Whilst the greenshank could not possibly be described as rare in Dorset it could not be deemed common either. It falls in to what I would call the 'interesting' group where seeing one is not out of the ordinary at the right time of year and in the right place but certainly worthy of a note in your book and worth a mention in a tweet. For that reason there are a good number of reports of greenshank in the Nature of Dorset database.
The distribution map shows that greenshank can be seen at many of the Poole harbour sites and they also occur in Christchurch harbour and at sites along the Fleet but by far the most reports come from Lytchett Bay and there are two reasons for this. Firstly, this is a well watched site with reports from there daily whereas other sites may, in general, not be as closely monitored. The main reason, however, is that it is a popular spot for greenshanks to feed, especially when the tide is not fully out and the birds are pushed off the exposed mudflats in the harbour and up to the shallow west end of the harbour. The lagoon on Brownsea Island is another site favoured by greenshank.
The greenshank is one of several wader species that breed in the Arctic and come south to Dorset when the extreme weather sets in further north and so one expects them to reported frequently during the winter and not at all in the summer but the weekly reports chart would suggest this is not the case. There are only a few reports each week from week 42 (mid October) through until week 15 in the following March whereas the bulk of reports come from weeks 32 to 41; that is mainly August and September. It would seem, therefore, that many greenshank reports are of migrating birds arriving, stopping to feed for a few days, before setting off further south. Just a few actually stay on longer and even they may move on if the weather gets really harsh here in Dorset.
This idea of them being more of a passage migrant than a wintering species is born out by a sudden peak of reports in week 16 (early April) and then some ongoing reports during the rest of April. This surely reflects the movement north from more southerly areas once the spring comes. A very small number of birds actually spend the entire summer here but they do not breed.
So greenshank can be seen all year round in variable numbers but even autumn the flock sizes are quite small, usually 12 or less and many sightings are of single birds. To add greenshank to your life list take the boat to Brownsea in late August or early September and you will be rewarded with excellent views right outside the hides overlooking the lagoon.