Tuesday 11 February 2014

Sexing Goldfinches: a photographic study (updated 18/02/14)

Lets start by stating the obvious - male and female Goldfinches are very similar in appearance. The standard reference that ringers use is the Identification Guide to European Passerines by Lars Svensson although most ringers simply refer to it as Svensson. In the guide males are described as having, on average, more extensive red on the head (or mask as I prefer to call it), black nasal hairs and lesser coverts that are black or brown black, sometimes narrowly tipped brown. While females are described as having, on average, less extensive red on the head (mask), grey or blackish grey nasal hairs and lesser coverts that are broadly tipped brown although occasionally as narrowly as some males. The illustrations give an indication of the differences in the extent of the red mask of males and females and also point out that the nasal hairs in 1Y males can be grey and in 1Y females light grey. Those descriptions and the use of the term 'on average' in particular allude to how slight some of the differences can be and to the fact that there can be a degree of overlap.

Ringers have different opinions on the reliability of the sexing criteria described above or perhaps I should say levels of confidence in them and I have even heard some say that the appearance of the lesser coverts is not a useful feature for sexing. If I had to estimate the general levels of confidence that ringers have for the various features I would expect the extent of the red mask to attract the highest levels, the colour of the nasal hairs would probably attract moderate levels and the appearance of the lesser coverts coming in last and only attracting low levels.

Photo 1. I have tried to show examples of male and female here that match the description in Svensson. If they were all like this they would be easy to sex and I wouldn't be writing this post. Note the red on the male extends well behind the eye and not just past the eye and also ends at a right angle. This differs slightly from the illustration in Svensson but is fairly typical of many males in my experience. The outer row of feathers in the lesser coverts of both birds is almost identical and this is not uncommon but the remaining lesser coverts are very different.
One of the problems we have with Goldfinches is that most of us don't get the opportunity to handle known sex birds to get a better understanding of the variation in the plumage features shown by the two sexes. I for one catch very few during the breeding season when the sex can be established from other features such as an incubation patch or cloacal protuberance. If you are lucky enough to catch Goldfinches during the breeding season the sample sizes are not likely to be big enough to get a full appreciation of the variation plus the feathers will be at their most worn which will affect the appearance of the width of any brown fringes to lesser coverts for example. When I asked a very experienced aviculturist for his opinion on sexing Goldfinches using plumage features his only response was 'hens lay eggs and cocks don't'!

Another problem with Goldfinches is that quite a large proportion have what appear to be contradictory and or intermediate sexing features based on the descriptions in Svensson. Of 92 birds I photographed and described in detail (extent of red mask, nasal hairs & lesser coverts) between 23/11/13 and 20/12/13 almost half (45) had one or more sexing features that I considered to be borderline, intermediate or were contradictory to a greater or lesser degree. I didn't sex or only provisionally sexed 16 of these which is around 17% or roughly 1 in 6 of the overall sample. I don't know what proportion of Goldfinches other ringers leave unsexed but I offer these figures to show how difficult I find it is to sex some of my birds using the criteria in Svensson alone. You may be wondering why I didn't leave more birds unsexed if all of the features didn't always fit on around half of the birds and now that I have reviewed all the descriptions and photographs so am I for a few at least. In the other cases the sex was confirmed from their wing length rather than any of the conflicting plumage characters.

So lets look at some of my 'ambiguous' birds. These are not aberrant individuals and are representative of about half of the birds that I caught in late autumn and early winter 2013. In my comments on each bird I may mention the wing length, general appearance and anything else I noted at the time. BWP gives wing lengths for British birds as 76-82 for males and 74-78 for females but I operate off 76 to 84 for males and 74 to 79 for females.

Photo 2. There are certainly some greyish white nasal hairs and the front row of forehead feathers are white suggesting female. The red mask is very bright and extends a good way behind the eye suggestive of male. The lesser coverts are quite broadly tipped brown, especially the inners which form a brown shoulder patch and lean towards female rather than male. Add to that the bill looks quite small and more female like and the wing length is 76mm. It is a juv (1Y ) not that I think we should let that sway us. I didn't sex this bird although I suspect it is a female but would everyone else agree?

Photo 3. The red mask on this bird extends well behind the eye and instantly suggests male but the nasal hairs appear pale greyish and more like a female. Then to confuse us even further the lesser coverts are almost a uniform brown forming a shoulder patch. If I now throw in that it is an adult, looks bigger billed than the bird in photo 2 and has a wing length of 78 and we are still none the wiser. This bird and the bird in photo 2 are just a couple of examples of why I don't believe considering the age really helps. This bird is a control and I don't have the original ringing details yet.
Photo 4. First impressions suggest male with bright mask extending a good way behind the eye. The lesser coverts are mainly black (shot suffers from over exposure) with just a few having very slight brown fringes also suggestive of male. You may be prepared to excuse the presence of some greyish white nasal hairs and sex this bird as a male (2 out of 3 ain't bad) and then you measure the wing and find it is only 74.5mm and under the normal range for males and therefore almost certainly a female.
Photo 5. The mask is a darker red on this bird but it still extends a moderate way behind the eye and an interpretation of Svensson could lead a novice to consider male. Nasal hairs appear grey which again could be ok for a male if a 1Y and this bird is a 1Y. Then you look at the lesser coverts and they are almost entirely brown and the median coverts even have brown centres and fringes. The wing length is 76 so doesn't quite exclude male. I provisionally sexed this bird as a female if you are wondering.

Photo 6. A similar head pattern and colour to the previous bird with the red extending a moderate way behind the eye. Most of the nasal hairs are dark grey or black but a few appear to be greyish white. The lesser coverts are very different to the previous bird and are mostly black with well defined narrow brown fringes although some of the brown fringes are a bit broader on the innermost (photo not at the best angle). However the wing length of 75mm almost certainly makes this bird a female and it is included to show how much the red can extend and how little brown there may be in the lesser coverts of some females.
Photo 7. The extent of the red is similar to the birds in photos 5 and 6 but is much brighter. Nasal hairs don't get any whiter and more obvious than this. Lesser coverts have moderate and quite well defined brown fringes with the innermost forming a small brown shoulder patch. It is an adult with a wing length of 77mm and I sexed it as a female. It was a retrap and when I checked the original ringing details I had ringed as a moulting adult female in Sept 2012 so at least I was consistent. The red mask can vary from reddish-orange and orange-red to dark red and through to crimson and some even have a mixture of those. The shade of red doesn't seem to relate to age or sex, at least not in any consistent and usable way.

Photo 8. In this bird the head looks male enough although the red doesn't extend as far as some. The nasal hairs are black so male but most of the lesser coverts are a yellowish-brown or broadly tipped yellowish-brown forming a shoulder patch and not what you normally associate with a male and certainly not an adult as this bird is. Throw in it has a wing of 81.5mm and there is no real doubt it is a male. I recorded quite a few birds with yellowish brown lesser coverts or yellowish-brown fringes to the lesser coverts and all of the ones I sexed were males. The yellowish-brown feathers and fringes contrast with the mantle much more than the usual brown lesser coverts but are really difficult to photograph.
Quite a bit of variation in photos 2 to 8 then and hopefully a few eye openers for some of you and perhaps photo 4 in particular given the short wing length. I could have just shown you loads of pictures of very obvious males and females that we could all agree on but what would be the point of that. I think I have read a comment somewhere that gave a figure for the discrepancies in the sexing of birds between ringers (in birds that have been controlled) and if I remember correctly it was quite high. A figure of around 20% comes to mind but I could be wrong although I am sure it was a significant figure. I suspect a lot of these discrepancies come from the notion that the red generally stops level with or short of the rear of the eye in females and only extends beyond the eye in males. Svensson doesn't actually say that but that is how it often seems to get interpreted from his illustrations.

If you haven't already realised this post aims to encourage ringers to think about leaving more birds unsexed or to only provisionally sex them because so many are far from straightforward and there is much overlap in the sexing features. It is much better to sex fewer and get them all right than risk getting a significant proportion wrong as seems to be the current situation. Lets have a look at another bird with that in mind.

Photo 9. The red doesn't extend much beyond the eye. There is some asymmetry with the red extending slightly further on the right side especially near the crown but it is still suggestive of female. The nasal hairs are mainly greyish-white again pointing towards female. The inner lesser coverts have broad brown fringes forming a small shoulder patch although the outers are dull black with narrower brown fringes but generally more suggestive of female than male. The wing length of 78mm is in the overlap between male and female so doesn't help. The black of the rear crown and nape has many brown feathers admixed. This was something I have looked at on all my birds to see if it is of any use in sexing birds as very obvious female types often seemed to have more brown feathers than males which were often all glossy black. Taking all things into consideration as shown above I sexed this bird as female. This bird lost a few small body feathers whilst being handled so I took the opportunity to send them away for DNA sexing. When the results came back I was surprised to find that it had been sexed as a male! So even with my cautious approach I have to admit to getting this one wrong.

Photo10. Another photograph of the male bird shown in photo 9. It was ringed on 24/11/13 and was obvious juv (1Y). It had no OGCs and had replaced all the tertials along with the central and outermost pairs of tail feathers. 
You will notice the bird above also has white sub-terminal spots on the outer 3 tail feathers. When I started photographing every bird I also trawled the Internet for any articles on sexing Goldfinches and came across a Spanish paper by Senar et al (link here) that looked at using the extent of yellow on the primaries and area of the sub-terminal white tail spots as a way of sexing juvenile Goldfinches. One of the things this paper put forward was that all birds with a sub-terminal white spot on T4 and therefore the outer 3 tail feathers could be safely sexed as male. I am not convinced of that but assuming it is correct for now lets have a look at the implications for the other sexing criteria we have been looking at on some of the other birds with 3 tail spots that I have caught.

Photo 11. The red mask is not very extensive on this bird stopping just after the eye and I think most would say female like and therefore contradicts the 3 white spots. The nasal hairs are greyish black although though some seem to have paler tips but certainly lean towards male. The lesser coverts are moderately tipped brown but are quite well defined and do not form a shoulder patch so are ok for male but are probably best thought of as intermediate. I provisionally sexed this bird as a female and largely based that on the limited extent of the red mask but having reviewed all the photos I wouldn't even provisionally sex this bird now. The wing was 77.5mm so that didn't help. If this is a male as the Spanish study suggests what does it say about the extent of red that males can show.
Photo 12. Another bird with 3 white tail spots and therefore a male according to Spanish study. The red extends a good way behind the eye suggestive of male but the nasal hairs are all greyish white and are about as female looking as they could be whichever way you look at them. The lesser coverts look female but are of the yellowish-brown variety rather than the usual brown. The wing was 79mm and too borderline for me to read anything into it so having taken everything into consideration I didn't sex this bird, not even provisionally.
One of the problems I have with the Spanish study is the suggestion that the white sub-terminal tail spots are oval and their area can be calculated by measuring the width and length and using the elliptical area equation π (Length/2) (Width/2). There is nothing wrong with the equation but there is a problem with thinking the tail spots are oval. In my experience it is not uncommon for tail spots to also be rectangular, circular, semi-circular and a variety if irregular shapes. Males may have more tail spots on average than females or they may be bigger in area but I doubt the number, shape and size of the spots have any practical uses that can be applied to sexing birds in the field. I certainly don't think that birds with 3 tail spots are always males from the many examples I have seen.

Photo 13. A few examples showing the variation in shape of the tail spots. It is not uncommon for the tail spots to be shapes other than oval. I hope to retrap some of the birds I have photographed after their next moult to as I am curious to see if the shape and size of the tail spots changes or stays the same. Similarly I will be able to see if the extent of the red mask, colour of nasal hairs and appearance of lesser coverts changes.
I did measure the length of the yellow on the 6th primary and the width and length of the white tail spots for around 100 bird but I haven't drilled the data yet and there wouldn't be much point as I am not confident about the sex of every bird. The yellow on P6 for the bird in photo 11 was 17.5mm and 18.8mm for the bird in photo 12 so both are relatively short, especially the bird in photo 11. The Spanish study is based on a relative small sample of birds trapped over 3 winters and may not have included any birds of the British subspecies. Ideally it needs repeating in the UK with the sex of each bird being verified from DNA or repeating on a good series of skins rather than going off plumage features that we can't always rely on.

Photographing and describing so many Goldfinches has allowed me to get a much better appreciation of the variation compared to just looking at a succession of birds in the hand. You can make comparisons that wouldn't have happened in the field and you are not relying on memory. At one point I had thought there was some mileage in looking at the rear crown and nape (as mentioned above) but even here you can see there is no clear cut difference between the 2 sexes as seems to be the case with all Goldfinch plumage features. While you can make some generalisations about the rear crown and nape of males and females there is a large degree of overlap and the odd exception on top of that.

Photo 14. Variation in the rear crown and nape. The age, sex and wing length is shown with the sex in brackets only being provisional. You really get to appreciate the variation when you can see them side by side like this.
So after all that how do you sex Goldfinches. Well I would have to say very cautiously and only if you look at every plumage feature. I don't think any one feature is better than another as there is overlap in all of them including the extent of the red mask and hopefully I have gone some way to showing that. I would steer clear of sexing any birds with contradictory and borderline features if we want to increase the accuracy of the data and consistency of sexing between ringers.

To be sure a bird is a male the red should extend well behind the eye to a length equivalent to or greater than half the width of the birds eye so at least 2mm beyond the eye. Also the red should remain beyond the eye all the way up to where it meets the black crown feathers and must not return back above the eye near the crown as with the bird in photo 7. The nasal hairs should be black or dark greyish black. Lesser coverts should be all black or have well defined brown or yellow-brown fringes (these fringes shouldn't form a brown or yellow-brown shoulder patch). Lastly the wing length must be 76mm or greater.

For a female the red shouldn't extend beyond the rear of the eye or it may just wrap around the rear of the eye by up to 1 or 2mm but if it does it must return back level with the rear of the eye or return above the eye towards the crown similar to photo 7 (it must not stay beyond the eye up to the crown). Nasal hairs should be greyish white or be mostly so. Lesser coverts (excluding the outer row) should be brown or broadly tipped brown forming a uniform or fairly uniform brown shoulder patch. The wing length should be 79 or under.

My version of the head patterns of male and female Goldfinches drawn in the style of the illustrations in Svensson's identification guide.
If features are ambiguous or the bird is in juvenile plumage you may be able to sex some birds on wing length alone <76 female and >79 male but you should record these as being sexed on size.

Adopting this or something very similar will reduce the number of birds that you sex but will improve accuracy and the repeatability between ringers. Even if you are certain of a birds sex but don't think other ringers would necessarily agree then provisionally sex it rather than giving it a confirmed sex. I have left approximately 30% of birds I have ringed this year unsexed or only provisionally sexed them since adopting this approach.

A final word of caution - don't start sexing 1Y birds if the post juvenile moult is not complete unless simply sexing them on wing length <76 female and >79 male. The feathers around the face are the last to be replaced and the post juvenile moult may not be completed until December for birds from the later broods. The extent of the red and the true colour of the nasal hairs may not have fully developed until well into December in these birds. Feather sheaths and debris from feather sheaths can also make nasal hairs look much paler than they actually are and even paler than the example shown below in some cases. This is not always obvious and easily overlooked if you are dealing with a lot of birds and are having to process them quickly. Also juvenile nasal hairs are pale greyish-buff so can give the impression of female if not fully moulted out.

Photo 15. Close-up of nasal hairs showing dusting of feather sheath debris making them appear paler than they really are. This is not the most extreme case I have seen.
Photo 16. The feathers of the face are the last to be replaced in both the post juvenile and adult moult and this can continue well into December. This bird gives the impression of being a female but the red may not have reached its full extent as the feathers behind the eye still seem to show some juvenile spotting. What appear to be greyish white nasal hairs may be that or they may include pale greyish-buff juvenile feathers that have yet to be replaced.
I have stuck my neck out with this post as there are likely to be as many opinions on sexing Goldfinches as there is variation in them. I have even owned up to the odd mistake as it is so easily done. There is clearly room for further study and this post may even prompt some of you to undertake your own or to publish what you already know. Until then I think ringers need to be more confident and comfortable with being uncertain and leave birds unsexed if there is any ambiguity in any of the features they present and the wing length falls in the overlap between males and females. I have learned a lot from my little photographic study and hope you will get something out of it too.

One more photo collage because it is a cracking adult male and to show how extensive the red can be or orange-red as it is in this case. The lesser coverts are also a little unusual on this bird.

Photo 17. An adult male with probably the most extensive red mask I have seen. This bird was originally ringed as an adult male in September 2011.The nasal hairs are a bit short and appear dusted grey because they are still growing and bits of sheath are present. A few of the nasal hairs were still in pin. The lesser coverts have quite well defined yellow-brown fringes although some of the fringes are much broader than you would normally expect with a male and especially an adult. Adult males are usually described as having glossy black lesser coverts with a few sometimes having well defined narrow brown fringes. There seems to be an exception to every rule but I think we can all agree on this one being a male.

and finally...........
Photo 18. Some Goldfinches are always going to leave us scratching our heads.

If you have read to the end of this post you may be interested in the summer update I posted on 4th July 2014. This shows a few birds caught during the breeding season and can be found by clicking here.

A further update posted on 21st August 2014 and showing more photographs of known sex birds can be found by clicking here.

An additional article on sexing Goldfinches posted on 20/06/15 showing photographs of breeding birds can be found by clicking here.