You can find an oystercatcher (Haematopus ostralegus) pretty well anywhere along the Dorset coastline, especially during the breeding season in May and June.
In winter, however, like many species of birds, they tend to flock together into areas where there is plenty of food and some shelter. Poole Harbour and the lagoon on Brownsea can be home to thousands of these birds during the harsher times of the year.
The numbers are certainly boosted by arrivals from further north and I have seen over 500 together on the shore of Green Island, opposite Arne beach, at high tide when feeding is difficult because the mud flats are covered. They huddle together, all with their backs to the wind and with their heads under their wings. I love the way they ignore the 'no access' signs and just stand their ground!
Waders can be tricky to identify, especially when starting out birding, so the oystercatcher is one you can get to know very quickly as it is quite distinctive in its black tie and tails attire and stout orange bill.