Brown-headed Nuthatch Sitta pusilla Scientific name definitions
Version: 1.1 — Published August 18, 2021
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A small nuthatch: length 105–110 mm, mass 10.2 g. Sexes monomorphic; first-year birds (Juvenile and Formative plumages) similar to adults (Definitive Basic Plumage). Crown dull brown. Distinct dirty whitish spot on nape where brown of nape meets blue-gray of back. Wings, rump, and tail also blue-gray, with darker slate-colored remiges and rectrices, the latter with white markings to the outer pairs. Light alula-patch often noticeable. Underparts from chin to undertail coverts generally dull whitish. In some plumages, throat and upper breast show buff hue, contrasting with grayish lower breast and belly. Indistinct blackish bar through eye, blending into brown of crown. Iris, bill, and tarsi blackish.
In all plumages, closely resembles Pygmy Nuthatch (Sitta pygmaea) of w. North America. Strict allopatry of the two species reduces chances of confusion, although some extralimital records have been uncertain as to species. In comparison to Pygmy, adult Brown-headed shows browner (less gray) crown; browner (less black) loral and postocular areas; generally more conspicuous nape-patch; paler tips to head feathers in worn plumage; less extensively white inner webs of primary coverts; darker base of middle pair of rectrices; and gray (not yellowish) pads beneath toes. Juvenile plumages are even more similar; Brown-headed shows greater wing coverts broadly edged or washed with pale brownish buff rather than distinct and narrow, dull yellowish-brown edging found in juvenile Pygmy. Pygmy reported to more regularly undergoes Prealternate Molt in spring (8). Brown-headed also has slightly lower body mass and tail-to-wing ratio, and vocalizations that are lower-pitched, more nasal, and less piping and staccato in quality (1).
Brown-headed Nuthatches have 10 functional primaries (numbered distally from p1 to p10, the outermost reduced in length), 9 secondaries (numbered proximally from s1 to s9, including three tertials, s7-s9 in passerines), and 12 rectrices (numbered bilaterally from r1 to r6 on each side of the tail). Geographic variation is very slight. The following plumage descriptions pertain to the widespread nominate subspecies (S. p. pusilla); see Systematics: Geographic Variation for slight reported variation between pusilla and populations in Florida and Grand Bahama Island. No geographic variation in molt strategies has been reported. See Molts for molt and plumage terminology. Sexes similar in all plumages; definitive appearance assumed at Second Basic Plumage.
Present primarily May-July, in the nest. Natal down is well developed in coronal, occipital, middorsal, and scapular regions, with light mouse gray color (1).
Juvenile (First Basic) Plumage
Present primarily April-July. Very similar to Definitive Basic Plumage but duller, crown somewhat paler, and body feathering loosely textured. Top of head and neck color averages grayer; white nape-spot indistinct or absent; upperparts less bluish; upperwing greater coverts edged or washed pale brownish buff; gray of sides and flanks duller brownish buff.
"First Basic" or "Basic I" plumage of Humphrey and Parkes (13) and some later authors; see revision by Howell et al. (14). Present primarily July-February or July-June if First Prealternate Molt absent (see Molts). Similar to Definitive Basic Plumage but molt limits present among upperpart feathers. Back feathers, scapulars, and some proximal secondary coverts formative, fresher and bluer, contrasting with browner-washed and more worn retained greater coverts and primary coverts; 1-3 tertials sometimes replaced, contrasting with more-worn retained juvenile secondaries and tertials; outer primaries and rectrices thinner and browner (12). Age determination can be difficult in this species.
First and Definitive Alternate Plumages
Present primarily March-August, perhaps not in all individuals. Similar to Formative and Definitive Basic Plumages, respectively, with most differences due to wear rather than molt. Brown head and neck areas lighter in color (15), sometimes with pale feather tips; underparts generally lacking buff color. Some newer alternate feathers may be present, reportedly primarily on underparts; alternate feathers the same coloration as un-molted feathers. Examination of images suggests that some back feathers may also be replaced and are brighter blue, contrasting with duller retained formative or basic feathers. Criteria to separate First and Definitive Alternate plumages similar to those described under Formative and Definitive Basic plumages although increased abrasion and wear to juvenile feathers may make molt limits more discernible.
Definitive Basic Plumage
Present primarily August–February or August-July if Definitive Prealternate Molt absent (see Molts). Crown, lateral hindneck, and upper auricular region brownish; lores and postocular region darker brown; median portion of hindneck dull white, forming noticeable spot; remainder of upperparts including upperwing coverts uniformly dull bluish gray. Tail black, but inner pair of rectrices dull bluish gray, slightly paler at base outer 3 rectrices (r4-r6) broadly tipped with gray, the 2 outermost (r5-r6) also with incomplete subterminal bands of grayish white. Primaries and secondaries dull or brownish slate, the outer webs edged with pale gray; outer primaries sometimes narrowly edged with white (9). Suborbital region, lower auricular region, malar region, chin, and upper throat whitish; remainder of underparts dull white, variably tinged with buff (sometimes strongly); sides and flanks dull white, tinged with buff, and/or light bluish gray; underwing coverts primarily whitish to pale gray.
Definitive Basic Plumage separated from Formative Plumage by having uniformly basic back feathers and wing coverts; dusky primary coverts showing broader gray edging, not contrasting in feather quality with greater coverts; tertials, secondaries and rectrices uniform in wear (without molt limits), the basic outer primaries and rectrices broader, duskier, and relatively fresher (12).
Molt and plumage terminology follows Humphrey and Parkes (13) as modified by Howell et al. 14, 16). Brown-headed Nuthatch appears to exhibit a Complex Alternate Strategy (Howell et al. 14, 17), including a partial preformative molt and absent-to-limited prealternate molts in both first and definitive cycles (18, 8, 1, 12, 19;Figure 4); it may be possible that prealternate molts do not exist (see below).
Prejuvenile (First Prebasic) Molt
Partial, July-September. Includes most to all body feathers and lesser and median coverts, sometimes (in ~33% of individuals) one to three inner greater coverts and (in ~43% of individuals) 1-3 tertials, but no primaries, primary coverts, secondaries, or rectrices.
First And Definitive Prealternate Molts
Limited (perhaps absent in some individuals), primarily February-April. Norris (1) stated that no such molt occurs, but did not elaborate or provide data. Banks (8) examined 105 specimens, of which 4 had feathers in sheath in Jan or Feb, and many others had fresher looking throat and breast feathers in February and March. Ages of these specimens were not published. It is possible that feather replacement reported by Banks was adventitious or based on protracted Preformative or Definitive Prebasic molts, although apparent increased feather replacement at this time of year in Pygmy Nuthatch (8) suggests prealterate molts have evolved in both species. It would be unusual among birds for prelaternate molts to include underpart but not upperpart feathers, but perhaps this is an artifact of the creeping behavior of nuthatches, which may result in underpart feathers becoming worn and in need of replacement twice per year (8,17). However, examination of images suggests that some back feathers may also be replaced as well.
Definitive Prebasic Molt
Complete, primarily June-September. It may occasionally begin in late May, intensifies in June, and occurs throughout July-August and into September, involving all feather tracts. Primaries are replaced distally (p1 to p10), secondaries are replaced bilaterally from the middle tertials (s8) and proximally from s1, with the last feather replaced usually s6, and rectrices are often replaced distally on each side of the tail, although variation in rectrix-replacement sequence may occur.
Bill And Gape
Young juveniles show yellowish color at base of rictus (1). Bill pale sepia brown through about November (18, 12). In adults, upper mandible slate blackish; lower mandible dusky terminally, pale bluish gray basally (20).
Legs And Feet
Table 4. Of mainland populations, northern individuals average larger than southern birds (2–7% difference in linear measurements between Maryland and s. Florida birds). Individuals vary in body proportions in a north-south cline; birds of Maryland and Virginia/N. Carolina show a distinct difference in proportions (1). However, size (and coloration) are uniform enough geographically that Norris rejected Bangs's (21) contention that peninsular Florida birds should be considered a subspecies, S. p. caniceps. Individuals on Grand Bahama are longer-billed and shorter-winged than mainland birds (Norris 1, n = 3; Hayes et al. 2004, n = 11), leading authors to support subspecific or species status, respectively, for S. p. insularis.
Data of Ridgway (20; n = 20 specimens) are generally consistent with Norris's larger data set. Data of Stevenson and Anderson (22; n = 50) show central and s. Florida birds have shorter wings and longer bills than n. Florida birds, leading them to support the notion of subspecies S. p. caniceps. Measurements by Howell (23; n = 18) showed Florida birds to have shorter wings but bill equal in size to that of birds from Maryland through the Carolinas. Measurements of Texas birds by Oberholser (24; n not specified) are generally larger in all dimensions than those of other authors, but Oberholser did not measure eastern birds for comparison.
Mean values for Georgia birds: breeding-season males, 10.3 g (n = 35); nonbreeding-season males, 10.1 g (n = 36); breeding-season females, 10.4 g (n = 33); nonbreeding-season females, 9.7 g (n = 19; 1; no variance measures provided).
For other Georgia birds: adult males, 10.11 g ± 0.19 SE (n = 17); adult females, 9.86 g ± 0.18 SE (n = 10); immature males, 10.26 g ± 0.29 SE (n = 11); immature females, 9.58 g ± 0.27 SE (n = 6; 25). Mass of fledged juveniles is similar to that of adults.