Behavioral and Morphological Diversification in Sharp-Tailed Sparrows (Ammodramus caudacutus) of the Atlantic Coast

Among Sharp-tailed Sparrows (Ammodramus caudacutus), a northern group of populations can be distinguished from a southern group. The northern group, comprising the races nelsoni, alterus, and subvirgatus, occurs in three disjunct populations distributed from the continental interior to the north Atlantic coast. The southern group, including the nominate race and diversus, is found only along the mid-Atlantic coast south to northern Virginia. The two groups, which are in limited contact in southwestern Maine, are defined by uniquely derived behavioral (song-related) characteristics, as well as by differences in morphology (bill size and ventral streakiness) and habitat. In the southern group, song-related behaviors are divergent compared to such behaviors in congeneric relatives. Instead of loud primary songs uttered repetitively in discrete bouts (the usual pattern in Ammodramus and other emberizines, and in males of the northern group within the species), southern males sing a continuous series of variable, muted phrases and sharp notes in a single display called "complex whisper song." They also have largely "suppressed" a typical emberizine flight song and display that are frequently employed by northern males. Males in both groups are nonterritorial and lack male parental care. A morphological reanalysis of two populations (Scarborough Marsh and Popham Beach) in the contact zone in Maine suggests that there is gene flow between the two groups, but that it is limited geographically, and perhaps in intensity. Phenotypic males of both groups, each exhibiting their characteristic song-related behaviors, co-occur at Popham Beach, with no evidence of behavioral intermediacy in the population. Since the two groups are distinct phylogenetic species, I suggest that the Maine contact is a zone of secondary intergradation. A cladistic hypothesis of relationship in the complex, together with geographic considerations, suggests a history of vicariance in which widely distributed interior-to-coastal populations were split apart during a Pleistocene glaciation. The northern group evidently evolved as an isolate in freshwater wetlands south of the glacial ice front in the interior of the continent, while the southern group differentiated in slat marshes along the Atlantic coast. Most recently, the interior isolate dispersed eastward during early Holocene time, perhaps via a James Bay/Gulf of St. Lawrence route to the Atlantic coast, where it met the northward-advancing southern group in Maine. I recommend that the two groups be treated taxonomically as allospecies of a superspecies.