This paper provides data on the breeding biology and social system of the Black Redstart Phoenicurus ochruros at a Tibetan site (4300 m elevation). Egg-laying occurred between early May and early July during which pairs bred once (10 out of 12 marked pairs) or twice (2). Clutch size varied between 4 and 5 eggs (4.56 ± 0.51 SE), but a seven-egg clutch was recorded. Incubation by the female lasted 12–14 d (13.0 ± 1.0 SE). Nestlings were provisioned by both parents for 13–18 d (16.7 ± 1.6 SE). Nestlings fledged when they were slightly heavier than adult females. Compared to their European counterparts, the high-elevation redstarts had similar clutch size, incubation or nestling periods and tended to produce fewer clutches of larger eggs. This suggests a life history strategy adapted to maximize offspring survival in the harsh highland environments. Most nests were attended by one male and one female, while at several nests the pair was accompanied by a third individual in female-like plumage. One of these helpers was molecularly sexed as a male and its plumage pattern indicated it to be a yearling. Such cooperative groups occurred in two of the 12 closely monitored nests and two additional nests based on opportunistic observations. One of the four trios was confirmed to raise two broods during a single breeding season and another produced the seven-egg clutch. This is the first confirmation of cooperatively breeding among the 11 Phoenicurus species.
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