On the lakes located on the Patagonian uplands, the main source of food is a small invertebrate, the snail Lymnaea diaphana taken in sizes from 8-15 mm in length. Several other items, mainly amphipods or arthropods, are taken in a lesser percentage. When feeding young they take smaller prey including midge pupae, copepods, bugs and larvae of water beetles until the chicks are old enough to feed on the snails (at about 14 days of age) or if the prey is scarce in the lake, in which case the chicks might be later abandoned. The dives in which they capture prey average 16 seconds but are considerably shorter when feeding chicks (Fjeldså 1986b, Llimona and del Hoyo 1992).
The most detailed report on feeding methods was realized by J. Fjeldså (1986b) in Lake Nevada, on the Vizcachas plateau, from which the following account largely is taken. Hooded Grebes tend to feed very close to the shore (4-15 meters) by diving (87.1%), instead of taking items from the surface as does the sympatric Silvery Grebes for a significant part of the time. They do not spend much time feeding from the surface or swimming through weed zones with submerged heads, preferring to exploit open leads. Their dives are a forward motion propelled by simultaneous strokes of their feet. After surfacing, they usually carry prey on their beaks and after maneuvering it for a while, presumably for removal of the shell of the snails, they swallow or feed their chick.
Although optimal prey types are common along globular algae near the shore and inside the water weed zones, Hooded Grebe prefers feeding among scattered submergents and along the vegetation borders. They might use a diversity of other foraging methods when the conditions and amount of large prey vary but in these cases there is a high overlap with the Silvery Grebe’s diet and it might only provide food for their survival and not enough for raising chicks.
The structure of the vegetation of the lakes leads to area concentrated feeding and although there might be a great abundance of its preferred food item, the microhabitat preferred by Hooded Grebe seems to be limited. Therefore in the areas where the grebes feed optimally, they seem to be able to remove almost all the large prey on which they feed, improving substantially since their arrival in the spring as the season progresses, to the point in which there might be a shortage of easily available big prey by the time they have young (Fjeldså 1986b). Accordingly, Fjeldså (1986b) suggested that the bottleneck in the life of the grebe is the breeding season, when they have to stay in the deep and less productive lakes which have dense milfoils used to build nests but limited prey to feed on.
Little is known about feeding habits on the wintering grounds and on the lagoons where the grebes stop during migration. Fjeldså (1986b) observed that Patagonian lowland lakes would be very poor feeding habitat for the grebes, an affirmation that seems to correspond with the fact that they are recorded in this lakes for very short periods of time and always in small numbers (Imberti et al. 2004, SI). Furthermore, there are no reliable records of grebes in the large Patagonian lowland lakes (namely Argentino, Viedma, San Martín, Buenos Aires) that are very close to their breeding lakes and could be used during migration but are seemingly very poor feeding grounds for such an specialist (SI).
On the estuaries, they seem to take advantage of the concentration of prey items in the channels formed by the strong movements of the tides, sharing this bounty with other species like Silvery Grebe, cormorants Phalacrocorax spp., Kelp Gull Larus dominicanus, Brown-hooded Gull Chroicocephalus maculipennis, South American Tern Sterna hirundinacea and marine mammals (Imberti et al. 2004, Albrieu et al. 2004). No studies on the diet of the grebe in this area have been done and only circumstantial evidence exists. One grebe found dead in the Gallegos estuary in May 2001 had a full stomach with contents being mostly fish, especially Falkland Sprat Sprattus fuegensis (70.81%), and Spider Crab Halicarcinus planatus (16.48%), with the remainder being algae (11.52%), mollusks (0.53%) and other smaller items including small rocks (Torres and Vargas 2005, Imberti 2006).