The only egret with a ‘sandy back’ is the Cattle Egret in summer plumage. Wandering individuals at the end of the breeding season can turn up in non-bovine areas. The large ‘brown-and-white’ heron was most likely a juvenile Black Stork from the breeding population in east central Portugal. The Purple Heron has bred near Alvor but it is slightly smaller than a Grey and even the juveniles lack white in the plumage.
His ‘small swifts’ could only have been Common or Pallid Swifts which both breed in the area. Size is deceptive and, counter-intuitively, birds seen at close range appear to be smaller than the same species seen high overhead. I saw many feeding low over the ground with rapid wing beats the last time I was at Alvor. His larger bird could only have been an Alpine Swift, an Algarve breeder which, despite its name, also occurs at low levels. This has white underparts but is darker-backed than he describes, although strong sunlight can render black and browns surprisingly pale to the naked eye.
The most obvious effects of climate change in the Algarve are more regular overwintering by species, like the Short-toed Eagle, which normally migrate to Africa, occurrences of increasing numbers of vagrant passerines from eastern Europe and Asia in autumn and arrivals of seabirds from the sub-tropics, Caribbean and even the South Atlantic, mainly in spring. It is an exciting time for birdwatchers, but largely detrimental to the bird themselves which cannot adapt to rapid changes in their environment at the rate required.