Autumn marsh harriers on Castlemartin Corse

Marsh harriers are highly variable between ages, sexes and individuals, and (adult males apart) can be confusing at the best of times. Add in 500m range, often poor visibility and rapidly expanding scrub, and getting a good look at individuals on the Corse, and thereby deciding how many birds are around,  can be challenging. Thanks to some post-match analysis of some outstanding photos by Caroline Pickett and Alan Merrett, we now know we have had at least 4, perhaps 5 individual marsh harriers on the Corse across September and October. though never more than two seen on a given day.

Caroline and I had a brief view of an adult female type on our September WEBS count (21.09). We later learnt that Alan Merrett had seen two together on 19.09. His photos show that these were both probable 1CY birds, one of them by its broad build almost certainly a female, the other probably a female, but neither of them our clearly adult bird from two days later. (Juvs in September - where have they come from? Our tagged bird from Norfolk in 2020 turned up in November). Caroline and I saw one adult female on our next WEBS count on 19.10: helpfully, it was moulting a primary. A couple of days later Caroline and Alan saw two birds together, both adult females. Finally, yesterday, Caroline photographed this female, clearly our bird from 19.10, as told by her missing primary.

So: two 1CY birds and one adult female in September, two adult females in October - one of which may or may not have been our September adult. Great work by Caroline and Alan.

1. Alan's birds from 19.09.2024 both likely 1CY types, the lower one a female by its broad build:

photos Alan Merrett

2. Caroline's and my bird from 21.09.2024, under typical conditions - 4-500m, poor visibility: buzzard on fence - cream-yellow extending down the sides of the neck, most likely a 3CY+ ♀ moulting a left primary: 

3. Caroline's pics of 2 birds, both adult female types, on 21.10.2024 - one of them (presumably) the one we'd seen on WEBS two days earlier. The lower c/u is of the top, further bird in the pic of the two:

photos Caroline Pickett

4. Lastly, Caroline's pic of ad. female type taken yesterday. From the missing left primary this appears to be the bird we saw on 21.09 (above):

photo Caroline Pickett

Great work by Caroline and Alan. Thoughts and comments on plumages welcome, this post makes no claim to be definitive! (Forsman devotes a page to separating female marsh harriers, black kites and dark phase booted eagles out of context/on passage, one problem we don't have at the Corse!)