Sparrows of the West

Recent decades have seen significant shifts in the classification of birds, particularly among the Passeriformes, the perching birds. These shifts have lead to the recognition of a number of major groups that were previously obscured. Among these recent elevations are the New World sparrows of the Passerellidae.

Gambel's white-crowned sparrow Zonotrichia leucophrys gambeli, copyright Gregory Smith.

The New World sparrows are part of a broader radiation known as the nine-primaried songbirds, along with such luminaries as finches, tanagers, and their Old World namesakes. The name 'nine-primaried' refers to the number of well-developed primary feathers (the long outer ones) in the wings; most other perching birds have ten distinct primaries. Though the nine-primaried songbirds have long been recognised as a coherent group, there has been a lot of disagreement over their subdivision. Historically, these subdivisions were strongly influenced by different bill shapes representing different diet specialisations, but recent molecular phylogenies have demonstrated that bill shape is more labile than previously recognised. The New World sparrows were usually regarded previously as a subgroup of the generalist seed-eating family Emberizidae, along with the buntings of the Old World, but molecular phylogenies have asserted the division between the hemispheres. Not all New World representatives of the old Emberizidae have shifted to the Passerellidae: a significant component of the Neotropical fauna (including the finches of the Galapagos islands) have instead proven to be closer to the fruit-eating tanagers of the Thraupidae. As currently recognised, the passerellids are a fairly coherent group of about 140 species distributed around North and South America.

Goldwn-winged sparrow Arremon schlegeli schlegeli, copyright Nick Athanas.

In general, the passerellids are small birds with simple, conical bills. Most are dull brownish in coloration though many are strikingly patterned, particularly around the head. Some are more distinctive: the South American sparrows of the genus Arremon often stand out as particularly colourful. Most passerellids are fairly retiring in their usual habits, foraging at or close to ground level. As noted before, they are mostly generalist feeders. Their short bills are excellently suited for milling the small seeds which make up a large part of their diet. However, they will also take insects and other small invertebrates. One widespread North American species, Ammodramus savannarum, has earned the vernacular name of "grasshopper sparrow" as a result. Notable outliers dietwise are the Neotropical bush-tanagers of the genus Chlorospingus which are primarily berry feeders. These largely greenish birds were previously classified with the Thraupidae as a result before molecular data led to their reassignment.

Common bush-tanager Chlorospingus flavopectus, copyright Becky Matsubara.

Whereas Neotropical members of the Passerellidae are mostly sedentary, North American species are often migratory, moving north with the approach of summer. However, migration is commonly related to environmental conditions. A number of species are migratory in the northern parts of their range but may be found in their southern territories year-round. In some species, migrating populations will leap-frog over resident populations, moving further south than any resident individuals during the winter months. Many passerellid species are strong singers and courting males will often select an exposed branch to sing from in contrast to their usual skulking habits. Other species, particularly those inhabiting open habitats where trees and shrubs are in short supply, may have prominent aerial displays. Males of one of these latter species, the lark bunting Calamospiza melanocorys, moult during the breeding season into black plumage with contrasting white patches on the wings and tail. During the remainder of the year, they are dull in coloration like their females. Nesting is conducted close to ground level like feeding with the nest often being a small cup in the ground concealed under vegetation. Where breeding has been studied in detail, passerellids are commonly what has been called "socially monogamous". Males and females will form what appear to be monogamous pairs with one male remaining close to one female (though construction of the nest and incubation are done by the female alone). However, genetic studies on nestlings have found that chicks are not uncommonly not the child of their apparent 'father', indicating that females have not remained faithful to their mate.

Yellow-striped brush-finch Atlapetes citrinellus, copyright Ron Knight.

Prior to molecular studies, authors had suggested a possible division of North American passerellids between two evolutionary lineages based on ecology and behaviour, the grassland and brushland sparrows. A molecular study of passerellids by Klicka et al. (2014) identified eight well-supported clades within the family. Two further species, the large-footed finch Pezopetes capitalis of Central America and the Zapata sparrow Torreornis inexpectata of Cuba, were not robustly assigned to a clade. Identified relationships were comparable to but not entirely congruent with prior hypotheses. For instance, most 'brushland sparrows' (of the genera Passerella, Zonotrichia and Junco) belonged to a single clade but the remaining 'brushland' genus Melospiza was placed in a clade mostly made up of 'grassland' species. The diverse South American genus Arremon was supported as monophyletic but others were not. In particular, the North American Ammodramus was divided between two widely separated clades. This lead to the resurrection of the genus Ammospiza for a group of saltmarsh-breeding species. Deeper relationships within the family deserve further investigation.

REFERENCES

Hoyo, J. del, A. Elliott & D. A. Christie (eds) 2011. Handbook of the Birds of the World vol. 16. Tanagers to New World Blackbirds. Lynx Edicions: Barcelona.

Klicka, J., F. K. Barker, K. J. Burns, S. M. Lanyon, I. J. Lovette, J. A. Chaves & R. W. Bryson, Jr. 2014. A comprehensive multilocus assessment of sparrow (Aves: Passerellidae) relationships. Molecular Phylogenetics and Evolution 77: 177–182.