Giant Cowbirds are brood parasites. Nests, incubation and parental care of young provided by host species.
List of known hosts totals 10 species, mostly caciques and oropendolas (Friedmann 1963, Robinson 1988, Jaramillo and Burke 1999, McCrary and Gates 2007). In accounts of hosts below, mass for female icterid hosts from Schönwetter 1981; female Giant Cowbird listed as 143.0 g. Summary of nest height, nest size, and lengths of incubation and nestling periods taken from Drury (1962; see also Jamarillo and Burke 1999).
Turquoise Jay (Cyanolyca turcosa). – Included as host based on report by Welford et al. (2007) of observations made of 2 cowbird young cared for by jays from February to June 2006 as nestlings and later as fledged young.
Green Jay (Cyanocorax yncas).– Initially included in host list on basis of single observation (Lehmann 1960: 273, cited in Friedmann 1963) in Colombia of 2 jays attending a young cowbird for several days . Four additional observations of Green Jays as a host presented by Ramoni-Perazzi et al. (2010).
Chestnut-headed Oropendola (Psarocolius wagleri).– Female mass: 128.2 g, 137.8 g (2 subspecies). Nest height > 35 m, nest 55 - 100 cm long, 20 cm diameter; incubation 17 d, nestling 36 d. Smith's (1968) work (which is questionable, see Jaramillo and Burke 1999) was at mixed colonies of Chestnut-headed Oropendolas and Yellow-rumped Caciques (Cacicus cela) and his reported data on cowbird parasitism did not distinguish host species; collectively 1115 cowbirds fledged from 1502 parasitized nests out of a total of 3270 nests with clutch data. At 2 mixed colonies of 107 Chestnut-headed Oropendola nests and 97 Yellow-rumped Cacique nests in Panama, parasitism rates for nests with eggs were as follows: 8 of 31 oropendola nests contained 9 cowbird eggs (Fleischer and Smith 1992). Host in Costa Rica (Crandall 1914, Chapman 1928).
Montezuma Oropendola (Psarocolius montezuma).– Female mass: 224.5 g. Nest height 15 - 35 m, nest 60 - 120 cm long, 17 - 23 cm diameter; incubation about 14 d, nestling about 30 d. This oropendola has been observed to eject a cowbird egg, perhaps minutes after it was actually laid (Skutch 1954). Host in Costa Rica (Skutch 1954, Crandall 1914).
Green Oropendola (Psarocolius viridis).– Female mass: 232.0 g. Nest about 1 m long; no information on incubation and nestling period. Apparently listed erroneously as host by Friedmann (1963) but since then reported as "common host" and that cowbirds successfully reared by this species (A. Jaramillo pers. obs. in Jaramillo and Burke 1999).
Crested Oropendola (Psarocolius decumanus).– Female mass: 157.0 g. Nest height 10 - 20 - 35 m, nest 125 - 137 cm long, 20 - 22 cm diameter; incubation 17 - 19 d, nestling 28 - 34 d or 31 - 36 d. First recorded host of the Giant Cowbird in Brazil (Goeldi 1894, 1897) and Guyana (Lloyd 1897). In Guyana, Young (1929) described this species as usually receiving 2 cowbird eggs and as being less often parasitized than Yellow-rumped Cacique. Female oropendolas respond to cowbirds that visit colony, most often by chasing them from the vicinity of their nests; males generally do not respond to cowbirds (Tashian 1957). Host in Venezuela (Schäfer 1957), Trinidad (ffrench 1980), Surinam (Hellebrekers 1942, Haverschmidt 1967) and Bolivia (Schönwetter 1981).
Russet-backed Oropendola (Psarocolius angustifrons).– Female mass: 158.5 g. Nest height 5 - 8 m, nest 76 - 140 cm long, 20 - 22 cm diameter; incubation 19 - 20 d, nestling 25 - 30 d. In Peru, 3 of 24 family groups of Russet-backed Oropendolas included Giant Cowbird young, all oropendola females were observed feeding only 1 (host or cowbird) young (Robinson 1988). In Venezuela, Schäfer (1957) did not find this species parasitized, even when nesting in mixed colonies with Crested Oropendolas.
Yellow-rumped Cacique (Cacicus cela).– Female mass: 65.0 g. Nest height > 15 m, nest 30 - 45 cm long; incubation 13 - 14 d, nestling 24 - 30 d. Usually only 1 or 2 cowbird eggs in nests of this cacique (Young 1929, Kuschel 1986, 1987); record of up to 6 eggs found in nest of this cacique (Kuschel 1896, 1897) is questionable because of egg description (Friedmann 1963). Smith's (1968) work (which is questionable, see Jaramillo and Burke 1999) was at mixed colonies of Chestnut-headed Oropendolas and Yellow-rumped Caciques and his reported data on cowbird parasitism did not distinguish host species; collectively 1115 cowbird fledged from 1502 parasitized nests out of a total of 3270 nests with clutch data. At 2 mixed colonies of 107 Chestnut-headed Oropendola nests and 97 Yellow-rumped Cacique nests in Panama, parasitism rates for nests with eggs were as follows: 13 of 24 cacique nests contained 24 cowbird eggs (Fleischer and Smith 1992). In Peru, 0 of 106 nests that could be checked and 0 of 168 family groups [= young with female] of Yellow-rumped Caciques included Giant Cowbird eggs or young (Robinson 1988). In Guyana, Young (1929) described this species as more often parasitized than Crested Oropendola. Host in Trinidad (ffrench 1980), Surinam (Hellebrekers 1942, Haverschmidt 1967) and Guyana (Lloyd 1897, Young 1929). Cacicus persicus, previously listed as host, is considered a synonym of C. cela.
Red-rumped Cacique (Cacicus haemorrhous).– Female mass: 69.0 g. Nest 38 cm long; incubation 17 d; nestling 25 - 28 d. Listed as a "hypothetical" host in Guyana by Friedmann (1929), possibly based on Lloyd's (1897) account, but found to be a host in Brazil (Pinto 1953).
Spot-breasted Oriole (Icterus pectoralis). – Female mass : 40 - 46 g. Nest up to 45 cm long; incubation likely 12 - 14 d; nestling likely about 14 d. Single observation of adult orioles feeding out-of-nest cowbird in Nicaragua over 5 d from 4 Sep - 8 Sep 1999; usual oropendola or cacique hosts not present in area (McCrary and Gates 2007).
Streak-backed Oriole (Icterus pustulatus). – Female mass 5: 39 - 50 g. Nest up to 25 - 50 cm long; incubation 12 - 14 d; nestling about 14 d. Single observation of adult orioles feeding out-of-nest cowbird in Nicaragua over 4 d between 14 Aug and 23 Aug 2002; usual oropendola or cacique hosts not present in area (McCrary and Gates 2007).
Other large species of icterids that are sympatric with Giant Cowbird may prove to be hosts with further study; potential “new” hosts include Black Oropendola Psarocolius guatimozinus (suggested by W. Meise in Schönwetter 1981), Casqued Oropendola Clypicterus oseryi (suggested by Jaramillo and Burke 1999), and Olive Oropendola Psarocolius bifasciatus yuracares (suggested by Fraga and Kreft 2007).
Egg dates: Brazil: Feb (Kuschel 1897); Trinidad, Jan, Feb, May (n = 5; ffrench 1980); Jan, Feb, Mar (n = 10; Belcher and Smooker 1937).
Does not build nest; see discussion of hosts above. Regular host species are colonial icterids that build pendulous nests. Characteristics of either host birds, host nest, host nest site, host habitat, host eggs, or some combination of host characters may influence selection by female cowbird of which particular host nest for parasitism.
Eggs elliptical to oval; mean size for 17 eggs in Crested Oropendola (Psarocolius decumanus) nests: 35.7 x 24.3 mm; extremes 32.5 x 24.3, 33.7 x 22.4, 40.1 x 24.2, 37.6 x 25.6 and for 34 eggs in Yellow-rumped Cacique (Cacicus cela) nests: 34.0 x 25.5 mm; extremes 31.5 x 24.3, 34.4 x 23.8, 35.5 x 27.3, 35.1 x 29.0 (Haverschmidt 1966 citing Hellebrekers 1942, 1945; see also Schönwetter 1981). Mean size for 12 eggs from Crested Oropendola nests: 32.5 x 26.3 mm; extremes 30.2 x 26.9, 34.3 x 25.1, 35.3 x 25.4, 32.5 x 28.0 (Haverschmidt 1967). Mean size 35.79 mm ± 1.87 SD x 24.86 mm ± 0.86 SD for 31 eggs from Panama (Fleischer and Smith 1992). Measurements given in Friedmann (1963) are incorrect (i.e., too small; Haverschmidt 1966). Egg mass: 9 g for 1 egg (Schäfer 1957); 8 g for 1 egg (Haverschmidt 1966); 11.7 (range 9.7 - 12.6, n = 6; Haverschmidt 1967). Egg mass: 10.59 g (n = 4) for M. o. impacifua; 11.20 g (n = 84) for M. o. oryzivorus; shell mass: 0.870 g (n = 4) for M. o. impacifus; 0.860 (n = 84) for M. o. oryzivorus (Rahn et al. 1988 citing Schönwetter 1981); 0.750 - 0.840 g (Kuschel 1897). Background color pale blue with small black spots and hair lines (Haverschmidt 1967). Egg "spotless white, rough in texture and slightly glossed" (Crandall 1914). Egg seen ejected was "... very pale blue, almost white, with a few scattered scratches of brown" (Skutch 1954). Generally distinguishable from host species (specifically Yellow-rumped Cacique and Chestnut-headed Oropendola) by thicker and rougher shell (Fleischer and Smith 1992). Surface texture slightly glossy. Eggshell thickness 0.197 mm (n = 4) for M. o. impacifus; 0.170 mm (n = 84) for M. o. oryzivorus (Rahn et al. 1988 citing Schönwetter 1981).
Egg laying. Multivariate assessment of egg measurements and markings indicate significant and non-overlapping differences in morphology between eggs of Giant Cowbird and 2 hosts (Yellow-rumped Cacique and Chestnut-headed Oropendola) in Panama. Electromorph and color morph differences showed that 2 or more female cowbirds definitely laid in 6 of 10 nests containing 2 or more cowbird eggs. In the other 4 nests, paired cowbird eggs could not be differentiated by color or electromorph and may have been laid by the same female (Fleischer and Smith 1992).
Young altricial (nearly naked and helpless) and nidicolous (confined to nest). Recently hatched ("2 or 3 days old") cowbirds have white skin, well covered with long, dark gray down; beak white and gape wide and yellowish; would gape "eagerly" when nest disturbed; nestmate Montezuma Oropendola contrasted in appearance (blackish and downless) and behavior (would shrunk to bottom of nest when disturbed; Crandall 1914). Another young at "about two weeks old" was taken as zoological specimen for the New York Zoological Society on 26 April from Chestnut-headed Oropendola nest; on 25 May, beak beginning to darken; on 25 June, anterior portions of face and lores covered with pin feathers; by 10 August, only faint traces of white at tip of upper mandible remaining (Crandall 1914).
No information on incubation period or nestling period, nor for how long young remain in care of host adults after leaving the nest.
History of the discovery of brood parasitism in the Giant Cowbird
Although we now accept this species as a cowbird, and its association with oropendolas and caciques is well known, this was not always the case. The brood parasitic nesting habits of the smaller species of cowbirds had been known for a long time, but documentation of the reproductive biology of the Giant Cowbird was much slower to materialize. Indeed, not until near the end of the 19th century was the matter solved. The discovery of the brood parasitic habits of the Giant Cowbird was made nearly simultaneously by two different ornithologists, Emilio Augusto Goeldi and M. Kuschel.
Goeldi (1894) apparently was the first to describe the brood parasitic relationship between the Giant Cowbird and oropendolas. In his book As Aves do Brasil, Goeldi (1894: 284) wrote of his experience with Giant Cowbirds and his suspicions of brood parasitic behavior. He was able to confirm his suspicions before publication of his book and had added a footnote to his text to document his discovery:
A maneira de vida do Melro [= Giant Cowbird] concorda com a da especie anterior. Entretanto tenho-o achado sempre mais circumspecto e arisco. Vejo-o muitas vezes aos 2 e aos 4, fazendo de observatorios os mais altos galhos das arvores que cercam os pastos; si alguem se approxima, vão logo para longe. Quanto á maneira de reproducção não conheço na litteratura outros dados além da observação do principe zu Wied, que esta Ave, segundo informação dos naturaes, apossa-se frequentemente do ninho de outras especies. Por minha parte suspeito que S. ater [= Molothrus orizivorus], á maneira de seus primos menores, introduz seus ovos entre os de outras Aves e não se occupa directamente com a incubação. [Translation: The lifestyle of the Giant Cowbird is much like that of the previous species [= Chopi Blackbird Gnorimopsar chopi]. However, I find it much more nervous and skittish. I often see them in pairs, or in groups of four, perched atop the highest branches of trees surrounding the pasture, on sentinel duty. If anyone approaches, they fly far away. As to their reproductive behavior, I know of no record in the literature, other than that of the Prince of Wied, that this bird frequently takes over nests of other species. However, I personally believe that S. ater [=Molothrus orizivorus], much like its smaller cousins, lays its eggs among those of other birds and does not do any incubating duties.]
Amended with the following footnote:
Mais cedo do que esperava decidiu-se esta questão e no sentido que eu suggeria. Em meiados de Dezembro foi-me trazido outro ninho de Osthinops cristatus [= Psarocolius decumanus]. No mesmo encontrava-se, além de um legitimo Japú [= P. decumanus], outra Ave quasi tamanha, na qual notava-se logo a falta de pennas amarellas na cauda. A duvida si se tratava de alguma anomalia não levou muito a resolver-se; dia a dia desenvolveu-se o exemplar em questão, apparecendo claramente um Melro, Scaphidurus ater; assim elucidou-se a historia natural de mais uma Ave d'aqui. Como seus primos menores insinuam os ovos em ninhos de Aves menores, o Melro procura no Japú um padrasto que se lhe approxime em dimens¿s (17 de dezembro de 1892). [Translation: Sooner than I expected, this question was resolved, and in the manner I suspected. In the middle of December, another nest of Ostinops cristatus [= Crested Oropendola Psarocolius decumanus] was brought to me. In it, besides a legitimate Crested Oropendola chick, was another chick of essentially the same size that lacked the yellow tail feathers. The chick in question developed quickly, into an unquestionable Giant Cowbird, Scaphidurus ater, clarifying the natural history of one more local bird. Much like its smaller cousins that lay eggs in the nests of smaller birds, the Giant Cowbird finds in the Crested Oropendola, a bird of similar dimensions to raise its chicks (17 December 1892).]
Perhaps because this important work was published in Portuguese, however, the news of this discovery was slow to reach other ornithologists. Kuschel (1896, 1897) also discovered brood parisitism in the Giant Cowbird, apparently independently of Goeldi; in response Goeldi (1897) published a summary of his observations in English, and received belated recognition (Sclater and Saunders 1897) for his discovery.