When Orioles Turn

69 Whe n Orioles Tur n Red Dan Derbyshire and Tom Flinn Figure 1 : Baltimore Oriole at Tommy Thompson Park Bird Research Station, Toronto, ON, 22 August 2005. Photo: Dan Derbyshire In August 2005, a Baltimore Oriole (Ict - involving the carotenoid rhodoxanthin. erus galbula ) that had unusual reddish To elucidate this possibility we will look hues to its plumage was captured at the at the following topics: the area where Tommy Thompson Park Bird Re search the bird was captured, a description of Station in Toronto. The authors believe the bird’s aberrant colouration, a gener - that this aberrant colouration was caus - al look at how birds attain the colours ed by something in the diet of this bird. they exhibit, the involvement of rhodo - While some degree of colour varia - xanthin in colour variation of Cedar tion is found in many species of birds, a Wax wings ( Bombycilla cedrorum ) and its search of the literature has revealed no suspected role in colour variation in described instances of diet-induced ery - some other species. thrism (reddening) in Baltimore Oriole VOLUME 25 NUMBER 2 70 Background the appearance of the second and much Tommy Thompson Park is the largest redder bird gave notice that further area of natural habitat that exists on the inves tigation was warranted. central Toronto waterfront. The park, In first basic plumage one would commonly considered Toronto’s “urban expect the areas that were significantly wilderness”, was designated an Impor - reddened would be coloured yellow to tant Bird Area (IBA) by Birdlife Inter - pale orange. Within most species there national in recognition of the signifi - is a range of colour variation that would cant numbers of nesting and mig ra tory be considered normal for a given plum - birds that can be found there. Tommy age. On a broad scale, this colour varia - Thompson Park Bird Re search Station tion can delineate recognizable forms, (TTPBRS) was est ab lished by Toronto such as races or colour morphs. On an and Region Con ser vation to deliver individual level, the colour(s) of some monitoring, re search and educational birds of a given species will appear programs focused around birds and the ‘bright er’ than others in the same plum - environment. The migration monitor - age. However, the bird in question was ing program at TTPBRS includes daily significantly redder than would be ex - standardized mist-netting, banding, pected in any plumage attained by Bal - census and point counts during spring timore Oriole. and fall seasons. Colour in Birds A Red Oriole The significance of colour in the life of An unusual looking first year Balt imore a bird cannot be overstated. The subtle - Oriole was captured and banded early ties of feather colour, or lack thereof, on the morning of 22 August 2005. have implications for mate selection, This individual showed pronounced red social status, camouflage and differenti - colour variably distributed throughout ation. the head, breast and undertail, with Colours in birds are mainly pro - some lighter red hue present in the duced in three ways, either structurally great er coverts, back and belly (see Fig - (blue, white), chemically (red, yellow, ure 1). The bird was briefly examined, orange) or both (green). There is no photographed and released, without pig ment that produces the vivid colour any clarity as to the nature of its aber - of the male Indigo Bunting ( Passerina rant colouration. Another bird with less cyanea ). The feathers only appear blue obvious red colouration had been cap - to the eye due to a complex structure tured earlier that same week. While it within the feathers and how that struc - had been dismissed as an isolated oddi - ture reflects light. This type of colour is ty, a bird that had perhaps been stained, therefore structural instead of chemical. ONTARIO BIRDS AUGUST 2007 71 Another example of structurally gener - birds. After ingestion some carotenoids ated colour is the shimmering green and are deposited into feathers in an un - purple iridescence of the Common Gra - mod ified form. Car otenoids may also be c kle ( Quisc alus quiscula ). chemically modified by birds to produce Birds exhibit a phenomenal range of new compounds, allowing a greater colours which in most cases are derived range of colour possibilities. Species from chemical compounds called pig - such as the Scarlet Tanager ( Piranga oli - ments. There are two basic types of pig - vacea ), North ern Cardinal ( Cardinalis ments involved, melanins and caro - cardinalis ) and House Finch ( Carpo - tenoids. Melanins are produced endoge - dacus mexicanus ) take caro tenoids ob - nously ( i.e. from within) and are the tain ed from their diet and modify them pigments that produce brown, black to produce red compounds called 4- and gray plumages. Car o tenoid pig - keto-carotenoids to attain the red hues ments are usually obtained exogenously in their plumage (McGraw et al. 2001). (i.e. externally) from food sources. Other modified compounds, called can - Thes e caro tenoid pigments are respon - ary xanthophylls, are responsible for the sible for the warmer colours of red, bright yellow colours of many species orange and yellow in North Amer ican including American Goldfinch ( Cardu - birds. There are many sources of caro - elis tristis ) (McGraw et al . 2001). tenoids and they can be found in both plant and animal material consumed by Origin of “Orange-Tailed” Figure 2: Orange tail band on Cedar Waxwing Cedar Waxwings at TTPBRS, Toronto, ON, 26 August 2005. Beginning in the early 1960s, some Photo: Dan Derbyshire Ced ar Waxwings started appearing with rectrices that were orange-tipped rather than the usual yellow-tipped (Figure 2). The initial appearance of this phe - nomenon was a mystery, although the suspicion was that it was related to a change in diet. The onset of the colour change roughly coincides with the species population doubling in size from 1965-1979, although aberrant coloura - tion was noted prior to 1965. The first orange-banded Cedar Wax - wing in the Cornell University Verte - brate Collection is from 1961 (Witmer 1996). VOLUME 25 NUMBER 2 72 By the late 1980s, chemical analysis ing shrubs or vines. Non-native species of orange-coloured tail tips had recov - of honeysuckle (“bush” honeysuckles) ered a carotenoid with a deep red hue, were first introduced to North America named rhodoxanthin. This pigment was in the 19th century. During the 1950s, generally believed to be acquired directly the shrubs were endorsed by the U.S. from the diet and deposited unmodified government as a viable plant for restor - (Hudon and Brush 1989). This discov - ing wildlife habitat, which led to a dra - ery was unusual in that 4-keto-caro - matic range expansion and increase in tenoids are responsible for most of the abundance of the plants. The increase in red-col oured feathers of North Amer - numbers of these fruit-bearing orna - ican birds. The only areas coloured red mental shrubs has been credited as being on Cedar Waxwings are the ‘waxy’ red a factor behind the population increase tips on the end of the secondaries, and of the Cedar Waxwing in the mid-20th the pigment responsible for that is a 4- century (Witmer 1996). keto-carotenoid named astaxanthin Confirmation that rhodoxanthin (Brush and Allen 1963). found in honeysuckle berries could red - Rhodoxanthin is present in small den the tail tips of Cedar Waxwing came amounts in several plant species native when Witmer (1996) conducted feeding to North America, including conifers experiments in the controlled environ - and yews. However, if the source of rho - ment of an aviary. Rectrices replaced doxanthin in the aberrantly-col oured when the berries of Morrow’s Honey - wax wings was from a native species, one suckle were fed to the waxwings were would expect that they would have orange-tipped, and those produced after always had orange tail bands. Analysis of a switch to a rhodoxanthin-free dog possible introduced sources of rhodo - chow diet were yellow-tipped. xan thin revealed that the chemical is In the case of the Cedar Waxwing the found in the berries of Morrow’s Hon ey - tips of the rectrices are normally pig - suckle ( Lonicera morrowii ), a non-native mented yellow by canary xanthophylls species (Brush 1990). The ber ries of Tar - (McGraw et al. 2001). Rhodo xanthin is tarian Hon ey suckle ( Lonicera tatarica ), so closely chemically related to the can - another introduced species, were not ary xanthophylls that it can be dep osited tested, but based on its close relationship unmodified using the same chemical to Morrow’s Honey suckle, and the fact pathways that would normally mob ilize that the two species readily hybridize in and deposit canary xanthophylls . Re - the wild, it is believed that they also con - search on ery thristic Cedar Waxwings tain rhodoxanthin (Mul vihill et al . 1992, has revealed that reddening occurs only Witmer 1996). at the onset of feather growth, when rho - Honeysuckles (genus Lonicera ) com - doxanthin (red) mixes with can ary xan - prise more than 180 species of fruit bear - thophylls (yellow) to produce orange- ONTARIO BIRDS AUGUST 2007 73 coloured tail tips. This type of diet-induced erythrism can only occur during a moult of feathers normally pigmented by caro ten oids and when the diet contains rhodoxanthin or some other red caro tenoid. The most common introduced source of rhodoxanthin is “bush” honey - suckles. Juvenile Cedar Waxwings with orange tail bands have been noted commonly over the past four years of banding at Figure 3: White-throated Sparrow with orange lores at TTPBRS, TTPBRS. Many of these indi - Toronto. ON, 1 October 2006. Photo: Dan Derbyshire viduals were so recently fledged that it is believed to involve rhodoxanthin is almost certain they were born at Tom my White-throated Sparrow ( Zonot ri chia Thom pson Park where bush honeysuck - albicollis ).

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