Hooded Grebes have been described as gregarious and peaceful. They tend to feed on dispersed flocks, mostly by diving (Fjeldså 1986b), staying under for 25 seconds on average. They do not congregate in very compact groups or rafts; nor do they take much food from the surface as Silvery Grebes do. Only occasionally, individuals join a ‘feeding frenzy’ of the later species, taking advantage of the surplus of available food close to the surface. On the winter grounds, the groups remain dispersed. The nonbreeders assemble in flocks in saline turbid lakes with hardly any vegetation.
As with other colonial birds, they do not seem to be very territorial except when protecting the eggs. In that case, there is always at least one adult on the nest, both of them taking turns to go away to feed (Lange 1981). Whenever a Kelp Gull Larus dominicanus is around, both adults stay close trying to protect the eggs. Outside breeding season they are not territorial at all.
Courtship is very complex, more so than any other grebe. Typically, small groups of up to 10-15 individuals gather in the more sheltered waters the lagoon or lake. Pairs can be found breeding on their own in smaller lagoons. Several movements and interactions have been described, namely ‘advertising’, ‘bouncy dive’, ‘cat display’, ‘sky jabbing’ (apparently unique to this grebe), ‘head turning’, ‘head flicking’, ‘habit preening’, ‘wing quivering’, ‘penguin dance’ and ‘discovery ceremony.’ These movements are described in detail by Storer (1981) and Lange (1981).
Social and interspecific behavior
A very gregarious grebe usually in small groups or even hundreds sometimes mixed with Silvery Grebes, an association that also happens during winter. During spring the Hooded Grebe performs very elaborate displays prior to the nesting period. This dancing and barging parallel with bodies raised almost out of the water (Fjeldså and Krabbe 1990) is characteristic and helps build pair bonds. These bonds seem to continue during winter as many couples have been observed performing courtship displays (or at least performing portions of these displays) in the estuaries on the Atlantic coast.
According to Storer (1981) Hooded Grebes fear Kelp Gull Larus dominicanus and when Kelp Gulls fly over, the grebes consistently look at them with their heads in an alert position. If the gulls fly too close the grebes tend to dive. Several interactions between these species have been recorded (see Predation). Red-gartered Coot Fulica armillata also interacts with the grebe, especially during the breeding season, when coots destroy nesting attempts to force the grebes to abandon the platforms, so that the coot can build their own nest or use the platforms for nesting or simply resting (Fjeldså 1986b, Johnson 1982, Lange 1981, Nuechterlein and Johnson 1981).
Before the expansion of the Kelp Gull Larus dominicanus, Hooded Grebe probably had only occasional problems with predators, due to their small population and the availability of other, more numerous, prey. Coots Fulica spp. likely were the main competitor of the grebe as they tend to use the grebes nesting platforms to build their own nests, in the process destroying the grebe’s nesting attempt. With the expansion of the farming industry into the hinterland of Patagonia, the Kelp Gull found a source of food in the most remote locations and has become fierce predator not only of the grebe but of many other species as well. The introduction of fish on Cardiel Lake, and the fishing industry that followed also gave the gulls more prey items, allowing them to increase their population and spread over the plateaus where the Hooded Grebe lives. A single Kelp Gull can easily destroy the nesting of an grebe colony in less than a day, having a considerable impact an the small population of the grebe (Beltrán et al. 1987, HC pers obs.).
The introduction of Rainbow Trout Oncorhynchus mykiss on the breeding lakes is a problem not only because of competition for the grebe's main sources of food, but also a direct preditor of the grebe, trout voraciously prey on nests, chicks and possibly even adults. This is a problem on the breeding grounds as well as the wintering estuaries, where large Sea-run Brown Trout Salmo trutta exist in large numbers. A netted 4 kilogram trout on the Gallegos estuary (well under the average size for the area), had an adult Silvery Grebe half digested in its guts (SI), a bird roughly the same size of a Hooded Grebe, a fact that indicates possible predation in the wintering grounds as well.
The American Mink Neovison vison was never mentioned as a possible threat for this grebe although its presence was knwon in several areas in Patagonia outside the plateaus. It has been mentioned as a main threat for grebe’s eggs and also for nesting adults, even for North American grebes (Cullen et al. 1999, Stedman 2000), which live in areas where the mink is native. In the case of Eared Grebe Podiceps nigricollis a surplus killing was reported (i.e. a killing of more individuals than the predator could eat) by American Mink involving of 50 adults in a colony of more than 150 pairs (Breault & Cheng 1988). Those authors considered surplus killing event as ‘casual and uncommon’ for Eared Grebe.
One of the only five colonies found in 2011/12, was destroyed by mink in a single event, leaving 33 adult Hooded Grebes dead and some 40 abandoned egg. This is the first report of predation by mink and a serious threath for an already diminishing population (Roesler et al in prep.).