The official emblem of one US state, the Baltimore oriole flourishes throughout eastern North America and is also fairly well known in aviculture. BILL NAYLOR finds it to be a fascinating representative of a uniquely New World family, the characterful Icteriidae
ICTERIDS, often known as the American blackbirds, are extrovert, highly vocal birds found only in the New World. Many have colourful plumage and few bird families surpass the icterids for variation of eye colour, which can be a striking blue, white, yellow, red or brown.
They vary from starling size to crow size and comprise several diverse genera. Some 106 species are recognised, including cowbirds, grackles, caciques, oropendolas, meadowlarks and marshbirds. About 20 species with dark plumage are known as this or that blackbird, almost exhausting the available adjectives.
Apart from the oropendolas and orioles (no relation to the Old World birds known as orioles), icterids are aggressive. Although easy to maintain and well adapted to captivity, they are infrequently kept outside America. In 1999, Jersey Zoo implemented a successful captive breeding programme for the critically endangered Montserrat oriole (Icterus oberi), which achieved the first UK breeding in 2000.
In personality, icterids recall starlings, corvids and jay-thrushes, but physically they are distinct. Their beaks are pointed – often tweezer-like – and in many the upper beak is extended at the base, most prominently in the oropendolas, where it forms a prominent casque.
As well as their main food of invertebrates, a number of species regularly eat fruits, flowers and nectar. Oropendolas are the most frugivorous, as well as the largest, of the family, and most of their food consists of fruit pulp.
Meet the orioles
The largest genus is Icterus – the orioles – with some 28 species. Three South American species are known as troupials, an alternative name for oriole. Except for two chestnut-brown species, orioles are black, combined with either orange or yellow. The Baltimore oriole (I. galbula) is the state bird of Maryland, and is named for its resemblance to the colours of the family coat of arms of Lord Baltimore, who first colonised that state. (It also gives its name to the State baseball team.)
Male Baltimore orioles vary from orange to yellow, based on the carotenoid content of their food. In captivity, icterid orioles are notorious for losing their colour and require dietary colour food. The female Baltimore is smaller than the male with faded yellow green plumage, though there is much variation. Some female Baltimores (and females of some other oriole species) are superficially similar to males. The female black-cowled oriole (I. prosthemelas), in its southern range, is almost identical to the male.
The Baltimore oriole has the distinction of being one of three icterid vagrants to the UK, and has been recorded 23 times, usually in autumn. It has also been recorded as a vagrant in Norway, Greenland and Iceland. The species’ regular distribution stretches as far north as Canada and it is a long-distant migrant, overwintering in Mexico and Central America. (Unlike other North American orioles, it rarely overwinters in South America.)
This is the commonest oriole in eastern North America, and it exploits both urban and agricultural areas. Its counterpart in the adjacent western area of the USA is Bullock’s oriole (I. bullockii). On the Great Plains where the ranges of these two species meet, they interbreed in a hybrid zone up to 200 miles wide. The further away from the centre of the hybrid zone, the more the hybrids resemble the nearest parental stock. They are good species, differing in size, vocalisations and moulting sequence. In fact, recent DNA analysis has revealed that the Baltimore isn’t even the closest relative of Bullock’s, but that the streak-backed oriole (I. pustulatus) of Mexico is its sister species.
This discovery is often highlighted as evidence that hybridisation in the wild doesn’t have to include closely related species. Nevertheless, Baltimore and Bullock’s were formerly considered to be one species and from 1973 to 1995 given the name northern oriole. They were restored to two species when research showed that hybridisation was naturally contained and not increasing.
Food and foraging
Icterids are among the most vocal birds in the Americas. Baltimore orioles chatter as they forage and males (nick-named “buglers”) produce a flute-like song after they arrive in their breeding area in spring. Females, arriving a few weeks later, also sing but less impressively. Many early colonists in America kept them as caged singing birds.
Their main activity while foraging in the upper tree canopy is to glean caterpillars – including those that are hairy or possess spines – from the undersides of leaves. Caterpillars form 30 per cent of their diet in spring and summer. Like most icterids, they use their feet as an aid when feeding.
Flower nectar is another main food and Baltimores are important pollinators for several species of flowers. In captivity, orioles – always active – become excited when presented with fruit. They are greedy eaters, and require spacious aviaries to exercise. Bananas are pierced to reach the fruit pulp inside.
Ripening cultivated grapes and cherries are targeted by Baltimore orioles as they migrate south through New York State in late summer. In winter, fruit is their main food. Fruit pulp is sometimes fed to nestlings, but like all orioles they also take an abundance of insect pests. As feeders they are opportunists. Various oriole species are recorded taking tadpoles and lizards, and feeding on roadkill. In captivity they readily adapt to inanimate foods such as softbill pellets.
It is not ideal to house orioles with smaller bird species. A Baltimore has been recorded to kill a hummingbird, presumably to eat. (Note, however, that Baltimore orioles are known to defend nectar sites against hummingbirds and other orioles.) Venezuelan troupials (I. icterus) have largely abdicated nest- building and regularly usurp other the nests of other birds, even those of their own species, destroying eggs and nestlings in their takeover.
Courtship for Baltimore orioles involves repeated bowing, a common ritual in all icterids as a greeting and bonding gesture as well as part of courtship ritual. Displaying males contort their plumage and spread their tail to highlight the orange breast and rump, while also singing.
“Hangnest” was once a common name for icterid orioles. In 1910, Arthur Denman recounted his experiences with captive orioles in an Avicultural Magazine article “In Praise of Hangnests”. The Baltimore oriole was bred in France by Delacour in 1970.
Other than weavers, orioles are the only birds that weave an entire nest. The female initiates and performs most of the work, building a framework of pliable plant tendrils secured to tip of a branch, then crocheting a deep purse-shaped nest which hangs 20-30ft from the ground. In urban areas, coloured string is often provided and is woven in the nests. Colour seems to be stimulating to Baltimore orioles, which often target brightly coloured flowers, especially red ones.
Males may breed in their first year but produce smaller clutches, and as they don’t moult out into adult plumage till the autumn of their second year, only measurements can confirm their sex. Females grow brighter with age but are always duller when breeding, due to the abrasion caused by the rigours of incubating duties.
Three to seven (usually four or five) blotched grey-white eggs are incubated by the female for 14 days. The nest’s height and location at the end of a branch result in limited predation, although this species is a target of North America’s most successful brood parasite, the brown-headed cowbird (Molothrus ater). (Cowbird nestlings do not eject the host’s nestlings. Instead, baby cowbirds are unwittingly reared alongside the host’s offspring.) Baltimore and Bullock’s orioles can usually identify a cowbird’s eggs and will remove and spike them with their beaks.
Just prior to fledgling, the female Baltimore leaves her mate to care for the young and quits the nesting territory to undergo her six-week annual moult. Ringing records of Baltimore orioles reveal a longevity of up to 12 years, with six or seven not uncommon. One captive bird survived for 25 years.
Bill Naylor has experience of numerous bird species from his career as a keeper in zoos and bird parks.
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