|Grey Heron and its victim at Rainham Marshes RSPB. Did you try and identify it?|
Looking again at the photos, in the first image we can get a feel for the overall body length of the bird in question: it seems very roughly equivalent to or a little shorter than the leg length of the heron (the feet of which obviously cannot be seen). Grey Heron’s tarsus (the leg below the knee) averages 151 mm in males and 141 mm in females, with an overall range of 132-172 mm (BWP). If the tarsus is about half the overall leg length, that gives a maximum range of 264-344 mm: this overall length fits well with Black-necked Grebe (280-340 mm) and overlaps with the lower end of Slavonian Grebe (310-380 mm) and the upper end of Little Grebe (230-290 mm). On this basis the two larger grebe species and Eurasian Coot can be excluded on size alone (as well as other characters).
The plumage also provides clues. In the first image the bird appears blackish above from head and neck right down over the upperparts. As pointed out in the comments, Slavonian and Black-necked differ in the ‘narrowness’ of the black line on the back of the head, but that is the case on swimming birds holding the head at right angles to the neck: in this image, with the head and neck held vertically in line by the heron, the feathering on the back of the head would be compressed into the neck and I don’t think any meaningful distinction would be visible.
The next three shots all show that the bird has clean white underparts, with no duskiness visible and a sharp division between white flanks and blackish upperparts. This fits better with Slavonian Grebe than with Black-necked, which has a tendency towards more extensive darkish ‘discolouring’ on the flanks. The whiteness of this area would seem to exclude Little Grebe, which in all plumages anyway appears more brown and buff.
|Slavonian (left) and Black-necked Grebes in non-breeding plumage: note the differences in the distribution of black and white on the head, neck and flanks.|
If we accept that the likely choices are Slavonian or Black-necked Grebe, can we narrow it down further? Although it’s hard to see here, enlarging the first image in Photoshop seems to reveal a fairly well-demarcated black cap above whitish cheeks; certainly, the bird seems to lack the typical darkly smudged cheeks of Black-necked Grebe. This feature, together with the whiter flanks and underparts, strongly suggests to me that the bird is (or rather was) a Slavonian Grebe. This was my initial impression when I first downloaded the images and remains my tentative conclusion, but it does seem to me to fit best. Could a tired and lost ‘Slav’ have fallen victim to a predatory adult Grey Heron? Although Slavonian Grebe is rare in London, coincidentally one was also found the previous day elsewhere in the London area, at Thorpe Park. Perhaps we’ll never know for sure, but it’s been an interesting and educational episode.