Birds

Ledges on sheer, well-shaded cliffs, often behind waterfalls, cradle Yosemite's Black Swifts. From their misty nesting habitats, these shadowy scimitars range far and wide, materializing over meadows, forests, lakes and even mountain peaks. In late July and August, when parental cares are probably past, flocks of over 30 have buzzed the summits of Mt. Conness (12,600 '-MC), Mt. Dana (13,100'-JH) and the Dana Plateau (11,700'-DG).
The ruggedness and inaccessibility of the Black Swifts' breeding haunts have defeated nearly everyone who has sought their nests. Not so Yosemite postmaster Charles Michael, who not only located seven nests in Tenaya Canyon (approx. 6000') in the 1920s, but also secured excellent photographs of nestlings. "It was the wild, erratic winging of a lone Black Swift," he wrote, "that first attracted my attention. Such bewildering speed, such coordination of mind and muscle... Somehow the thought came to me of a great winged spider gone mad... While I watched, the bird suddenly swooped and fairly seemed to plaster himself to the wall not fifty feet from where I stood... The nest site was in one of the most inaccessible sections of a gorge as grand as any in all the Sierra".
"The nest was composed of the delicate pinnae of the five-fingered fern. Great banks of these ferns hung from neighboring walls, and it would be quite possible for the swifts to procure material while on the wing. Perhaps, though, the swifts may gather nesting material while clinging to a wall, as I have often seen swifts alight on a ferny ledge above Vernal Pall... The nest was placed on a bit of projecting rock ...located within the shelter of an overhung wall, thirty feet directly above a deep pool in the creek. The inner chasm is here very narrow; the vertical walls stand not fifty feet apart. The channel is dark and cool; in the long summer days the sun lights its depth for but a brief hour. And at no time or season does the sun ever play on the nest of the swift-cramped quarters, I should say, for birds of the wide skies" (Michael 1927).

Passerine extrapair mating dynamics: A Bayesian modeling ...

One striking paradigm shift in the study of avian mating dynamics has been the realization that monogamy in bird species is not the rule (Lack 1968) but the exception (e.g.,Griffith et al. 2002). Many broods consist not only of off-spring sired by the social father (the male providing care)but also of extrapair young (EPY) sired by a male who has engaged in one or more extrapair copulations (EPCs) with the focal female but provides no care for her offspring.Variation in the frequency of EPY is dramatic, varying from species without any EPY to species where,on average,almost 80% of offspring are EPY (Griffith et al. 2002). The current consensus explanation of this variation recognizes a hier-archy of processes affecting the probability of producing EPY (WestneatandSherman1997;ArnoldandOwens2002;Bennett and Owens 2002; Griffith et al. 2002; Westneat and Stewart 2003). Differences across species are probably due to differences in the division of parental care and in the cost scenario (for both parents) that unfolds when an EPYis produced (Mulder et al. 1994; Birkhead and Møller 1996;Gowaty 1996; Møller 2000; Møller and Cuervo 2000). An EPY entails costs to the social father when he provides care for an unrelated offspring, and he may therefore reduce his care or stop caring for the brood altogether, which will have repercussions for the female. Life history (especially longevity) probably plays an important role in mediating such costs, because the social male is more likely to desert a brood in case of uncertainty in paternity in long-lived species than in short-lived species (e.g., Mauck et al. 1999).