Thursday, March 31, 2011

Red-winged Blackbirds

male Red-winged Blackbird (c) John Ashley
Male Red-winged Blackbird performs "song spread" display
In Montana, our birds are much more reliable than our weather.

Because it can (and does) snow in July, our most accurate messenger of spring is the boisterous conk-la-ree! song of the male Red-winged Blackbird. Males sing this song gently in winter while roosting in same-sex flocks -- sort of like groups of male opera singers practicing the scales. But spring is when they stage virtuoso performances that are accompanied by exaggerated displays of their namesake, red and yellow wing patches.

Red-winged Blackbirds (RWBB) are likely the most common bird in North America, estimated at almost 200 million individuals. Large, winter flocks can number several million and stretch for miles. Northern populations (in Canada and some snowy states) spend the winter 400-800 miles south of their breeding areas, while those in more southern areas tend to be non-migratory.

Red-winged Blackbirds migrate out of eastern Montana, but a few overwinter in western Montana in mixed flocks with other blackbirds and starlings. When day length ("photoperiod") grows long enough to trigger those familiar, hormonal changes of spring, the older males begin leaving their flocks in agricultural areas and heading off to cattail marshes. Arriving in small groups, they set up breeding territories and slowly begin to court in earnest.

Red-wings are among our earliest migrants, and mature males are the first to arrive in Montana during March and early April. Each breeding male excludes other males from a 1/8 to 1/4 acre patch of cattails, and from this stage he will begin singing and displaying his colorful wing patches ("epaulets") for females who haven't arrived yet.

The adult male ratchets up his performances a few weeks later, when females and younger males start showing up. Each female studies this cacophony of studliness from perches surrounding the marsh and, after a few inspections, she chooses one male's territory and settles in. All of the females join in this dance, but most of the first-year "floater" males do not establish territories, do not breed, and continue to roost together at night with other young males.

male Red-winged Blackbird (c) John Ashley
           Male Red-winged Blackbird performing a "flight song" display            
Red-winged Blackbirds are polygamous, and breeding females outnumber breeding males. A virtuoso male reigning over high-quality habitat might end up with as many as 15 nesting females, though the average is closer to 5, depending on the quality of his habitat. Pair bonds form quickly as, over several weeks, more and more females arrive and choose a territory.
Songs are learned, not hard-wired, and males may add new song types as they grow older and more experienced. When a female arrives on a territory, the male performs an energetic "song spread" display (singing with spread wings and tail extending downward, epaulets exposed) and "crouching" display (similar exaggerated posture, but with head held low and without song). Upon female arrival, or when a nearby male displays or flies over, a male on territory will also perform a "flight song" display. This is a low-speed flight with tail feathers spread and lowered, slow and deep wing beats, and with his red and yellow epaulet feathers on full display.

female Red-winged Blackbird (c) John Ashley
Female gathering grass for final nest layer
When settling on a male's territory, a female is silent at first, but she will frequently answer the male's song once a pair bond forms. At this point, they begin "sexual chasing," where a male flies full-speed after his mate, temporarily ignoring the boundaries of his territory. This can cause several other males to join the chase, but the two instigators are usually already paired. The young "floater" males will also chase females, but they lack a territory for the female to nest in.

This carefully choreographed courtship and breeding, however, is not as straightforward as it appears. While most activity takes place within the territory, males and females both leave the territory to find food and seek extra breeding opportunities.

Some females actively seek extra-pair copulations, usually from a neighboring male, while other females accept advances from neighboring males without seeking them. Genetic studies have shown that 22-48% of RWBB nestlings are sired by a male other than the territory owner. Natural selection appears to favor this behavior, as females who participated in extra-pair matings produced more young than females who were faithful to their mate.

Red-winged Blackbird nest and eggs (c) John Ashley
Red-winged Blackbird nest with two eggs
Here in Montana, female Red-wings begin sculpting nests in early May. She weaves together wet materials that form a platform low over the water, and builds a rounded nest cup on top. This effort usually takes several days, but it can be completed in one day if she chooses not to add a mud layer to the inner cup. The final layer is a lining of fine grasses. (One carefully disassembled nest was built on a platform of 18 reed stalks and contained 34 strips of willow bark, 142 strips of cattail leaves and 705 pieces of grass.)

During nest construction, the fully-feathered females begin loosing breast feathers to form a patch of bare skin that is used to warm her eggs. In this "incubation patch," her skin thickness doubles and the network of blood vessels increases seven-fold. (The increased thickness and vascularization begin to recede a few weeks later, after the eggs hatch, and the lost feathers are replaced during her next molt.)

Once the nest is complete, egg laying may begin as soon as the next day. One egg is laid each day, usually just after sunrise, and a typical clutch size is three eggs. Incubation often starts when the next-to-last egg is laid, and lasts for 11-13 days. Only the female incubates, and she does not allow her mate near the nest. While she incubates, the male is in anti-predator mode, using call rate and call switching (the speed and number of different calls given) to keep his females informed about the presence of potential nest predators. If a predator approaches, both male and female go into mobbing mode to drive the threat away from their nest. Also, the nests around the edges of the marsh are often parasitized by female Brown-headed Cowbirds, which secretly lay an egg in up to three-quarters of the RWBB nests.

male Red-winged Blackbird mobs subadult Bald Eagle (c) John Ashley
Male Red-winged Blackbird mobs a subadult Bald Eagle
In Montana, RWBB chicks start hatching in mid-June. Blind and helpless at hatching, the chicks can maintain their own body temperature by day eight, and they increase in weight 10-fold during the first ten days. The males often join the females in bringing food to the nestlings and, if the female dies, some males are able to raise the chicks alone. Adults continue feeding the juveniles for two weeks after fledging and for another three weeks after moving off the territory. Once on their own, around mid-July, the young birds often flock with adult females while the adult males once again form same-sex flocks.

After the breeding season, all of the adult RWBBs suddenly disappear from their breeding marshes during July and August. They leave for more secluded areas where they stay hidden from view while molting their worn breeding feathers. By October, they return with new plumage and regroup into the segregated flocks of winter (adult males with a few young males, adult and young females with many young males).

In the large winter flocks, adult male Red-winged Blackbirds quietly practice their conk-la-ree song while waiting for the arrival of spring, and the chance to show off their virtuoso voices and colorful wing patches.
winter flock of Red-winged Blackbirds (c) John Ashley
A Red-winged Blackbird winter flock passes the rising moon while heading for evening roost