Shiny Cowbird Molothrus bonariensis

  • Order: Passeriformes
  • Family: Icteridae
  • Polytypic: 7 subspecies
  • Authors: Peter E. Lowther


Shiny Cowbirds are brood parasites.  The nest, though selected by a female cowbird, is that of another species.  Care for cowbird eggs and young provided by host species and reflects characteristics of the host species.  In a study of host selection by Shiny Cowbirds, Wiley (1988) made several observations: (1) Hosts were not parasitized in proportion to their abundance; (2) breeding season of Shiny Cowbird corresponds to breeding season of its "major" hosts; (3) food habits of cowbirds and major hosts overlap; (4) egg sizes of Shiny Cowbird and its hosts are similar; (5) cowbirds find nests by watching and closely monitoring nest status through frequent visits so as to facilitate synchronized egg-laying; and (6) host nest structure has no influence on cowbird selection. In another study in Argentina, Shiny Cowbirds seemed to prefer hosts larger than themselves but use of "large" and "small" hosts was related to structure of host community (Mason 1980, 1986b). No evidence that females, which lay different egg morphs (see egg descriptions below), show any differences in host selection (Mason 1980, 1986b).

Host species show variable responses to parasitism by Shiny Cowbirds. Many host species accept cowbird eggs, but species that can distinguish and respond to cowbird eggs in the nest (by ejecting foreign eggs or deserting nests) exert selective pressures for coevolutionary adaptive responses by brood parasites (see Mason and Rothstein 1986, 1987).

About 250 species are victims of Shiny Cowbird parasitism, 93 of which are known to have reared its young (see Host list of the parasitic cowbirds).  Most commonly reported hosts, based on the about 900 records assembled by Friedmann (1963), were as follows: Rufous-collared Sparrow (Zonotrichia capensis), Common Diuca Finch (Diuca diuca), Fork-tailed Flycatcher (Muscivora tyrannus), Rufous Hornero (Funarius rufus), White-banded Mockingbird (Mimus triurus), Chestnut-capped Blackbird (Agelaius ruficapillus) and House Wren (Troglodytes aedon).

Specific studies of interactions with single host species include work on the following species: Rufous-collared Sparrow (Sick 1958, King 1973, Fraga 1978, 1983), Chalk-browed Mockingbird (Mimus saturninus; Fraga 1982, 1985, Salvador 1984), Yellow-shouldered Blackbird (Agelaius xanthomus; Post and Wiley 1976, 1977b), Yellow-hooded Blackbird (Chrysomus icterocephalus; Cruz et al. 1990) and Brown-and-Yellow Marshbird (Pseudoleistes virescens; Mermoz and Reboreda 1994).

General studies of host-parasite interactions include those by Cruz et al. (1985) in Puerto Rico, Manolis (1982) in Trinidad and Tobago, Post et al. (1990) in St. Lucia, and Fraga (1982, 1985) and Mason (1980, 1986a, 1986b) in Argentina. Collectively these studies document geographic variation in which species serve as favored hosts; a host favored in one region may not be selected in another (see also Wilson 1979). Pattern of host choice stable in Trinidad and Tobago over 60 yr period (comparing period 1920-1940 with 1950-1984; Cruz et al. 1995).

No information or phenology of pair formation.

First clutches per season.  In Dominican Republic, oviducal egg reported mid-April (Arendt and Vargas Mora 1984). In Puerto Rico, mid-March-July (Wiley 1988); 10 March-28 October, peak in June and July (Pérez-Rivera 1986). In Venezuela, early May to mid-November, most active June and July (Ramo and Busto 1981). In western Ecuador, February-April (Marchant 1960). In Argentina, September-February (Friedmann 1963); 26 September- 7 February (Fraga 1986); 15 October- 22 February (Salvador 1983).

Does not build nest; characteristics of host birds, host nest, host nest site, host habitat, host eggs or some combination of host characters may influence selection by female cowbird of particular host nest for parasitism. Hosts selected include species which nest in cavities or which build enclosed nests. Female cowbirds find nests by silent vigil, active searching and flushing behavior (short noisy flights into potential nesting area, apparently intended to flush hosts from nest; Wiley 1988). Perches used for silent vigils were 9.4 m ± 0.59 SD (range 2-23; n=49) from host nests which were visited (Wiley 1988).

At colonies of Yellow-hooded Blackbirds in Trinidad, Surinam and Venezuela, Wiley and Wiley (1980) observed female cowbirds at lookout perches next to, or within, the marsh colony; female cowbirds would fly across marsh and often slow to almost a hover over blackbird nests; there was little apparent attempt at stealth and male blackbirds would chase cowbirds. At study site in Colombia, with House Wren as primary host, female cowbirds made inspection visits throughout nesting cycle (3.4 visits/h [during 20 h observation] when wrens were building nest foundation; 1.8/h [21 h observation] when building nest cup; 1.4/h [12 h observation] during egg laying); of visits made before 12:00, most (39 of 51) were by single females and included brief (<15 s) entry into nest cavity; once a female was observed to inspect 2 wren nests in sequence (Kattan 1997). In Puerto Rico, rate of visits by Shiny Cowbird to host nests were as high as 1.5 cowbird visits/h at both Yellow Warbler (Dendroica petechia) and Greater Antillean Grackle (Quiscalus niger) nests, and about 9 cowbird visits/h at Yellow-shouldered Blackbird nests (Wiley 1988). In Argentina, male seen to follow nest-searching female only 2 times (Fraga 1978). In Venezuela, males seen to visit Stripe-backed Wren (Campylorhynchus nuchalis) nests alone (Piper 1994).


Shape. Usually ovate.

Size. Eggs of 2 morphs; spotted and immaculate (see description of color, below): For M. b. minimus in Puerto Rico and Hispaniola: mean dimensions 20.65 mm ± 0.93 SD (n=235) x 16.46 mm ± 0.59 SD (n=235; Wiley 1988, Cruz and Wiley 1989). In Argentina (Buenos Aires Province), spotted eggs (n=73) from Chalk-browed Mockingbird nests -- mean 22.41 mm ± 1.20 SD x 18.06 mm ± 0.59 SD; immaculate eggs (n=5) -- mean 22.89 mm ± 0.97 SD x 18.16 mm ± 0.79 SD; Fraga 1985). Spotted eggs (n=18) from Rufous-collared Sparrow nests -- mean 22.91 mm ± 1.06 SD (range 20.7-24.7) x 17.91 mm ± 0.72 SD (range 16.6 - 18.9); immaculate eggs (n=22) - mean 22.56 mm ± 0.94 SD ( range 20.7-24.3) x 18.35 mm ± 0.89 SD ( range 16.8-19.8); immaculate eggs more rounded (Fraga 1978). In Argentina (Córdoba Province): spotted eggs (n=197) from 11 hosts -- mean 24.16 mm (range 20.7-27.6) x 20.17 mm (range 18.2-20.9); immaculate eggs (n=29) -- mean 24.36 mm (range 22.8-25.1) x 19.18 mm (range 18.4-21.2; Salvador 1983). In Brazil: mean 22.28 mm ± 0.95 SD (n=31) x 17.51 mm ± 0.51 SD (n=31; Cavalcanti and Pimentel 1988).
Mass. In Argentina, mean of 19 eggs 4.0 g ± 0.3 SD (Fraga 1985); of 31 eggs 4.71 g (range 3.9-5.3; Salvador 1983). In Brazil, mean 3.75 g ± 0.6 SD (n=34; Studer and Vielliard 1988). In Colombia, mean 4.3 g ± 0.4 SD (range 3.5-5.2; n=49); mean energy content of eggs measured as 14.54 kJ ± 0.19 SD (n=10; Kattan 1995).

Color. Spotted and immaculate color morphs exist.  Immaculate eggs. Usually white, rarely pale bluish white.  Spotted eggs. White, pale gray, pale blue or pale buff ground color and marked with variable patterns of brown to reddish-brown blotches or spots; often with underlying marks in gray or pale lilac. Eggs seemingly intermediate are infrequent (10 of 71; Lyon 1997) or rare (1 - 2% of 250; Fraga 1985).
Eggshell texture slightly glossy.

Eggshell thickness. From Spaw and Rohwer 1987: for M. b. minimus, mean 0.126 mm ± 0.004 SD (n=4); for M. b. bonariensis, mean 0.138 mm ± 0.011 SD (n=11). From Rahn et al. 1988: for M. b. minimus, mean 0.113 mm (n=29); for M. b. bonariensis, mean 0.131 mm (n=295).

No information on clutch size.

Eggs laid before noon (Hoy and Ottow 1964, Cruz et al. 1990). One observed egg laying: At 06:00 female flew straight to nest of Yellow-hooded Blackbird when male blackbird was absent, laid egg and left after only 30 s (Wiley and Wiley 1980: 161). Lays eggs before sunrise (06:00) in nests of Forbes' Blackbird (Curaeus forbesi; Studer and Vielliard 1988). Some Furnarius ovenbird nests reported receiving 15, 17, 20, 25, 26, 37 Shiny Cowbird eggs (Friedmann 1963); nest with 25 eggs possibly result of 12 females laying (on basis of egg appearance; Miller 1917). Shiny Cowbirds regularly puncture or remove host eggs (Fraga 1986). Of 267 eggs laid in House Wren nests, 82 were laid in 20 abandoned nests, 65 before wren's egg laying, 87 during wren's egg laying, and 33 after wren's egg laying (Kattan 1997).

Shiny Cowbirds do not develop incubation patch.

Incubation period. Lasts 10 - 11 d (n=3 eggs; Wiley and Wiley 1980); 11 d (at 1 nest of Rufous Ovenbird; Hermann 1966); 11 d (n=4; Studer and Vielliard 1988); 11.5 d (n ≈ 8; Woodworth 1997); 11-12 d (n="some"; Friedmann 1929), 11.5 d (n=1) or 12 d (n=9; Fraga 1978); 12 - 13 d (n unknown; Salvador 1984); 11-13 d (n=22 nests; Mermoz and Reboreda 1994). For eggs incubated at 38 C, mean 11.7 d ± 0.5 SD (range 11.2-12.1, n= 11); in the field, 12.0 d ± 0.8 SD (n=7; Kattan 1995).
For some comparison with some hosts, incubation period for Rufous-collared Sparrow is 13 d (n=1; Fraga 1978); for Yellow-shouldered Blackbird, 13 d (n=2 eggs; Post 1981); for Brown-and-yellow Marshbirds, 14 - 15 d (n=22 nests; Mermoz and Reboreda 1994); and for Puerto Rican Vireo (Vireo latimeri), 15 d (n=26 eggs; Woodworth 1997).

Hardiness of eggs against temperature stress; effect of egg neglect. Sclater and Hudson (1888, see also Bendire 1893, Hudson 1920, Friedmann 1929) give report of a Shiny Cowbird egg, buried in nest lining of Yellow-browed Tyrant (Satrapa icterophrys), apparently for 5 wk, containing a viable and fully developed embryo.

No information on preliminary hatching events.  No information on time of day. Host species would provide any assistance at hatching or disposal of eggshells. Time from shell-breaking to emergence from egg, 2 - 3 h (Kattan 1995).


Condition at hatching. Altricial (nearly naked and helpless) and nidicolous (confined to nest). After dried from hatching, skin light orange pink; tarsi and toes light orange; claws light yellow; bill light grayish yellow; eye-skin bluish, with greenish yellow slit; rictal flange white; interior of mouth orange-yellow, with red on middle of roof; neossoptiles grayish; mass about 2.5 g (Friedmann 1929). Neossoptile counts on 3 nestlings: 200, 112, 84 (Collins and Minsky 1982). Mean mass at hatching, 3.5 g ± 0.18 SD (n=5; Kattan 1996).

Growth and development. Nestlings have yellowish skin, and sparse tufts of pale gray down; oral flanges range from white to yellow (Fraga 1978, 1985). Sheaths for primaries and secondaries emerge at end of day 3; eyes begin to open day 4 and fully open day 5; sheaths of wing feathers begin to open day 6; "usually" leaves nest at day 10 (Friedmann 1929). Growth constant K = 0.568/d and asymptotic mass 42.0 g; mean mass at 5 d, 21.5 g ± 1.9 SD (n=9), at 10 d, 39.4 g ± 3.6 SD (n=9; Kattan 1996). See also Wiley (1986), Figure 4. Young cowbirds in nests of Grassland Yellow-Finch (Sicalis luteola) did not survive, most likely because of unsuitable nestling diet (Salvador and Salvador 1986).

Behavior. At hatching, will right itself if turned over, beg for food and call peep; initially young birds maintain a flat position (lies on belly with head on nest floor); by day 5, young assume sitting position; preening observed about 1 wk after hatching (Friedmann 1929).

In contrast to behavior of host nestlings, "anything approaching ... is welcomed [by Shiny Cowbird young] with fluttering wings and clamorous cries, as if all creatures were expected to minister to its necessities" (Hudson 1874: 158, see also Hudson 1920: 82-86). Cowbird nestlings show "unusually aggressive and vigorous begging behavior, at least in comparison to their meadowlark hosts [Loyca or Greater Red-breasted (Sturnella loyca) and Pampas S. defilippi meadowlarks]" (Gochfeld 1978: 44). Up to age 4-5 d, cowbirds give short, loud peep's; in older nestlings, the calls are longer, tremulous calls with hissing quality, 4-9 kHz (see Figure 3 in Fraga 1985 for sonograms of 2-d-old and 7-d-old young). Initial peep becomes "a beady, sibilant zeee ... [then] more beady, slightly more husky, and a little lower in pitch" (Friedmann 1929: 131).


Departure from the nest. Nestling period 12 - 14 d in Colombia; leaves nest at about 65 - 75% of adult mass (Kattan 1996). Studies in Argentina report nestling period 12-13 d in Rufous-collared Sparrow nests or 13-15 d in nests of other hosts (Fraga 1978); 14-15 d (Fraga 1985); 12-13 d (Salvador 1984).

Growth. No information once young leave nest. Location call is a loud klip which is different from young's begging call (Sick 1985, 1993).

Association with parents or other young. Continues to be cared by host adults for about 2 wk; selects conspicuous perch from which to loudly beg for food (Friedmann 1929). Three surviving young, of 2 broods, remained with Chalk-browed Mockingbird hosts for 20-21 d after leaving nest (Fraga 1985).

No information on ability to get around, feed, and care for self.

Recommended Citation

Lowther, P. E. (2011). Shiny Cowbird (Molothrus bonariensis), version 1.0. In Neotropical Birds Online (T. S. Schulenberg, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA.