Eastern bluebird (Sialia sialis) survival and dispersal in areas with high and low densities of red imported fire ants (Solenopsis invicta).
Lang, Jason Delbert
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Eastern bluebirds (Sialia sialis) and fire ants (Solenopsis invicta) may compete for food, ground arthropods. I describe eastern bluebird survival and dispersal for the 2001 to 2005 breeding seasons at two locations, Athens, Georgia (higher fire ant density) and Clemson, South Carolina (lower fire ant density). Using program MARK to model bluebird survival rates and OpenBUGS to model dispersal distances, I found no evidence that fire ants affected eastern bluebird survival or dispersal for these two populations. Survival: Adult bluebirds had higher survival rates than juveniles; sexes did not differ (confidence intervals: AHYAthens 0.4832 - 0.5560; AHYClemson 0.4867 - 0.5566; HYAthens 0.1417 - 0.1820; HYClemson 0.0996 - 0.1280). Mean survival rates at Clemson varied up to 23% for adults and 10% for juveniles, while only ~1.5% for adults and 1% for juveniles at Athens. At Clemson, survival was negatively correlated to growing-degree days + precipitation. No variables correlated to Athens bluebird survival rates. Breeding dispersal: On average 3.2 ± 1.3 % of breeding adults emigrated each year. Adults dispersed almost twice as far after nest depredation than after other nest failure causes ( predation = 176 ± 288(SD); other = 91 ± 66(SD)). Individuals that changed mates also moved farther (95% credible intervals; changed mates: 80-233 m, ~ 1 to 3 territories; same mate: 33-141 m, ~ 0 to 2 territories). Natal dispersal: About 24% of natal dispersers emigrated. A majority of individuals (91%) stayed within 1500 m of their natal nest (range 0 to ~ 93 km; = 826 m; 95% Credible Interval: 666 – 1024 m). Natal males and females dispersed similar distances. Individuals from spring broods moved almost twice as far as those from summer broods ( spring= 1146 ± 208 m (SD); summer= 620 ± 86 m) and showed a positive correlation to adult survival and an inverse correlation to hatch-year survival. Individuals from summer broods remained closer to their natal nest when they were in better condition (more weight relative to tarsus length). Natal dispersal distance distributions support the resource competition hypothesis and suggest search strategies may differ by sex.