Migratory connectivity in the Loggerhead Shrike (Lanius ludovicianus)

We combine genetic and stable isotope data to quantify migration patterns in Loggerhead Shrike (Lanius ludovicianus), a species of conservation concern in North America, to assess how connectivity differs and impacts population evolution, ecology, and conservation.

We sampled shrikes across the majority of their nonbreeding range, from the Atlantic Coast to the western United States east of the Rocky Mountains and throughout Mexico.

Our study used a Bayesian framework using δ²H_f from a breeding season origin feather and nuclear genetic microsatellite markers to distinguish between co-occurring migratory and nonmigratory individuals on the wintering grounds and, for migrants, to assign individuals to a breeding ground origin and genetic group.

Migratory shrikes were present throughout the nonbreeding range but the proportion differed among sample areas. Four main wintering areas were identified. Connectivity ranged from weakly negative in birds wintering on the Atlantic Coast to strongly positive between wintering grounds in the southwestern United States and Mexico and northwestern breeding populations. Connectivity was weakest in L. l. migrans, and strongest in L. l. mexicanus and L. l. excubitorides. Although believed to be nonmigratory, long-distance movements of individuals were observed in L. ludovicianus and L. l. mexicanus. Our data support a pattern of chain migration, again most notable in the western half of the species nonbreeding range, and differential migration based on age.

Our study provides of one such of the first quantitative measures of migratory connectivity and is among the first studies of a short-distance migratory passerine in North America. The higher migratory connectivity among western, versus eastern populations, and less severe population declines attributable to habitat loss or reproductive success, may result in more localized and/or less severe limiting factors for western populations and more severe on the Atlantic coast and Mississippi Alluvial Valley wintering grounds.