age ratio, dispersal, Lanius ludovicianus, leg banding, loggerhead shrike, nesting success, population size, population trend, site fidelity, site reuse
We monitored a breeding population of loggerhead shrikes (Lanius ludovicianus) in the Midewin National Tallgrass Prairie from 2005 to 2016. Demographic data were compiled annually, including information on population size, age structure, and reproductive success. We banded adult shrikes, with some additional effort focused on nestlings (2005–2007) and independent hatch-year birds (2014–2016). We measured return rate as the number of birds that were previously banded. Site reuse rates were quantified as the use of a breeding territory in multiple years. Site fidelity, the use of a site by the same male or female in consecutive years, and dispersal distance of banded birds from their natal or previous breeding territory were assessed in 2015 and 2016, after inception of a color-banding program in 2014. Population size ranged from 4 to 14 breeding pairs, with additional single nonbreeding birds noted in most years. The percentage of the population that successfully fledged at least one young each year ranged from 50 to 100%. First nests were more often unsuccessful than later nests. Site reuse was high, and appears related to nesting success in the preceding year, suggesting that shrikes use postbreeding information when selecting nest sites. The ratio of second-year (SY, first year breeder) to after-second-year (ASY) birds varied by year and among sex, with largerthan- expected numbers of SY female birds. Average natal dispersal distance within Midewin was 0.97 km, and average adult dispersal distance was 1.6 km. Female dispersal distance was slightly greater than that of males (2.1 km vs. 1.3 km). By the end of each breeding season, the majority of the known adult population was banded, but approximately only one-third of the banded population returns at the start of the subsequent breeding season. Population size and trend appear correlated with number of ASY birds, and immigration rates of unbanded birds include first-year breeders and those that have bred at least once before, likely from outside of Midewin.
Chabot, Amy A.; Harty, Fran; Herkert, James; and Glass, William, "03. Population Demographics of the Loggerhead Shrike: Insights into the Species Decline from a Long-Term Study in the Midewin National Tallgrass Prairie" (2016). North American Prairie Conference Proceedings. 13.