Thu, Aug 05, 2021:On Demand
Background/Question/Methods
Avian seed dispersers potentially play an important role in assisted migration for range-expanding tree species. The eastern red cedar (Juniperus virginiana) is native to the eastern United States, but has been expanding its range into western grasslands in recent decades, leading to altered ecosystems and increased management costs. Several avian species are known to feed on cedar cones during the winter seed production season (November-March), and gut passage has been found to improve germination rates. We expect that the winter occurrence of potential avian seed dispersers is contributing to the western expansion of the eastern red cedar through local movements and seasonal migration. We evaluate 12 years of eBird occurrence records (2008-2019) for one non-migratory bird species (Cyanocitta cristatus) and four migratory bird species (Bombycilla cedrorum, Turdus migratorius, Hylocichla mustelina, and Setophaga coronata) to 1) document species’ occurrence within the eastern red cedar range during the cone production season, 2) identify spatio-temporal patterns of multiple potential seed dispersing species overlap, 3) estimate migration timing and speed as proxies for potential maximum seed dispersal distance, and 4) evaluate temporal shifts in avian occurrence and migration.
Results/Conclusions All species occurred within the eastern red cedar range during the seed production season. The co-occurrence of potential seed dispersing species increased during the period 2008 to 2019 from a median of two to three species. We calculated daily population-level centroids to estimate migration timing for the four migratory species (B. cedrorum, n=1,069, 826; H. mustelina, n=399,012; S. coronata, n=1,108,016; T. migratorius, n=1,998,750). All four species exhibit long-distance migration into the first 1-2 months of the seed production season (median end autumn migration ranges 9 November to 21 December) and three of the four species exhibit long-distance migration during the last 1-2 months of seed production season, excluding B. cedrorum (median onset of spring migration ranges 20 February to 17 March). Estimated migration dates were consistent across years, except for S. coronata which had significantly earlier onset of spring migration (p = 0.048, R2 = 0.27), advancing approximately 1.5 days per year, indicating an increased potential interaction with eastern red cedar propagules. Maximum population-level migration speeds ranged from 17.1 (T. migratorius) to 82.8 km/day (H. mustelina), with the fastest speeds occurring during spring migration.
Results/Conclusions All species occurred within the eastern red cedar range during the seed production season. The co-occurrence of potential seed dispersing species increased during the period 2008 to 2019 from a median of two to three species. We calculated daily population-level centroids to estimate migration timing for the four migratory species (B. cedrorum, n=1,069, 826; H. mustelina, n=399,012; S. coronata, n=1,108,016; T. migratorius, n=1,998,750). All four species exhibit long-distance migration into the first 1-2 months of the seed production season (median end autumn migration ranges 9 November to 21 December) and three of the four species exhibit long-distance migration during the last 1-2 months of seed production season, excluding B. cedrorum (median onset of spring migration ranges 20 February to 17 March). Estimated migration dates were consistent across years, except for S. coronata which had significantly earlier onset of spring migration (p = 0.048, R2 = 0.27), advancing approximately 1.5 days per year, indicating an increased potential interaction with eastern red cedar propagules. Maximum population-level migration speeds ranged from 17.1 (T. migratorius) to 82.8 km/day (H. mustelina), with the fastest speeds occurring during spring migration.