Wednesday, August 19, 2020

Mourning Doves

Mourning dove (internet download)
Recently, our backyard revealed a ‘puddle’ of feathers about a yard across. Best guess is that a hawk had caught and killed a mourning dove, and then proceeded to pluck feathers before flying away with the carcass.

Mourning doves – older names turtle dove and Carolina turtledove – are a common bird across America, with a population estimated at 300 to 400 million birds. Doves are a light grey and brown in color, with only subtle differences in appearance for males and females. Mated pairs are monogamous. Two eggs are laid per nesting cycle. A pair will raise several broods each year. Both parents participate in nest building, egg incubation and feeding the chicks (also referred to as squab). Although doves feed on seeds and grains, very young chicks are not fed seeds, but rather a protein- and fat-rich secretion from the parent birds’ crop, called “crop milk.” [The crop is a pouch just below the throat. Many species of birds can fill this with food to digest later or else regurgitate into their chicks’ mouths.]  As chicks get larger, seeds the parents have swallowed into their crop are combined with crop milk. With such a rich diet, chick fledge – are able to fly – within two weeks from hatching. The parents will continue to feed them for an additional week or two, but will have also laid eggs for the next brood.

In captivity, lifespan can approach 20 years. This, plus all the fecundity would cover the countryside knee-deep in mourning doves, except that predators find them easy to catch. In the wild, average lifespan is less than two years. Ravens and crows take chicks from nests, fledged chicks are easily caught by many predators, and adults end up in the food chain because they are ground feeders. Prior to European colonization, the major predators of adult dove were hawks and falcons, attacking from above. Add to that cats (pet and feral), and the life of a dove can be brutally short.  

Feathered evidence of a hawk kill
The predator that left us the feathered evidence was most likely a Cooper’s hawk. This is a relatively small but very agile hawk that preys primarily on birds. Red-tailed hawks are larger, but a bit too slow to be bird predators, preferring instead rabbits and squirrels. The other possible culprits would have been peregrine falcon, or sharp-shinned hawk, but these are relatively rare in eastern Massachusetts. In addition to doves, Cooper’s hawks will also prey on robins, starlings and sparrows – also ground feeders. Attacked birds are taken either on the ground or while trying to take off. One estimate has each hawk killing on average two birds a day.

In describing mourning doves, John J. Audubon wrote “Whenever it starts from a tree or the ground, on being unexpectedly approached, its wings produce a whistling noise, heard at a considerable distance.” This is an example of “sonation,” defined as sounds produced by birds, using mechanisms other than the vocal organ. Humans make sounds by means other than voice: we snap our fingers, clap our hands, slap our thighs, stomp our feet and click our heels. For some bird species the sound making is voluntary, typically related to males claiming territory or their mating display. The male palm cockatoo of New Guinea uses a claw-held stick to drum on a hollow tree he has selected as a possible nest site. The female, attracted by the noise, checks out the real estate.

For mourning doves, the taking off sound is involuntary but mallable. Birds often form flocks to increase collective vigilance, thus allowing early detection of predators. This benefit of sociality relies on rapid transfer of information. For many species, an alarm call, but for pigeons and doves, one of the flight feathers on each wing is modified so that on take-off, with the wings fully extended for extra lift, the modified feather rapidly flutters against the next feather during the wings’ downstroke, making an easily heard noise. This noise is present whenever mourning doves take off, but if the cause is alarm from detecting a predator, wings beat faster and the pitch is higher. Taped recordings of regular and alarmed take-offs confirm that other doves respond more to the sound of alarmed take-offs.

Is this altruism or self-interest? Assuming this evolved under predation by hawks, one bird taking off from a group on the ground may just be making itself the best target. But if all the birds take off, the visual complexity of the group may cause the attacking hawk to lose sight of an individual bird, and perhaps even miss all of them. Once in the air, mourning doves can reach speeds in excess of 50 miles per hour. Bye Bye Birdie.   

For mourning doves, much like robins, migration is optional. Given enough food, males of both species will remain behind, forming loose flocks of 10-30 birds. Their reason for not leaving is to have dibs on the best territory when the females return in the spring. Robins will find and plunder berries such as winterberry and Asian bittersweet. Mourning doves will seek out remnant grains in farm fields and cluster under birdfeeders to benefit from what perching birds have spilled to the ground.

Lastly, from James Lipton’s “An Exaltation of Larks”: a group of doves is called a "dule," (pronounced ‘dool’) taken from the French word deuil (mourning), because “The soft, sad ululation of the dove has always evoked the sense of mourning.”

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