Among hummingbirds, virtually all of the activities associated with nesting and the rearing of young are the sole responsibility of the female. No other major avian family seems to have adopted so overwhelmingly this trend toward male emancipation from nesting responsibilities and consequent promiscuous mating tendencies. J. J. Audubon ironically believed that the beauty of hummingbirds must cause one to “turn his mind with reverence toward the Almighty Creator.” Yet, were it not for their remarkable mating system and a high degree of associated territorial advertisement behavior, these birds might well have been no more esthetically attractive than their drab relatives the swifts, which have consistently held to a monogamous mating system. In adopting an adventurous and specialized life-style, involving a high degree of nectar-dependency, a prodigal expenditure of energy during flight, and a seemingly devil-may-care mating system, the hummingbirds epitomize a unique kind of high-risk but potentially high-reward strategy for survival.
Female hummingbirds are among the most tenacious and persistent of mothers. They often build or rebuild their nests in the most vulnerable locations, and audaciously attack any human or animal that ventures too near, including large hawks that might easily consume the bird in a single swallow. For their part, the males are no less admirable in their stalwart defense of foraging or mating territories, and on a few rare occasions have been observed incubating eggs or helping to feed the young. The most notable examples of male incubation were reported by two independent observers of wild individuals of the sparkling violet-ear (Moore, 1947; Schäfer, 1952). More recent investigators have also studied the species, however, and have been unable to confirm participation in incubation or parental feeding as a regular pattern of male behavior either in the wild or in captivity.
There have been reports of males of a few other tropical species of hummingbirds helping in incubation or feeding the young, including members of the genera Glaucis and Phaethornis. A few scattered observations of male incubation have been reported in North America, as for example in the ruby-throated (Welter, 1935) and the rufous hummingbirds (Bailey, 1927). Both of these species are most northerly of all hummingbirds in breeding distribution; in such a climate, with relatively cold environmental temperatures and limited food resources, a monogamous mating system with male participation in incubation and brooding would be most advantageous. There is also a single reported observation of an adult male Anna hummingbird feeding young (Clyde, 1972).
Although the male fiery-throated hummingbird does not defend or feed the young fathered by him, he allows females with which he has mated to forage within his territory, probably because of the considerable sexual segregation in foraging behavior exhibited by this species (Wolf and Stiles, 1970). This species does not exhibit a definite pair bond, but does show this remarkable cooperation of the sexes in their reproductive biology. Perhaps the pair-bonding system in this species should be considered polygynous rather than promiscuous.
The first step in hummingbird nesting is construction, which normally occurs well before fertilization and produces some of the most remarkable of all avian structures. Almost invariably the nest contains extensive wrappings of spider webbing or similar silken materials, which are used to bind it together and to lash it to a solid substrate. In addition, nests of all species contain a very soft inner lining, usually made of a cottony seed material, the wooly surface material of some leaves, or soft bird feathers as may be locally available. Finally, in most species the nests are “decorated” (camouflaged) on the outside with fragments of lichens, bark, moss, or other similar materials, which blend them almost perfectly with the immediate environment.
Although the hummingbird nests are relatively similar in composition, they are placed in a wide variety of locations and substrates. They may be saddled on horizontal branches, partially suspended in a fork or in crotches of trees, adhered to the walls of rock faces, or suspended from above by pendant strands (Figure 21). In the subfamily Phaethornithinae, the typical substrate consists of a hanging leaf, such as that of a palm, with the nest supported on its underside along the leaf and with a long “trailer” of leafy matter hanging downward from the nest. Such seemingly precarious locations are probably quite secure, and protect the nest from rain and from most terrestrial predators.
A few hummingbirds, including the sooty-capped hermit, enhance the equilibrium of the nest by incorporating small bits of clay or pebbles into its bottom and sides to counterbalance the weight of the sitting female (Figure 21). Similarly, the Andean hillstar increases the nest materials unequally on one side, achieving the same result (Ridgway, 1890). The nests of the latter species are otherwise unusual in being remarkably large and thick-walled, which increases the insulating value of the nest for these high Andean birds (Carpenter, 1976; Dorst, 1962). They nest in extremely well-protected and inaccessible locations, sometimes in shallow caves of deep ravines, and as many as five active nests have been found within a radius of only 2 meters in such favored locations (Smith, 1949)—an amazing concentration for any hummingbird, given the bleak environment. An equally remarkable breeding concentration of crimson topaz hummingbirds was reported by Ruschi (1979), who found 10 occupied nests of this species in an area of 100 square meters.
21. Nesting sites and nests of hummingbirds, including palm-leaf nest of long-tailed hermit (A), hanging nest of blue-throated hummingbird (B), saddled nest of black-chinned hummingbird (C), doubly supported nest of Costa hummingbird (D), pine-cone clump nest of calliope hummingbird (E), fork nest of vervain hummingbird (F), counterbalanced nest of sooty-capped hermit (G), and crotch-supported nest of white-eared hummingbird (H). (After various sources)
The length of time required to construct the nest probably varies greatly, but in a few observed cases the work has been virtually completed in a day or two (Bailey, 1974; Welter, 1935). More often it takes about a week, and sometimes the work may be spread out over two weeks (Legg and Pitelka, 1956). Frequently the female continues to add materials to the nest after she lays the eggs, and sometimes she continues this behavior well into incubation.
22. Eggs and nestlings of hummingbirds, including egg (A) and nestlings of blue-throated hummingbird at 1 day (B), 3 days (C), 6 days (D), 12 days (E), 24 days (F), and final bill length (G). Also shown are eggs of vervain hummingbird (H) and giant hummingbird (I). (Mostly after Wagner, 1952)
The eggs are pure white and almost invariably two in number. The tiny bee hummingbird of the West Indies probably lays the smallest egg of any species, but measurements are not available. However, the eggs of the slightly larger vervain hummingbird of the same area measure approximately 7.0 by 5.0 millimeters (Ridgway, 1890) and weigh only about 0.37 grams each. A normal clutch of two such eggs would thus be equal to about 34 percent of the weight of the adult female (Lack, 1976). The largest hummingbird eggs are those of the giant hummingbird, which average 20 by 12 millimeters (Figure 22) and probably weigh very close to 1.5 grams each; a clutch of two would thus represent about 15 percent of the weight of the adult bird. So, in common with other birds, the energy drain of laying eggs is probably less severe on females of larger species than of smaller ones.
In many species (or individuals) of hummingbirds, the female begins to incubate immediately after laying the first egg, and the eggs thus hatch in the same approximate time sequence with which they were laid. The eggs typically are laid in the morning and deposited about 48 hours apart. However, they are frequently laid on subsequent days and sometimes three days apart. When the eggs hatch synchronously or nearly so, incubation probably did not begin until the laying of the second egg.
Incubation periods of hummingbirds have commonly been seriously underestimated, perhaps because of their very small size; some published estimates of periods of as little as 9 to 12 days have appeared. In spite of the eggs’ small size, incubation periods are actually long, perhaps because females usually have to leave the nest for extended periods of time to forage every day. This factor may cause a general prolongation of the minimum incubation period to 15 to 17 days (with a few reliable observations of 14-day periods). The longest-known incubation periods are those of the Andean hillstar, which average about 20 days, and sometimes require 22 to 23 days (Carpenter, 1976; Dorst, 1962).
Young hummingbirds are hatched in a nearly naked, blind, and totally helpless state. At the time of hatching they seem to be nearly all “head,” but their eyes are tightly closed and the beak is barely indicated. Yet even newly hatched hummingbirds have a well-developed crop, and shortly after they hatch the female begins to “inject” extraordinary amounts of food into her young. She inserts her needlelike bill into the nestling’s mouth and regurgitates food from her own crop to that of the young. Even nestlings but a few days old are fed large quantities of tiny insects and probably also nectar, which soon causes their crops to protrude from the sides of their necks like gigantic goiters (Figure 22C–E).
Hummingbird youngsters lack a distinct downy feather stage; instead, the definitive contour feathers emerge directly from the pinfeathers. Yet, in spite of the lack of downy insulation, the young birds are remarkably tolerant of short-term temperature fluctuations. Moreover, by the time they are about 12 days old, before they are well-feathered, they have often acquired a considerable degree of temperature control (Calder, 1971). Depending on the species, the female may continue to brood them until they are from 12- to about 18-days old. Species that rear their young under relatively cold conditions may have a prolonged fledging period; the Andean hillstar, for example, usually requires about 38 days, but favorable conditions may lessen it to as little as 22 days (Carpenter, 1976). Even after fledging the tail feathers and bill of young hummingbirds continue to grow for some time before they reach their adult length (Figure 22G), and maternal care and feeding of the young often continues for a while after the young leave the nest. Skutch (1973) has summarized information on the duration of parental care in various hummingbirds, and for five species the observed range of the last observed feeding was 40 to 65 days after hatching.
Sometimes adult hummingbirds attempt to feed young that are not their own. Thus Wagner (1959) observed wild adult white-eared hummingbirds feeding both nestling and fledgling birds that were not their own offspring. Under aviary conditions there have also been instances of adult birds “adopting” young not their own.
Even in tropical areas, most hummingbirds do not breed the year round, but rather exhibit seasonality in breeding that is probably associated with the relative intensity of blooming of preferred flower sources during wet or dry seasons. However, some equatorial species do breed throughout the year, as does the Andean hillstar in Ecuador (Smith, 1949) but not in southern Peru or northern Chile (Carpenter, 1976). Year-round breeding has also been reported for the Anna and Allen hummingbirds in southern California (Wells et al., 1978).
Although the incidence of multiple brooding still remains to be studied thoroughly, it is probably of relatively widespread occurrence in tropical hummingbirds. It has also been reported for several North American species, including the blue-throated hummingbird, where it seems to be fairly common. In other species such as the Anna, Allen, and black-chinned, it is less frequent, but probably all species attempt to nest a second time if their initial clutch or brood is lost prior to fledging.
In the species that sometimes exhibit multiple brooding, the female typically begins building a second nest while still feeding young from the first brood. Several instances of such concurrent care of two nests have been described for various North American species, including the white-eared (Skutch, 1973), ruby-throated (Nickell, 1948), and black-chinned hummingbirds (Cogswell, 1949).
In spite of the great perseverance and courage shown by female hummingbirds while defending their nests, the reproductive success of these birds in general is relatively poor. Such low rates for hatching and fledging young (Tables 8 and 9) are probably the result of high vulnerability of hummingbird nests to loss of eggs or young from accidents, weather-associated catastrophes, and predation. Indeed, one of the most successful species of nesting hummingbirds is the Andean hillstar, which avoids high predation losses in its cold and unfavorable nesting environment. In other tropical species, as well as North American ones such as the Anna hummingbird (Stiles, 1972b), predation accounts for much of the nest mortality (Carpenter, 1976; Baltosser, 1986).
In addition to their persistent efforts at nesting, hummingbirds have long potential breeding spans. Very few have been banded in any number, but at least one banded female ruby-throated hummingbird survived at least 9 years (Baumgartner and Baumgartner, 1992). A female broad-tailed similarly survived to 12.1 years, and a male to 8.0 years (Calder and Calder, 1992). Two banded female calliope were recaptured six years after banding, and a male after five years (Calder and Calder, 1994).
Although reliable data on mortality rates in hummingbirds are not yet available, Baumgartner (1981) obtained some recapture data on ruby-throated hummingbirds. Of the 384 hummingbirds she captured between 1977 and 1979, she recaptured 88 birds (23 percent) the following year, 31 of 268 birds (11.5 percent) the second year after banding, and 10 of 110 birds (9.9 percent) the third year. These figures indicate a minimum annual survival rate of 23 to 46 percent, and, because undoubtedly some survive but are not recaptured, the actual rate must be considerably higher. Calder and Calder (1992) estimated a 50-percent annual survival rate for the broad-tailed hummingbird.
