He has looked at the energy expenditure of migratory birds and the physiology of feather moult, by tracking wild barnacle geese (Branta leucopsis) from Svalbard to Scotland, using biologging technology.
[4][5][6] Portugal has studied the etiquette of aggression in Siamese fighting fish (Betta splendens),[7][8][9] and the developmental energetics of avian brood parasites.
His research has shown how birds fly in V-formation,[14][15][16][17][18] using Northern bald ibis (Geronticus eremita) that were part of a European reintroduction scheme.
His team has worked extensively with homing pigeons (Columba livia), demonstrating how individuals compromise on speed to stay together as a flock, and the increased energy expenditure required to fly as a group.
[20][21][22] Work with secretary birds (Sagittarius serpentarius) demonstrated their capacity to deliver a kick 5 times their own body weight,[23] while his studies on African harrier-hawks (Polyboroides typus) showed their unique visual system designed to support their unusual foraging technique of extracting food from tree holes.