In the last ten years spoonbill have become something of a permanent feature in Dorset having been very scarce prior to that. I wrote some time ago that " In 2008 six young birds stayed in the area all year" and at that time there was great anticipation that they might start to breed here but, as far as I am aware, despite regular birds here and in good numbers too I believe we are still waiting! They have started to nest elsewhere in the UK I believe so perhaps it is just a matter of time.
There were more tweeted reports of spoonbill in 2017 than in 2018 but this just could be that they are just becoming accepted as part of the native fauna now and not always worth reporting, I certainly don't think they are in decline although the significant numbers reported in the winter of 2017/8 have yet to be matched this year. On the 17th October 2017 there were 60 on the lagoon on Brownsea alone and there probably others present in the harbour elsewhere. So far this winter I think the highest count I have seen mentioned is 36 at Arne in November 2018
The spoonbill is certainly a resident species now with reports from somewhere every week but the number of reports increases in the autumn so the numbers here in the summer are certainly boosted by arrivals from elsewhere for the winter months. As spoonbills can be found just across the North Sea in Holland, Germany and Denmark, and they are known to be a migratory species, it seems quite likely that a 'short' hop across to Dorset is preferable to the long trek south to the Mediterranean or even into Africa.
The distribution map shows them clustered around Christchurch harbour, Poole harbour and the Fleet but the biggest population is undoubtedly to be found at sites in Poole harbour with Lytchett Bay, Arne and Brownsea the leading reporting areas. Elsewhere it seems just the odd one turns up every now and then, possibly migrants just dropping in for a quick snack before continuing onwards with their journey.
To see spoonbill the best option is to head for the Shipstall hide at RSPB Arne where a good number often congregate on the spit there at high tide. If they are not there then head to the Middlebere hide as they could be there feeding in the channel at low tide.