Habitat use and movement patterns by dependent and independent juvenile Grasshopper Sparrows during the post-fledging period

Abstract

The post-fledging period is a critical life stage for young grassland birds. Habitat selection by recently fledged birds may differ from that of adults and may change as juveniles transition from the care and protection of parents to independence. To describe patterns of habitat selection during these important life stages, we studied habitat use by juvenile Grasshopper Sparrows (Ammodramus savannarum) in a Conservation Reserve Program grassland in Maryland. We used radio-telemetry to track daily movement patterns of two age classes of Grasshopper Sparrows during the post-fledging period. Sparrows were classified as either dependent (<32-d-old) or independent (ࣙ32-d-old). We characterized the vegetation at 780 vegetation plots (390 plots where birds were located and 390 paired random plots). Microhabitats where dependent birds were found had significantly more bare ground, litter, and plant species richness than paired random plots. In addition, dependent birds were found in plots with less bare ground, more warm-season grass cover, more total vegetation cover, and more forb cover than plots used by independent birds. Plots where independent birds were located also had significantly more bare ground than random plots. Dependent birds are less able to escape from predators because their flight feathers are not fully grown so they may benefit from remaining in areas of greater vegetation cover. However, juveniles transitioning from dependence to independence must forage on their own, possibly explaining their increased use of more open areas where foraging may be easier. To properly manage habitat for grassland birds, management strategies must consider the changing needs of birds during different stages of development. Our results highlight the importance of diverse grassland ecosystems for juvenile grassland birds during the transition to independence.

Social interactions do not drive territory aggregation in a grassland songbird

“…Aggregation could reflect decisions of adult sparrows that prioritize survival and future reproductive potential over the current breeding effort (Hinde et al 2010, Dudeck et al 2018. However, the most isolated territories were only~2 km from aggregation hotspots, distances unlikely to restrict flocking of independent juveniles who move >100 m/d (Small et al 2015). We think this unlikely at this site, however, as sparrow settlement decisions are unrelated to food availability (Williams 2016).…”

Postfledging habitat selection and survival of Henslow&#8217;s Sparrow: management implications for a critical life stage

“…Because of limited battery life for transmitters that can be fit to small songbirds, few researchers have followed fledglings into the independent period. However, those studies that have examined this period have shown that fledglings may use habitat with different vegetative characteristics than those utilized by nesting adults (Vitz and Rodewald 2010, Dittmar et al 2014, Small et al 2015. If this is the case, then management that focuses on providing habitat suited to adults may not meet the needs of fledglings, thereby potentially limiting productivity.…”

Juvenile Sandhill Cranes exhibit wider ranging and more exploratory movements than adults during the breeding season

“…The first phase of dispersal has been well studied for some avian species; however, most studies have focused on small‐scale, post‐fledging movements of species that exhibit short‐duration parental care in only the first year of life (White & Faaborg , Small et al . ). Short‐lived avian species, such as songbirds, spend relatively little time between the onset of independence and successful breeding.…”

“…Juvenile dispersal is considered to have three distinct phases: (1) departure from parental care, (2) transience, during which individuals investigate the landscape, and (3) settlement, where individuals first defend a breeding territory (Bowler & Benton 2005, Murphy et al 2017). The first phase of dispersal has been well studied for some avian species; however, most studies have focused on small-scale, post-fledging movements of species that exhibit short-duration parental care in only the first year of life (White & Faaborg 2008, Small et al 2015. Short-lived avian species, such as songbirds, spend relatively little time between the onset of independence and successful breeding.…”