In the somewhat mysterious world of birdwatching, “species pairs” can be a source of great frustration or of great joy. A “species pair” consists of two species with very similar morphology (form and structure). Not being able to easily separate the species can be very frustrating but finding the ‘key’ is usually a great joy. Two examples of “species pairs” are Wandering Tattler/Grey-tailed Tattler and Brown Goshawk/Collared Sparrowhawk.
As the easy and accurate identification of the individual species in each pair can be quite difficult it is not unreasonable for birdwatchers to seek out “silver bullet” identification features for each of the species involved. Unfortunately, incorrect information, missing information, incorrect interpretation of information can all lead to dubious or wrong ‘rules’ for identification being applied. This can easily lead to wrong or erratic identification of the species being observed.
For more insight into Wandering Tattler/Grey-tailed Tattler identification see the identification guide “ID Wandering Tattler” elsewhere on this web-site.
The discussion on this page concerns one commonly used ID feature for determining Brown Goshawk (Accipiter fasciatus) or Collared Sparrowhawk (Accipiter cirrhocephalus).
It is often claimed that the lengths of the middle toes of these species can be used reliably to separate Brown Goshawk from Collared Sparrowhawk. The theory is that Collared Sparrowhawks have obviously longer (elongated) middle toes than that of the Brown Goshawk. The insinuation is that Brown Goshawks do not have noticeably elongated middle toes but Collared Sparrowhawks do and that Collared Sparrowhawks have the longer middle toe.
I have heard this theory espoused for something like 30 years and actually believed it for much of that time. More recently a close study of the entries for Brown Goshawk and Collared Sparrowhawks in the “Handbook of Australian New Zealand and Antarctic Birds” (HANZAB) has shown me that the theory is incorrect.
Briefly, the measurement tables for the two species in HANZAB show that (male to male and female to female) Brown Goshawk has, on average, the longer middle toe. Furthermore, the shortest Brown Goshawk middle toe is longer than the shortest Collared Sparrowhawk middle toe. Also, the longest Brown Goshawk middle toe is longer than the longest Collared Sparrowhawk middle toe. However, there is a considerable overlap in the ‘middle’ lengths.
In my opinion this makes the actual lengths of the middle toes irrelevant and of no value in determining the species.
However, the length of the middle toe in relation to the lengths of the other toes, particularly the outer toe, seems to be potentially a good indicator of species.
This identification point mainly involves the position of the base of the claw of the outer toe in relation to the distal joint of the middle toe. (The distal joint is the outermost joint.) To see this relationship accurately it would be best to have the bird in the hand with the toes extended and side by side. This positional relationship could be difficult to accurately determine if the foot is gripping a branch and the toes are wide apart but it can be seen in reasonably good quality digital images of the birds in overhead flight. (See the two images below.)
As stated in HANZAB, in Accipiter cirrhocephalus (Collared Sparrowhawk), distal joint of middle toe lies beyond base of outer claw while in Accipiter fasciatus (Brown Goshawk) it is level with or shorter than base of outer claw.
This different positional relationship of distal joint and claw base is probably what makes the middle toe of the Collared Sparrowhawk appear to be much longer than that of the Brown Goshawk.
The two images below are of the one bird and, going by the statement above, I would identify this bird as a Brown Goshawk. (The plumage has me guessing but I am guessing it is a juvenile.)
Unfortunately I don’t have any similar shots of Collared Sparrowhawks for comparison but at least these images should explain the concept and they show what is meant by ‘distal joint’ and the ‘base of the outer claw’.
HANZAB also gives figures for the percentage the length of the outer toe is of the length of the middle toe.
Collared Sparrowhawk = 63 - 67%
Brown Goshawk = 70%
Thus adding to the perception that the middle toe of the Collared Sparrowhawk is very long.
Once again the best way to measure this would be with the bird in the hand but it may be possible with images similar to the first one below. In that image the outer toe appears to be about 70% of the middle toe (disregarding the claws on both toes).
Of course, this is a very difficult ID feature to assess without the bird in the hand so, as the lengths of the middle toes cannot be used as a positive ID point, it would probably be best to leave the toes alone and concentrate on other more easily assessed ID features.