“Broadly speaking, species that are covered in British Birds’ annual review of scarce species (and forms of similar rarity).”

Not quite county description species, but worth twitching, a good find, and birds that would generally make a good days birding –

Cory’s Shearwater
Great Shearwater
Leach’s Petrel
Grey Phalarope

Hoopoe
Wryneck
Yellow-browed Warbler (except when overwhelmed by numbers!)
Blue-headed Wagtail 

White-fronted Goose
(Greenland White-fronted Goose)
(Eurasian White-fronted Goose)

Whooper Swan 
Garganey
Velvet Scoter 
Little Auk

The below species are more discretionary –

Long-tailed Duck 
Red-necked Grebe 
Slavonian Grebe  
Black-necked Grebe (away from Carrick Roads?)
Spoonbill 
Bittern (new birds)

Osprey 
Red Kite
Marsh Harrier
Hen Harrier

Avocet (away from Kingsmill area)
Jack Snipe (if twitchable)
Spotted Redshank 
Wood Sandpiper
Little Stint 
Curlew Sandpiper 

Little Gull (sometimes)
Glaucous Gull
Iceland Gull (sometimes)
Yellow-legged Gull (occasionally)
Little Tern (sometimes)
Black Tern
Pomarine Skua (sometimes)

Turtle Dove 
Short-eared Owl

Crested Tit (we can hope!)
Woodlark
Cetti’s Warbler (away from regular sites?)
Ring Ouzel (sometimes)
Pied Flycatcher (maybe)
Water Pipit 
Lapland Bunting 
Snow Bunting

When seabirds are numerous in reports/mid-season perhaps no need to highlight every time.

Other ‘scarce’ UK birds such as Goosander and Scaup may also be highlighted when the first(s) of the season arrives, but not usually subsequently.