Bird Facts: Dickcissel
The Dickcissel (Spiza americana) is a small American seed eating bird. It belongs to the family Cardinalidae (or American finch)is only member of the genus Spiza.
This bird is sometimes known as the Black Throated Bunting. It has the characteristic cone shaped bill that is associated with finches and sparrows but has to be placed in a genus of its own because it does not share a blue colouration that is found in its taxonomical near neighbours. The females and juveniles are sometimes confused with House Sparrows but can be distinguished because they have streaked flanks. The adult males have a black throat patch, a yellow breast and grey cheeks and crown. During the breeding season the male colours are particularly strining and are said to resemble the Eastern Meadowlark.
The Dickcissel is very vocal.Their song sounds like a sharp dick dick followed by a buzzed cissel from which they get their name. In flight their song sounds like an electric spark - fpppt.
The Dickcissel breeds in the fields of midwest North America. They overwinter in southern Mexico, Central America and northern South America and arrive in North America in late May and early June. The birds seek for insects and seeds in the fields. In the summer they are so numerous that they are considered a pest by many farmers. The birds nest close to the ground in dense grass, shrubs and bushes. Four to five eggs will be laid in each nest. The cowbird sometimes lays her eggs in these nests. The eggs hatch after two weeks and are fully fledged within ten days. During the summer most males will find one or two mates, the more prolific may find six. The birds bond for the summer season then depart for winter quarters in August.
Strangely, Dickcissel the population and range can fluctuate wildly. In the early 19th century Dickissels could be seen in New England and the mid Atlantic states where they are no longer found. These fluctuations are probably related to changes in land use.
Although the bird is not threatened in North America has a large worldwide populaton, which is estimated to exceed 22 million birds there are and grounds to worry about this bird. The Dickcissel is being heavily persucuted in its Venezuelan wintering grounds. During the winter the Dickcissel forms very large flocks and up to 10 to 30% of the total world population can cluster in a single roost. In Venezuela these enormous winter roosts are being poisoned with toxic agrochemicals that are applied by crop duster planes.
A problematic specimen is often discussed under the name of Spiza townsendi (or Spiza townsendii, the original misspelt specific name proposed by John James Audubon). This individual was collected on May 11, 1833, by Audubon's colleague John Kirk Townsend in New Garden Township, Chester County, Pennsylvania. The specimen remains unique and nothing is known about what it represents with certainty; it has thus even been suggested to be an extinct relative.
It is commonly called "Townsend's Dickcissel" (or "Townsend's Bunting", "Townsend's Finch") in reference to the collector whom the scientific name honors. Rather than a distinct species or subspecies, it is (as certainly as this can be said in absence of direct proof) a color variant. Comparing the birds, it is immediately obvious that the yellow lipochrome pigments are entirely absent in "Townsend's Dickcissel". The specimen has foxed today, giving it an altogether beige hue, but when originally shot, the olive areas of the head were grey as the cheeks, and the yellow and buff on face and underside was pure white. The brown wings and tail were rufous, due to the pheomelanins not being tinged by lipochromes.
A color mutation?
Thus, this bird is very likely certainly the result of a simple genetic change, perhaps just a single point mutation, affecting some part of the carotinoid metabolism - essentially the same thing that happens in albinism but in a different metabolic pathway. Though the bird seemed to be healthy and had survived to maturity when it found its untimely end through Townsend's gun, no other such specimens have been documented before, nor ever since. Albinism and other pigment aberrations are not infrequently seen in birds, and the lack of further specimens is somewhat puzzling in that respect.
No specific details are known about the Dickcissel's lipochrome metabolism; it may be that it happens to be more fine-tuned than in other birds, so that most mutations therein will be lethal and Audubon's bird was simply one of the very few individuals that survived. It stands to note that in wild birds, varying from species to species some color aberrations are less frequently seen than others, and that in captive birds such as canaries, some color mutations have only arisen a handful of times at most during several centuries of dedicated breeding and screening for novel color variants (see also Budgerigar colour genetics). While only a complete molecular biological study of the Dickcissel's metabolism and the specimen's ancient DNA stands any reasonable chance to resolve the question with certainty, the hypothesis of an extremely uncommon color mutation is plausible, and such phenomena certainly occur in other Passeroidea.
Alternatively, the bird was considered a hybrid, but the present state of knowledge of the Dickcissel's relations makes this not very plausible - there are a number of species with which Spiza could conceivably produce hybrids - such as Passerella -, but the lack of even the slightest hint of blue structural colors in Townsend's specimen and it moreover being not different from a Dickcissel in habitus makes the hybrid theory suspect. Regardless, Townsend noted observed the bird making vocalizations reminiscent more of an Indigo Bunting (Passerina cyanea), and by comparing mitochondrial and nuclear DNA sequences of the specimen with those of the Dickcissel, the Indigo Bunting, and perhaps other Passerina, the hybridization hypothesis should be far more easy to prove or reject than a color aberration. On the other hand, there is not enough known on whether Dickcissels pick up their characteristic vocalizations from conspecific males or whether they are innate, and thus no firm conclusion regarding Townsend's observations is presently possible.
An extinct species?
The notion that "Townsend's Dickcissel" represents an extinct species appears entirely without merit. Hybridization e.g. in Passerina is well-documented to occur often, and the hybrids are often fertile to some extent. As the capability to produce intergeneric hybrids is almost always the same among closely related bird genera, there is nothing to indicate that the Dickcissel and a hypothetical congener would not have been able to produce fertile hybrid males at least (see Haldane's Rule). Considering that, and the time and place where Townsend got his specimen - at a popular stopover region for Dickcissels returning from their South American winter quarters, and right at the time when the species is present in this region - it is hardly conceivable how "Townsend's Dickcissel" should have been, as has been suggested, "the very last remnant of a now extinct species".
That a closely related congener of Spiza americana, with which it almost certainly would have hybridized to some extent as well as shared at least a migration route and habitat, possibly even the breeding range, would even have evolved in the first place almost begs belief. Though there are numerous sister species of songbirds occurring west and east of the Appalachian Mountains for example, these do not differ in such a striking but easily explained way. Rather, their differences are usually more subtle but deep-rooted, for example vocalizations or details of the color pattern. Of course, it must be said to Audubon's credit that at his time neither theories of evolution nor of biogeography or phylogeny were accepted, and so his error is excuable. This cannot be held in favor of those 20th century writers who still claimed "Townsend's Dickcissel" to be a recently-extinct species.