Goshawks
Whereas most of the information contain within this wiki is related to the railway, a bit also covers the geological commerce of the area (i.e., Slate quarrying). A volunteer has written on the subject of the local bird life.
Goshawk, or Gwyddwalch ( = Goose hawk), Accipiter gentilis, a medium to large hawk. The male weighs 1½ to 2½lb (600g to 1200g) and the female is much larger at 2 to 4lb or more (900g - 2kg). The maximum body length is 2ft (600mm) and the wingspan up to 5ft (1.6m).
The female can be much the same size as a buzzard but goshawks have more curved wings and are a gorgeous light blue colour in sunlight.
They have a pronounced white eyebrow and, I'm told, a 'fierce red eye.' Goshawks fly weaving and twisting 'sometimes fast' between the trees along the river looking for dinner; they eat smaller birds and the usual mice and so forth, having a curious habit of folding or 'mantling' their wings around their prey so that you can't see what they have. They seem to like the larch plantations, lay three or four eggs, incubate them for five weeks and the chicks fledge, 'take their first flight'. seven or eight weeks later.
For some reason they are still persecuted by gamekeepers, taken illegally for falconry or predated by egg-collectors, so I'm not saying where I've seen them, but they may be seen from the train if you're very lucky.
Understandably, but unlike buzzards, they are chary of humans. There are thought to be about 400 pairs breeding in the UK.