WRW#12B: Oystercatcher
The cache, a camo-taped bison, tube, is hidden just off the riverside trail in an area of developing (wild cherry?) woodland. As a suitable hide location was being sought, the unmistakeable call of this distinctive wader was heard and a pair seen briefly heading upriver flying quickly and close to the water.
Although the bird is recorded as occurring along the Wharfe, this was the first time I had heard or seen it.
To reach the cache: park @ or near N 53 55.807 W 1 50.233 in the car park of the sports centre and follow the Dales Way footpath, which continues from the north-west corner of the car park, past the location of WRW #12 and then at N 53 55.836 W 1 50.433, instead of heading left along Dales Way, continue straight on and head towards and then along the riverside path to the cache location.
The (Eurasian) oystercatcher (Haematopus ostralegus) aka the common, pied or palaearctic oystercatcher, is the most widespread oystercatcher and one of the largest waders in the UK. It is an obvious and noisy plover-like bird. The scientific name the Greek haima (blood), pous (foot) Latin ostrea (oyster) and legere (to collect or pick).
It is 40–45 cm including a bill of 8–9 cm, and has a wing-span of 80–85 cm. It has black and white plumage, red legs and strong broad red bills used for smashing or prising open molluscs like mussels or for finding earthworms.
Despite its name, oysters do not form a large part of its diet. The bird still lives up to its name, as few if any other wading birds can open oysters.
It is unmistakable in flight, with white patches on the wings and tail, otherwise black upperparts, and white 
underparts. Young birds are more brown, have a white neck collar and a duller bill. The call is a distinctive loud piping (listen here).
The bill shape varies - birds with broad bill tips open molluscs by prising them apart or hammering through the shell, whereas pointed-bill birds dig up worms. Much of this is due to the wear resulting from feeding on the prey. Individual birds specialise in one technique or the other which they learn from their parents.
It is a migratory species over most of its range. The European population breeds mainly in northern Europe, but in winter the birds can be found in north Africa and southern parts of Europe. Although it is present all year in Ireland, GBand adjacent European coasts, there is still migratory movement: the large flocks found in the estuaries of SW England in winter mainly breed in northern England or Scotland. They are highly gregarious outside breeding season.
The nest is a bare scrape on pebbles, on the coast or on inland gravelly islands. 2–4 eggs are laid. Both eggs and chicks are highly cryptic.
Because of its large numbers and readily identified behaviour, it is an important indicator species for the health of the ecosystems where it congregates.
For more information on this handsome and distinctive bird, see here and here.