- Order: Podicipediformes
- Family: Podicipedidae
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 Larus 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).
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.).
Hooded Grebes nest in clear water volcanic lakes that have cliffs on at least one side, providing some protection from the extreme winds that blast the steppes of southern Patagonia. They form colonies of several up to a hundred pairs on dense floating aggregations of Myriophyllum elatinoides, laying two eggs between December and February but never attempting to raise more than one chick (Fjeldså and Krabbe 1990). After courtship mating takes place on the platform they built; occasionally some reverse mounting is observed (Nuechterlein and Storer 1989). Colonies usually have an elongated shape and are located on the sheltered side of the lagoon on account of the winds, which have the potential of destroying the whole colony in a single storm (Beltrán et al. 1992). Some Silvery Grebes (Podiceps occipitalis) might share the colony, and at least one hybrid between the two species has been observed (Storer 1982).
Due to the late development of the plant material over which they build their nests, the grebes nest late compared to other birds of the area. Laying may start as early as December and last until the end of February. Incubation lasts 20-21 days. Hatching is asynchronic and the second egg is abandoned. If the first attempt fails early in the season, a pair may make a second attempt at breeding. Chicks are fed very small items at first, and after two weeks the diet of the young switches to snails. If the density of these items is low a complete colony may fail and the chicks abandoned to starve (Fjeldså 1986b).
Populations and Demography
After its discovery in 1974, it was estimated that the population of the Hooded Grege was a relic confined to Escarchados lagoon, numbering less than 150 individuals. Fundación Vida Silvestre Argentina dedicated its efforts towards studying this species, with discouraging results regarding its general condition. After a few nesting failures it was decided to search for other populations in nearby lakes. These searches extended over most of the plateaus in Santa Cruz province, mainly due to the efforts of the late Andrés Johnson (1982, 1985, 1986, 1994, 1997, Beltrán et al. 1987). Estimates after these efforts pointed to a population of around 3000 to 5000 (Beltrán et al. 1987, Fjeldså 1986b). Its winter habitat was unknown until part of the population was found in 1994 (Johnson and Serret) but the searches did not resume until the period 1999 - 2004, when Imberti (et al. 2004) were able to do more work on its wintering grounds. They reported surveying the Chilean Fiords and the southern part of Santa Cruz where some new sites for the species were discovered. However, the numbers they found suggested that the grebe’s population might be declining. Clearly, summer information was needed and accordingly, Asociación Ambiente Sur organized expeditions to the plateau in January and March 2009. The four plateaus (Strobel, Buenos Aires, Siberia and Viedma) identified during the 1980s as having the large majority of breeding grebes were visited. Over 50 lakes and lagoons, which included the six key water bodies thought to host 40% of the total population in the 1980’s (Johnson 1997), were surveyed. Of the six key lakes from the 1980s, three were entirely devoid of grebes and the others had far fewer grebes than during that decade. In the 1980s, the total mean number of adults found on these lakes was 1,832; the recent surveys found only 117 individuals. The equivalent figures for nests were 581 and zero. These findings were sufficient evidence for BirdLife International, in its 2009 edition of the IUCN Red List, to uplist the Hooded Grebe by two whole categories of threat due to a possible decline of 40% (see Conservation Status).
Subsequent field work in the 2010/11 summers produced a more extensive coverage of the plateaus with over 250 lakes visited and many more were found dry. Due to this factor, the discovery of the mink as a serious problem and the rest of the currently known threaths, the total population of the grebe may have declined as much as 60% and it is estimated to be under 1200 individuals.
Imberti, S., H. Casañas, and Ignacio Roesler. 2011. Hooded Grebe (Podiceps gallardoi), Neotropical Birds Online (T. S. Schulenberg, Editor). Ithaca: Cornell Lab of Ornithology; retrieved from Neotropical Birds Online: http://neotropical.birds.cornell.edu/portal/species/overview?p_p_spp=91111