Behavior

Both sexes of Andean Hillstar often perch atop shrubs for extended periods on hillsides (Fjeldså and Krabbe 1990).

Well known for being the first species of hummingbird proven to go into torpor (Pearson 1953), Andean Hillstar often roosts in caves, limply clinging to vertical rock surfaces and propping itself up by the tail. It can drop and maintain its body temperature to within 0.5º C of the ambient temperature (down to around 7º C), thereby reducing the gradient of heat loss to the environment (Carpenter 1974).  The frequency and duration of torpor both increase significantly during the winter, and Carpenter suggests a photoperiodically controlled rhythm which would allow birds to preemptively conserve energy for later expenditures during the night (Carpenter 1974).

This species persists on high Andean plateaus, where elevations rarely drop below 3,700 m for up to 70 miles in any direction, preventing any daily altitudinal retreat, a strategy favored by many small birds. As such, Andean Hillstar roosts and nests in caves (which maintain a warmer and more consistent temperature than open air) and become torpid to endure near freezing temperatures. Since breeding corresponds with the wet season, cloud cover often acts as temperature buffer, preventing severe drops during the night, but hail during the day can compound the metabolic challenges of torpor by preventing feeding and adding a 15-hour fast on top of the next night of torpor - these birds' abilities to survive such conditions are remarkable.

As denizens of extreme Andean elevations, Andean Hillstars also are known for perching on flowers while feeding, instead of hovering. Despite this habit, however, Altshuler et al. (2004) suggest that the relatively minimal energy demands of hovering are never out of reach, even at the highest elevations; more taxing efforts such as evasive or pugnacious maneuvers are often quite costly at higher elevations.

Carpenter (1976) describes males as being relatively aterritorial, with females aggressively defending suitable nesting sites. Rocky areas with suitable nest sites tend to have the highest density of nectar, so males often are forced to open hillsides, where food is sparse; the males’ larger body size may potentially offset this cost allowing more efficient, level flights between feeding sites (Carpenter 1976).

Females defend access to suitable nest sites and nectar sources. Oobservations of several caves having only one nest, and a high proportion of caves in an area being occupied, suggest that competition for these sites may be intense, and constrain population densities (Pearson 1953).

While males tour female territories, they perform steep display flights, spiraling 30-40 m above the ground and diving to just above a perched female. A low, undulating flight display has also been described (Fjeldså and Krabbe 1990).

Andean Hillstars typically are solitary.

Andean Hillstars (despite energetic costs noted above – see Behavior) are aggressive birds, and have been known to even attack Giant Hummingbirds (Patagona gigas) (Fjeldså and Krabbe 1990). Within species aggression also is common, wherein females will fiercely defend territories, driving away conspecifics of both sexes (Carpenter 1976).

No descriptions of predation events were available. However, in addition to thermal regulation, their habit of roosting deep in caves (in nearly complete darkness) almost certainly functions to reduce predation rates during torpor (Carpenter 1974).