Editor's Note: Phylogenetic analyses of sequences of mitochondrial and nuclear DNA indicate that all species formerly placed in Dendroica, one species formerly placed in Wilsonia (citrina), and two species formerly placed in Parula (americana and pitiayumi) form a clade with the single species traditionally placed in Setophaga (ruticilla). The generic name Setophaga has priority for this clade. See the 52nd Supplement to the AOU Checklist of North American Birds for details. Future revisions of this account will reflect these changes.
This pine specialist breeds from mountains of the southwestern United States to the Caribbean coast of northern Nicaragua. One of the smallest warblers, it is an active forager, hopping and flitting rapidly among the outer foliage of high branches, perching on twigs and needles in search of insects and spiders, and occasionally fly-catching or hovering to obtain its prey. Northernmost populations, from northern Mexico and the United States, are migratory, whereas populations from central Mexico south to Nicaragua are only partially migratory or resident. The species prefers parklike stands of mature pines, a habitat that has declined over time through forestry practices of logging and fire suppression, at least in the northern parts of its range. Little is known about Mexican and Central American breeding populations.
Distribution and habits make Grace's Warbler one of the least known North American passerines. Its territories tend to be large, its nests well hidden, and individuals a challenge to follow. No comprehensive studies exist, and much basic information is lacking. Few nests have been found, few individuals have been collected or measured, and no studies have thoroughly documented basic information such as molt, migration, or population dynamics. Only 59 individual Grace's Warblers have ever been banded, far fewer than any other North American wood-warbler.
Its ecology has been studied mainly as a member of the bird community inhabiting pine or pine-oak forests in Arizona ( Marshall 1957c , Balda 1969 , Szaro and Balda 1979a , Brawn et al. 1987 , Block et al. 1992 , Rosenstock 1996 , Block and Finch 1997 , P. Beier unpubl.), Mexico ( Marshall 1957c ; Block et al. 1992 ; Nocedal Nocedal 1994b , Nocedal 1994a , Nocedal 1995 ), and Nicaragua (Howell Howell 1971b , Howell 1972 ). These studies have documented the range of habitats occupied, microhabitats used for foraging, and densities of some populations. The only studies to date solely on Grace's Warbler focused on the singing behavior of individuals in northern Arizona ( Staicer 1989 ), geographic variation in songs of the northern subspecies (CAS), and nesting behavior of a few pairs in southwestern Colorado (S. Hutchings unpubl.).
This account compiles information from the studies mentioned above, from various regional compendiums and atlases, and the Breeding Bird Surveys and Christmas Bird Counts. A synthesis of the ecology and dynamics of southwestern pine forests ( Block and Finch 1997 ) helps to place information for Grace's Warbler into a larger and historical context. Some gaps are filled by unpublished information, but there is still much to learn about the species, especially those populations breeding south of the U.S. border.