary and moulting proceeds centrifugally from this moult centre. In autumn, after the moult, second calendar year birds have four to nine juvenile-type secondaries, six to ten primaries and four outermost and sometimes (always?) some innermost primary coverts left. About 10 % of birds of this age class had retained all primaries.

Older birds have a similar moult each year in which they shed some flight feathers. The last juvenile secondary (1st or 4th) and last primary (1st or 10th) will usually fall when the bird is 3 or 4 years old. However, one Great Grey Owl, completely in adult plumage, had replaced in one moult 7 primaries and 5 secondaries. This indicates that it may be possible for Great Grey Owl to moult all the flight feathers in two years. The last primary coverts are moulted probably when the bird is 2 or 3 years old. The Swedish records of birds with known age support our theory. Two birds living their 3rd winter (3Y/4Y) had still one or two juvenile primaries (1st or 10th) and one secondary (1st or 4th) left. Two birds, living their 4th and 5th winter, had no juvenile feathers remaining.

Distinguishing feathers

Pattern, shape and by some means colour distinguish juvenile feathers from adult feathers (Figure 2). Dark bars are narrower and the outermost bar is closer to feather tip in juvenile feathers. Further more, these feathers are often also slightly lighter brown, white tipped, and slightly narrower and more pointed than adult feathers.

Primaries and secondaries

Secondaries have the most significant identification mark for ageing; an "extra" narrow and fuzzy dark band, which forms a characteristic tapering figure to the tip of the feather . Especially on the inner web this band does not reach the edge of the feather. The second band is almost as far from the tip of the feather as the first band in adult feathers (about 25 mm)

Primaries can be identified with same principles as secondaries.

Tail feathers

Tail feathers of adult type are broad and the tip is dark and round. Juvenile feathers are narrower, white tipped, more pointed and the tip also frays easily. Due to wearing out, the white tip of the feather is less visible after January - March. The dark bands are quite similar as they are in secondaries of corresponding type.

Primary coverts

Primary coverts of juvenile type have a narrower outermost dark band and it is closer to the feather tip than in adult feathers. In adult feathers, the end of the feather is almost as dark as the dark bands are, but in juvenile feathers the end has the same colour as the light bands have.


The growth of the wing can be used to determine the age of the nestling. This reference data is collected from nest first visited during egg laying or hatching time and the hatching date of each nestling is calculated based on egg laying interval, incubation time and hatching interval collected from literature. The nestlings whose age is known based on true hatching date are marked with * to the figure. The accuracy of the age determined based on the wing length is roughly +- 4 days. The length of the wing is measured with maximum method, and the number of measurements in the data is: 15 broods, 36 nestlings, 53 measurements, and the age of 11 of these nestlings is known based on hatching date.

Age counter

Computes the age of great grey owl nestling, when you input the length of the wing. Note that the result is not reliable if the wing is longer than 250 mm.

Wing [mm]   Age [d]

Ageing owls in autumn after moult and in spring before moult

1Y autumn / 2Y spring

In the first winter (1st calendar year autumn and 2nd calendar year spring) all secondaries, primaries, tail feathers and primary coverts are juvenile.

Figure 3a & 3b. Wing and tail of a young Great Grey Owl in autumn (1Y). Note white tipped feathers and a outermost dark band which is close to feather tip.

In the picture below young Great Grey Owl in spring (2Y). (By clicking the picture opens bigger image.)

2Y autumn / 3Y spring

In the second year's autumn a part of secondaries and usually a part of primaries and primary coverts have been moulted to adult feathers. Adult wing feathers which all are of the same colour form a clear contrast against brownish juvenile feathers. The number of juvenile secondaries normally varies between 5 and 9 and often these are in one group, although it is quite common that either the 5th or 6th secondary has changed but the 7th remains unchanged. None or some primaries (normally up to 5) has changed. New ones are among the middlemost primaries. Usually 4 outermost primary coverts (and probably some innermost ones too) has remained unchanged.

All the tail feathers have usually changed but rarely some juvenile feathers have remained.

The bird in the figure has partially shed it's primaries secondaries and primary coverts. It has not shed primaries 1-3 and 7-10, secondaries 1-8 and ore some of the outer and inner primary coverts.

3Y autumn / 4Y spring

This age class has changed normally 5 - 8 primaries and also the number of changed primary coverts is greater than with the previous age class. In secondaries there are still 1 - 4 juvenile feathers usually separated by adult feathers from each other. In secondaries, and in most cases also in primaries, there are adult-type feathers from two moults, which can be distinguished from each other from their abrasion and fading i.e. from the shade of colour.

The figure shows a Great Grey Owl that has moulted in the first moult secondaries 5, 6, 9 and 10 (-12) and primaries 4 and 5. In her second moult she has shed secondaries 2,7 and 8 and primaries 2, 3 6-10.

4Y autumn / 5Y spring

When the bird is 3 years old there may still be single juvenile feathers remaining in wings. If above and beyond, it is possible to distinguish three age classes of adult-type feathers one can use age classification 3Y/4Y.

Authors have captured two Great Grey Owls in spring who had only one single juvenile-type feather (1st and 2nd primary) and the adult-type feathers were from three moults.

Other age classifications

+1Y autumn / +2Y spring

If the wing is a mixture of adult and juvenile-type feathers, but it is unclear how many age classes the adult-type feathers present, one should age the owl as +1Y / +2Y. This means that the bird is living its, at least, third calendar year in autumn, and at least fourth in spring. This classification should also be used if the moult pattern of sub-adult bird does not conform to what is described in "2Y autumn / 3Y spring" or "3Y autumn / 4Y spring". By studying only the tail one can distinguish two age classes 1Y / 2Y and +1Y / +2Y.

Figure 6. Tail of an adult +2Y Great Grey Owl in spring. Tips of the feathers are dark and round and feathers are broader than in Figure 3. More accurate ageing requires that also the wings are studied.

+2Y autumn / +3Y spring

If all primaries or all secondaries or all primary coverts are moulted adult-type feathers but it is unclear how many age classes these present one should age the owl as +2Y / +3Y. This means that the bird is living its, at least, third calendar year in autumn, and at least fourth in spring.

Figure 7. Sub-adult (+2Y) Great Grey Owl in June. Four outer secondaries as well as three outer and three inner primaries are juvenile. Note the contrast between adult and juvenile feathers.

+3Y autumn / +4Y spring

One can use age classification +3Y / 4Y if there is no juvenile-type feathers left and the adult-type feathers are from three moults and can be distinguished from each other from their abrasion and fading i.e. from the shade of colour.

Figure 5. A Great Grey Owl that has moulted at least once all flight feathers (+4Y spring). Also the palest feathers are adult type feathers. It is possible to distinguish three age classes of adult-type feathers: P2 and P3 dark, P4 pale, P5 intermediate. Note broad bands in primary coverts. (February).

Although we know that is possible that some owls have feathers from four moults, we do not recommend to try to find this many age classes of adult-type feathers.


Many thanks to Oulu University and specially to Risto Tornberg there by making it possible to study and photograph owls.

Litterature of the article:

1. Cramp, S. & Simmons, K.E.L. (toim.) 1985: Handbook of the Birds of Europe, the Middle East and North Africa. The Birds of the Western Palearctic. Vol. 4. - Oxford University Press, Oxford.

2. Dement'ev, G.P., Gladkov, N.A., Ptushenko, E.S., Spangenberg, E.P. & Sudilovskaya, A.M. 1951: Birds of the Soviet Union (Ptitsy Sovetskogo Soyuza) Vol. 1. - Israel Program for Scientific Translations, Jerusalem (1966).

3. Gluz von Blotzheim, U.N. & Bauer, K.M. 1980: Handbuch der Vögel Mitteleuropas, Band 9. - Akademische Verlagsgesellschaft, Wiesbaden.

4. Hunter, D.B., McKeever, K., McKeever, L. & Crawshaw, G. 1987: Disease Susceptibility in Owls - Teoksessa: Nero, R.W., Clark, R.J., Knapton, R.J. & Hamre, R.H. (toim.): Biology and Conservation of Northern Forest Owls. 3.-7.2. Winnipeg, Manitoba, s. 67-70. - U.S. Department of Agriculture Forest Service, General Technical Report RM-142. Fort Collins, Colorado

5. Niiranen, S. & Haapala, J. 1987: Huuhkajan iän määrittäminen. - Lintumies 22: 112-116

6. Suopajärvi, P. & Suopajärvi, M. 1994: Lapinpöllön iän määrittäminen. - Linnut 29: 2/1994 27-30

7. Suopajärvi, P. & Suopajärvi, M.  Mittaaminen kannattaa aina. Kanahaukan, tuulihaukan, lapinpöllön ja helmipöllön poikasten iänmääritys siiven pituudesta. Sirri 2003, vuosijulkaisu, 23 vsk, 54-59.