IUCN threat status:

Critically Endangered (CR)

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Biology

With its life cycle wholly dependent on infrequent showers of rain, the Raso lark has a particularly precarious existence (2). During periods of drought, one of the few sources of food and water for this species is provided by the small subterranean bulbs of the nutsedges, Cyperus bulbosus or Cyperus cadamosti. In order to reach the bulbs, the Raso lark excavates shallow burrows in the sandy soil using its strong bill (2) (4). Males appear to consume more bulbs than the females, and the largest dominant males usually form territories containing several productive burrows, which they aggressively defend (4) (6). In contrast, females are more reliant on surface food sources such as grass seeds and insects (4). While the difference in bill size between the sexes has previously been thought to account for the differences in feeding behaviour (2) (3) (4), more recent research has shown that this may not be the case (6). Despite its smaller bill size, the female appears to be equally efficient in digging for bulbs, and, as such, its lower bulb intake appears to be due to competitive exclusion from burrowing sites by males. This added obstacle to finding food in Raso Island's harsh conditions may explain why the Raso lark's adult population is mostly comprised of males. Not only are the females more likely to starve during droughts, but the increased time spent foraging relative means that less time is spent keeping a look out for potential threats, making losses due to predation more common (6). Raso lark breeding coincides with the onset of rain showers, with the males courting the females by quietly singing, raising the crest and hopping up and down on the spot with the wings held open. After mating, both sexes collect nesting material such as dried grass, which the female then uses to line a small, three-centimetre deep scrape in the soil, while the male defends the nesting site from intruders. A clutch of up to three eggs may be produced over a period of several days, with individual eggs sometimes laid over a day apart. These are incubated by the female for short, ten-minute periods, interspersed by preening and feeding breaks (4). Few chicks appear to survive to fledging, as newly-hatched chicks and eggs appear to be heavily predated by the Cape Verde giant gecko (Tarentola gigas) (2) (3).

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Source: ARKive

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