The Bobolink (Dolichonyx oryzivorus)-General Biology

APPEARANCE

Male bobolinks have large, flat heads, short necks, and short tails. Males are black on the anterior portion of head, wings, and tail. The back, scapulars, lower back, rump, and upper tail-coverts are white/pale gray. They have a  yellow nape and a glossy black bill (which is relatively long compared to their head size.) After breeding season, the males shed their plumages and lose bill pigmentation, looking more like female bobolinks.

Females have the same flat heads, short necks, and short tails, but are a warm brown/yellow color below, with brown and white streaking on the flanks, sides and under tail-coverts. Females have a dark eyeline, with a broad yellowish brown crown stripe bordered by brownish black stripes on either side.   

HABITAT

Bobolinks are primarily found in open areas and wetlands across the northern United States and southern Canada. They prefer to nest in fields with a mixture of grasses and broad-leaved plants, like dandelions. When they migrate, they like to visit rice fields, which has earned them the nickname “rice birds”.

They have a lot of other nicknames, too, depending on geographical area. Bent (1958) writes, “in New England it is sometimes called by the pretty name ‘meadow-wink’ and the less complimentary name ‘skunk blackbird’…on fall migration it is recognized as ‘ortlolan,’ ‘reed bird,’ and ‘rice bird,’ on account of its haunts and habits, and, in Jamaica, where it has grown exceedingly fat, they call it ‘butter bird.’”

 

Bobolinks formerly nested in the tall-grass or mixed-grass prairies of midwestern United States and South-central Canada, but after the area was transformed for agricultural use, Bobolinks were displaced into clearings where deciduous forests used to stand.

Bobolinks begin migration in early August. Eastern populations of Bobolinks typically head southeast. Western populations are smaller and not much is known about their migration path.

Eastern populations migrate across the south-eastern United States, through the Carribean, and conclude in South America. They stay here from about late April to September, within a range extending through Brazil, Paraguay, and Argentina.

Aside from the South American mainland, Bobolinks are known as the only migratory landbird species that stops every year in the Galapagos. During Darwin’s stay in the Galapagos, he collected a single Bobolink specimen in San Cristobal. It is theorized by Perlut (2016) that Bobolinks may be a way for seeds to disperse from the Galapagos to South American mainland.

After attending migration, bobolinks tend to re-nest in the same area every year.

DIET

In the breeding season, Bobolinks eat weed seeds, insect larvae, adult insects, and other arachnids. They feed their nestlings with invertebrates exclusively, and a meal for them can consist of Lepidoptera, sawfly, arctiid larvae, and mayflies. In May, male bobolinks eat dandelion seeds with the occasional cutworm. By June, males begin to eat cinquefoil seeds, yarrow, Canadian thistle, false lupine, dock, and mallow.

When foraging for seeds, Bobolinks often perch on the tops of nonwoody plants and carefully extract seeds for ingestion. They have been observed jumping from the ground to get at mature dandelion seeds and other plants that aren’t strong enough to support the Bobolinks’ weight. Bobolinks tend to swallow their food whole. Following the ingestion of milky grains or insects, they will wipe their bill on nearby vegetation.

During migration and in the winter, Bobolinks will feed on rice, oats, other small grains, tassels, weed seeds, and small insects. They look for insects on the underside of soybean and sunflower leaves.

While typically daytime foragers, during migration they will feed in rice fields on bright nights, to build up fat reserves for long overwater flight.

BREEDING BEHAVIOR

After a flock of Bobolinks arrives at a breeding ground, the males become extremely territorial and fight for ground by singing, dancing, and chasing each other. After the females arrive, males rarely leave their territories unless it’s to chase away intruders, and remain defensive until they start feeding nestlings. Females, on the other hand, are rarely aggressive towards each other and do not defend the male’s territory from other females.

COURTSHIP AND MATING

When females pass overhead, males give obvious, short aerial song displays to get their attention. If a female lands within a male’s territory, he proceeds with a repetitive courtship sequence, which includes brief song, hovering circle-flight, and abrupt drops to the ground. During the drops, the male emits 1-3 rasping buzz notes. If the female stays, the male repeats this sequence up to 5 times a minute for 30-40 minutes.

Bobolink display
https://www.youtube.com/watch?v=1jo3U5-OQ-o

Fertile females make a peeping “whine” to get males’ attention, with these whines often followed by complex aerial chases. During male courtship sequences, the female is usually passive. Occasionally a female can initiate sexual chase by flying over the male with a zeep sound and/or whining. Chases can involve up to 8 males, with the flight of the female rapid and as elusive as possible.

COPULATION

Courtship typically lasts 2 to 3 days before copulation starts. Male Bobolinks engage in precopulatory crouch displays, which consist of partial song, accentuated wing spread, and tail fanning. Bobolinks are polygynous, meaning that males mate with multiple females, so males often pair with another female 3-8 days after the initial mating.

NESTING

The female chooses the nesting site, typically on wet soil, at the base of large non-woody plants like clover, meadow rue, and golden alexander. The female then gathers materials within 100 yards of the nest and builds the nest herself in 1-2 days. First, she plucks bare a patch of soil and makes a depression, and then weaves a floorless outer wall of dead grasses and weed stems. The inside is then lined with grasses and sedges.

EGGS/HATCHLINGS

The female lays one egg a day within 1 or 2 days of mating. A female can lay anywhere from 1 to 7 eggs. Incubation is 10-14 days, and feeding begins within the first hour of the first nestling hatching and continues until the fledglings become independent (10-11 days).

In a male’s primary nest, both parents feed the young equally, but in a male’s secondary nests, the amount he helps feed the young varies. In some cases, polygynous males show adaptive behavior by turning some or all attention to secondary nests if the female isn’t delivering sufficient food.

SOCIAL SYSTEM

While males are extremely aggressive and territorial during the breeding period, they are very sociable the rest of the year. Bobolinks live socially in flocks, with some flocks even involving hundreds of birds. Interfamily flocks usually form within the first week after the young fledge. 

WHERE SHOULD FUTURE RESEARCH FOCUS?

It would be interesting to know how the western population of Bobolinks migrates. Since Bobolinks tend to return to the same area every year post-migration, it would be relatively easy (compared to other species, that may not return to the same area every year) to tag individual birds with trackers and retrieve them when Bobolinks come back to nest.

WORKS CITED:

Bent, A. C. 1958. Life histories of north american blackbirds, orioles,

tanagers, and allies. U.S. Natl. Mus. Bull. 211: 1-549.

Bobolink. Bobolink – | Birds of North America Online

Bobolink Life History, All About Birds, Cornell Lab of Ornithology. , All About

Birds, Cornell Lab of Ornithology.

Bird ID Skills: Field Marks. All About Birds. 2015 Aug 11.

Martin, S. G. 1967. Breeding biology of the Bobolink. Master’s Thesis, Univ.

of Wisconsin-Madison, Madison.

Perlut, N. G., & Renfrew, R. 2016. Stopover on galapagos during autumn migration of bobolinks (Dolichonyx oryzivorus). The Wilson Journal of

Ornithology 128: 935+.

Simon, A. 2014. Dolichonyx oryzivorus (On-line), Animal Diversity Web.

Accessed April 29, 2018 at http://animaldiversity.org/accounts/Dolichonyx_oryzivorus/

Wiens, J. A. 1969. An approach to the study of ecological relationships among

grassland birds. Ornithol. Monogr. no. 8:1-93.