Icterus bullockii x galbula
Status: Uncommon regular summer visitor west and west central, rare casual elsewhere.
Summer: May 9, 9, 9 <<<>>> Jul 25, 26, 28
Later dates are 4 Aug 2012 Red Willow Co, and 7 Aug 2020 Keith Co.
The last dates above are about a month prior to departure of Bullocks’ (11-12 Sep) and Baltimore Orioles (4-6 Sep in the west); perhaps hybrids are at a disadvantage finding mates and tend to leave early.
There are numerous records in the western half of the state, with these easternmost: 14 May 2014 Antelope Co, 4 Jun 2019 Merritt Reservoir, Cherry Co, 11 Jun 1986 Thomas Co, and 4 Aug 2012, Red Willow Co.
Comments: Hybridization occurs between this species and Baltimore Oriole (Rising 1970, 1996; Carling et al 2011; Walsh et al 2020), causing at one time their taxonomic status to be controversial. AOU (1983) lumped them as a single species, Northern Oriole, but returned both to full species status (AOU 1998) based on absence of free interbreeding at some locations resulting in a stable hybrid zone (Rising 1996, Carling et al 2011, Walsh et al 2020). Furthermore, “the two species differ in plumage, vocalizations, physiologic traits, molt, nesting ecology, and allozyme frequency to a degree greater than in any other birds generally treated as single species” (DeBenedictis 1996). Recent genetic studies suggest that the closest relative of Baltimore Oriole is actually Black-backed (Abeille’s) Oriole (Icterus abeillei) and that Bullock’s Oriole is a member of the Streak-backed Oriole complex (Flood et al 2020). One factor limiting expansion of the hybrid zone is lowered fitness of hybrids; one cause of this is that Baltimore Oriole molts in summer and Bullock’s in fall, but hybrids molt twice, a significant energy drain (Rohwer and Johnson 1992, Flood et al 2020), and another is the putative difference between the two species in ability to thermo-regulate (Carling et al 2011, Rising 1996).
Walsh et al (2020) found that the center of the hybrid zone has moved westward since the 1950s and narrowed to less than 100 miles, its center located around Crook, Colorado, south of Cheyenne Co, Nebraska. Carling et al (2011) estimated hybrid zone width nearer to 200 miles. The presence of phenotypically pure adults of both Bullock’s and Baltimore Oriole in the hybrid zone suggests assortative mating is occurring (Carling et al 2011, Flood et al 2020); Walsh et al (2020) found that genomic mixing is prevalent in the hybrid zone, some 41% “recent-generation (F1/F2) hybrids”.
Short (1961) considered the hybrid zone rather wide in Nebraska, with its center a line from Big Springs, Deuel Co to just west of Valentine, Cherry Co (see map above). Short found pure Baltimore Orioles west to Hastings, Adams Co, St. Paul, Howard Co, and Spencer, Boyd Co, and pure Bullock’s Orioles east only to Chadron, Dawes Co, and Big Springs. However, hybrids outnumbered pure Baltimore Orioles east as far as Blair, Washington Co, and “obvious hybrids” occur in the northwest in summer (Rosche 1982). Youngworth (1955) and Mossman and Brogie (1983) found Baltimore Orioles predominant in the Niobrara Valley Preserve, although hybrids were seen during breeding season. Both Baltimore and Bullock’s Orioles have been reported at NNF Bessey, Thomas Co (Bray 1994); a hybrid was there 11 Jun 1986. Jaramillo and Burke (1999) stated “If anything, there is evidence for an eastward invasion of Bullock’s genes, but not the opposite”. In Keith Co, Brown et al (1996) found that 116 of 176 birds examined (not all were nesting birds) were phenotypic Baltimore Orioles, and 41 showed evidence of hybridization, the remainder apparent Bullock’s Orioles. Of six orioles seen on the Dalton BBS route 18 Jun 2006, five were hybrids, the other a female Bullock’s Oriole. It is possible that the incidence of Bullock’s Oriole and introgressants east of the Panhandle has been reduced since the westward spread of Baltimore Oriole (Corbin and Sibley 1977).
Observations in Scotts Bluff Co suggest that as Baltimore Orioles moved west in the 1980s and came in contact with Bullock’s, hybrids were common, possibly since the the first birds westward may have been mostly males (as was the case with Blue Jays after they entered northeastern Colorado, Bill Kaempfer, pers. comm.) but as numbers of Baltimore Orioles increased in western Nebraska and assortative mating became an option, hybrids became less common. This sequence was observed in a Mitchell, Scotts Bluff Co yard by Kathy DeLara (pers. comm.); there was a Bullock’s paired with a Baltimore in 2007 and numerous hybrids for several years, but only one hybrid in 2017 and none in 2018 and no phenotypic Baltimores during those two years.
Acknowledgements: Photograph (top) of a Bullock’s x Baltimore Oriole (hybrid) in Keith Co 20 May 2017 by Noah Arthur.
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Bray, T.E. 1994. Habitat utilization by birds in a man-made forest in the Nebraska Sandhills. M.S. thesis, University of Nebraska-Omaha.
Brogie, M.A., and M.J. Mossman. 1983. Spring and summer birds of the Niobrara Valley Preserve, Nebraska: An annotated checklist. NBR 51: 44-51.
Brown, C.R., M.B. Brown, P.A. Johnsgard, J. Kren, and W.C. Scharf. 1996. Birds of the Cedar Point Biological Station area, Keith and Garden Counties, Nebraska: Seasonal occurrence and breeding data. Transactions of the Nebraska Academy of Sciences 23: 91-108.
Carling, M.D., L.G. Serene, and I J. Lovette. 2011. Using historical DNA to characterize hybridization between Baltimore Orioles (Icterus galbula) and Bullock’s Orioles (I. bullockii). Auk 128: 61-68.
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DeBenedictis, P.A. 1996. Fortieth supplement to the AOU check-list. Birding 28: 228-231.
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Rising, J.D. 1970. Morphological variation and evolution in some North American orioles. Systematic Zoology 19: 315-351.
Rising, J.D. 1996. The stability of the oriole hybrid zone in western Kansas. Condor 98: 658-663.
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Rosche, R.C. 1982. Birds of northwestern Nebraska and southwestern South Dakota, an annotated checklist. Cottonwood Press, Crawford, Nebraska, USA.
Short, L.L., Jr. 1961. Notes on bird distribution in the central Plains. NBR 29: 2-22.
Walsh, J., S.M. Billerman, V.G. Rohwer, B.G. Butcher, and I.J. Lovette. 2020. Genomic and plumage variation across the controversial Baltimore and Bullock’s oriole hybrid zone. Auk 137: 1–15. DOI: 10.1093/auk/ukaa044.
Youngworth, W. 1955. Some birds of the Quicourt Valley. NBR 23: 29-34.
Silcock, W.R., and J.G. Jorgensen. 2021. Bullock’s x Baltimore Oriole (hybrid) (Icterus bullockii x galbula). In Birds of Nebraska — Online. www.BirdsofNebraska.org
Updated 26 February 2021