As yesterday’s post indicated, the progression in which signs of spring appear remains much the same, but the timing of this progression is changing. Ornithologists have determined that modern climate change has resulted in an advancement of spring phenology throughout the Northern Hemisphere.
Many birds are arriving on their breeding grounds earlier in response to these changing conditions. Past research has focused on correlating climatic changes on breeding grounds with early arrival. However, it appears that climate variability on the wintering grounds of temperate species also plays a part in these short-distance migrants’ arrival on their breeding grounds.
Many climatic factors are involved in this phenomenon. The annual variation in temperature on the wintering grounds of American robins was found to be strongly related to their first-arrival date. Red-winged blackbirds’ first arrival dates were most influenced by precipitation during winter and spring months.
These and other changes in migratory patterns can have life or death consequences — birds arriving early on their breeding grounds face the possibility of adverse conditions and limited resources.
Regardless of the deep snow that remains on the ground and the frigid temperatures we’ve had recently, spring has announced itself with the arrival of red-winged blackbirds this week. Most redwings that breed in New England migrate approximately 500 miles further south to spend the winter. In the spring, males begin migrating first, flying north in flocks during the day to their breeding grounds. (In the fall, females depart first.) While they eat primarily insects during the breeding season, redwings subsist mainly on seeds and grain this early in the season. Before you know it, females will return, and up to fifteen of them will be nesting on the territory of each male (though he doesn’t sire all of the nestlings).
During the breeding season, insects make up the bulk of a Red-winged Blackbird’s diet, but during the rest of the year, plant seeds are preferred. While the seeds of ragweed, corn, oats and smartweed are more staple food sources, cattail seeds are not overlooked. At maturity, and under dry conditions, the cattail spike bursts, releasing the seeds (some estimates are as high as 228,000 seeds/spike). When this happens, blackbirds take advantage of the easily accessible source of food, but the minute size of each seed (.0079 inches long) means obtaining a meal is a labor-intensive endeavor.
Male red-winged blackbirds have returned to Vermont, and their most notable features are the brightly-colored reddish-orange feathers on their “shoulders”, referred to as epaulettes. In the military, an epaulette is a shoulder ornament that indicates, through its position, color, length and diameter, the bearer’s rank. Some birds, including male redwings, also possess this badge, or visual cue, which indicates the social status of the bird to other birds of the same species. Studies of male red-winged blackbirds and their epaulettes indicate (through dyeing their bright orange/red shoulder patches black) that epaulettes play a significant role in the male’s defense of his territory. Over 60% of the redwings that had their epaulettes dyed black lost their territories to other males. Further research revealed that aggression by a territorial male redwing is proportional to the epaulette size of the encroaching male redwing. It also indicated that male redwings intruding into redwing-occupied territories greatly limit the exposure of their epaulettes by covering them with black feathers. (Female red-winged blackbird plumage is brown and lacks epaulettes.)