As with so many aspects of the life history of Black Swift, nesting was a mystery until 1901, when Vrooman (1901), while collecting Cormorants’ eggs on a sea cliff a few km west of Santa Cruz, California, flushed a Black Swift (C. n. borealis) from a nearby crevice. Upon closer inspection, he discovered a single egg in a depression in the mud. A week later he returned and collected the egg. He concluded that this species lays a single egg, as the egg he collected had undergone several days of incubation, and the bird was difficult to flush, as would be expected as incubation advances. The authenticity of his finding was questioned by many, so in 1905 he returned to a similar area and obtained an egg and nest, and also collected the adults. He estimated incubation of the single egg to be two-thirds advanced, further supporting his idea that this species lays a single egg annually (Vrooman 1905). Yet, it wasn’t until 1914 that Vrooman’s discoveries were truly accepted when he was accompanied by W.L. Dawson, a California ornithologist who witnessed the adult, nest and egg, and captured a photograph of the nest (Dawson 1915).
Dawson recognized that Black Swift was "a bird of the high mountains", and had observed them at Kearsarge Pass in the Sierras of California in 1913 and in the basin of the Little Yosemite River below Nevada Falls in 1914 (Dawson 1915). Previously there were a number of reports of sightings of Black Swifts ranging from the mountains of western North America, mainly on the Pacific Coast; north to British Columbia; east to eastern Washington and Colorado; and south to California (Bendire 1895). It wasn’t until 1919, when the first nest supporting Dawson’s statement was found near in Johnson Canyon, Banff, Alberta and described by Bent (1940). Subsequent to that first inland nest, others were reported (Michael 1927; Smith 1928; Dixon 1935; Thompson 1937; Hall 1948; Hunter and Baldwin 1962; Grant 1966; Knorr 1950) and others.
It is still unknown at what point a pair-bond is established. Does it develop on the wintering grounds, during spring migration, or at breeding colonies? There is little information on copulation in this species although it has been speculated to occur in the air (Rathbun 1925) and apparently observed by Foerster (1987).
Characteristics of occupied sites were described by Knorr (1961) and included five requirements: presence of water, which also allows moss to grow for use in nest building; high relief, or a commanding position above the surrounding terrain; inaccessibility to terrestrial predators; darkness or shading of nest niches; and unobstructed flyways for approaching the nest. Predictions of occupancy based on these requirements were made by Levad et al. (2008) which found that analysis of 291 sites supported the assumption that increasing stream flow, number of potential nest platforms, amount of available moss, shading of potential nest niches, topographic relief of surrounding terrain, and ease of aerial access to potential nest niches contributed to a higher probability the site would be occupied by Black Swifts.
Nest microhabitat (relative humidity and temperature) was studied in Colorado, New Mexico and California and determined that median temperature and relative humidity at nine Colorado and New Mexico sites were 9.4° C and 89.7%, respectively, and at one California site were 13.4° C and 92.8%, respectively; these values remained relatively stable throughout the nesting period with slight changes reflecting the ambient temperature and humidity of the surrounding macroclimate (Gunn et al. 2012).
There is also scant information on nest-building activities. Hirshman et al. (2007) reported that most nest-building activity apparently occurred early or late in the day, outside of observation periods, but she did observe that existing nests were repaired and nests were rebuilt at sites where nests had previously been placed. She reported that it took 13-15 days to repair or build nests. On one occasion, Marín (1997) observed one bird gathering moss from the wall at a nesting colony by clinging to the wall, wings extended, and gathering moss with its bill.
Materials used to make the nest appear to vary with locale and opportunistic use of available components. On sea cliffs, Vrooman (1901, 1905) reported the nests to be slight depressions in wet mud, with trampled green grass from nearby tufts of grass growing on the rocks which were constantly dripping with water. At another nest that was discovered a decade later in sea cliffs near Santa Cruz the egg was in contact with the moist floor of a "clinging root-lashed earthen bracket" with wiry green grass growing around the edges (Dawson 1915). Legg (1956) reported a thick-walled, compact, well-built saucer in a sea cave made primarily of live, green seaweed (Enteromorpha sp.) with smaller amounts of marine flowering plants, red algae and two kinds of moss. Nests found in the interior mountainous regions of western North America are composed of rounded cups of moss (Smith 1928; Thompson 1937) often mentioned as very moist or within the spray zone of waterfalls. Mud is mentioned as a possible component of some of these nests (Smith 1928; Lack 1956). At other sites, the nests were made of ferns found growing nearby (Michael 1927). Dixon (1935) also found the nests to be made firm cups of green resurrection moss, pressed down but not stuck together with saliva, placed on and supported by five-fingered fern. Hall (1948) reported a well-rounded, high-lipped nest made completely of growing moss. Hunter and Baldwin (1962) reported on five nests in Montana; those which could be examined appeared to be composed of mosses, liverworts, and brownish materials with some broken pine needles and several fibrous twigs at the bottom of the nest cup. All nests observed by the authors in Colorado and New Mexico are cups made of moss found at the associated waterfall. Fir and spruce needles are not uncommonly found at the base of the cup, but it is unclear whether these have been placed during nest building, repair or maintenance, or fell or blew in from surrounding trees. Occasionally, a fresh flower, strand of long grass or a green leaf has been observed (S.E. Hirshman personal communication). Collins and Landy (1968) reported a nest composed of mosses and ferns and lined with a few pine needles at Cofre de Perote, Veracruz, Mexico. Rowley (1966) reported a Black Swift nest at La Cima, Oaxaca, Mexico, that was placed on a recessed shelf with heavy water seepage over the nest, which was made completely of moss. C. n. niger near Laudat, Dominica produced bulky, cup-shaped nests of moss situated on ledges or in holes on the side of a small, damp ravine in dense forest (Bond 1941). In the tropical rainforest of el Yunque National Forest in Puerto Rico, a nesting C. n. niger was recently found and appeared to be composed of mud, moss and lichen (N. Gonzalez personal communication). A Black Swift (C. n. niger) nest was found in La Batata cave, Topes de Collantes Protected Area in the Sierra de Escambray area of south-central Cuba in 2009; the nest was composed of lichen and moss (Espin and Rivera 2010). Marín and Sánchez (1998) reported that C. n. costaricensis nests found in the upper Río Tiribí in Costa Rica were half-cup shaped and made with living mosses and liverworts with bits of dry material as lining and were attached to rock walls with some mud at the base.
Lack (1956) stated that all swifts, including Cypseloidine swifts use saliva in nest building, and all have enlarged salivary glands in both sexes in the breeding season. However, Ridgway (1911) stated that the nests of Nephoecetes (Cypseloides) swifts were "loosely put together and not held together by salivary secretion". Johnston (1958) also questioned whether all swifts had enlarged salivary glands in the breeding season and used saliva in construction of nests. Additionally, saliva has not been reported to be used in the construction of nests (Dixon 1935; Legg 1956; Johnston 1961), and the authors have not found evidence for this. In nests that are continually saturated with water, saliva would have no value in nest construction.
Nests can range from 7.6-20 cm (3-8 inches) across and 7.6 cm (3 inches) deep (Baicich and Harrison 1997). Nest size and shape can vary based on the size of the crevice, niche or ledge upon which it was built, and whether it is a freshly built nest or one near the end of the nestling period, the latter tending to be flattened by the activities of the nestling and adults. The authors have also noted fresh nests built on top of previous years’ nests, with the older nests having a faded brown or gray color to the moss and the newer nest showing a brighter green; at moist nests, the moss appears to have taken root and is growing.
The clutch of the Black Swift is invariably made up of a single egg. Exceptions to this were noted by close observation by Hirshman et al. (2007); in ten years of observation the only time a second or third egg was produced was when the first or second eggs disappeared from the nest. The egg is a smooth, non-glossy white. It is a long elliptical or long oval shape averaging 29 x 19 mm (Baicich and Harrison 1997). The egg surface has no markings, but can be stained by moist nest materials.
The nest is attended by both adults throughout the entire breeding season, but there is scant information on sharing of incubation, brooding or feeding duties. At Box Canyon Falls, Ouray, Colorado, a surveillance camera secured photos from one nest shortly after incubation started through fledging, and revealed presence of both adults, as well as periodic visitation of the nest or nearby roosting by other birds (Gunn 2010).
The most extensive study of phenology of C. n. borealis was conducted at a large colony in Colorado indicating a nesting success of 72% (Hirshman et al. 2007). The results of this study are given in the following table:
average and (range)
|Arrival||13 June (31 May-19 June)||11|
|Interval between arrival and egg laying||9 days (1-22 days)||11|
|Egg laying (does not include 2nd or 3rd eggs laid)||28 June (19 June-16 July)||73|
|Egg laying (includes 2nd and 3rd eggs laid)||30 June (19 June-29 July)||81|
|Incubation onset||1 July (16 June-16 July)||83|
|Incubation length||26 days (22-32 days)||56|
|Hatching||26 July (17 July-9 Aug)||59|
|Nestling phase||48 days (40-58 days)||56|
|Fledging||13 Sept (31 Aug-7 Oct)||117|
Prior to this study, there was scant information on the nesting phenology of Black Swift. Boyle (1998) reviewed egg and hatching dates based on two published reports and only four data points collected during surveys for the Colorado Breeding Bird Atlas. Bailey and Niedrach (1965) reviewed all known nesting information, which at that time represented 35 individual nest reports in Colorado, only 13 of which had phonological events observed and recorded. Outside of Colorado, the most thorough Black Swift nesting studies were in southern California. Marín (1997, 1999) reported 20 direct field observations and reviewed 67 egg data cards, and Foerster (1987) observed breeding of 13–14 pairs over a 2-year period. Other studies presented extrapolated or estimated phonological events (Murphy 1951; Hunter and Baldwin 1962).
From surveillance cameras (Gunn 2010) and direct observations (Hirshman et al. 2007) it has been established that at the maiden voyage of fledging Black Swifts is the advent of southward migration. On very few occasions it has been observed that a fledged swift returns to the nest, and in these instances, intense storms coincided with fledging (S.E. Hirshman personal communication).