Do American Dippers Obtain a Survival Benefit from Altitudinal Migration?

Abstract

Studies of partial migrants provide an opportunity to assess the cost and benefits of migration. Previous work has demonstrated that sedentary American dippers (residents) have higher annual productivity than altitudinal migrants that move to higher elevations to breed. Here we use a ten-year (30 period) mark-recapture dataset to evaluate whether migrants offset their lower productivity with higher survival during the migration-breeding period when they occupy different habitat, or early and late-winter periods when they coexist with residents. Mark-recapture models provide no evidence that apparent monthly survival of migrants is higher than that of residents at any time of the year. The best-supported model suggests that monthly survival is higher in the migration-breeding period than winter periods. Another well-supported model suggested that residency conferred a survival benefit, and annual apparent survival (calculated from model weighted monthly apparent survival estimates using the Delta method) of residents (0.511 ± 0.038SE) was slightly higher than that of migrants (0.487 ± 0.032). Winter survival of American dippers was influenced by environmental conditions; monthly apparent survival increased as maximum daily flow rates increased and declined as winter temperatures became colder. However, we found no evidence that environmental conditions altered differences in winter survival of residents and migrants. Since migratory American dippers have lower productivity and slightly lower survival than residents our data suggests that partial migration is likely an outcome of competition for limited nest sites at low elevations, with less competitive individuals being forced to migrate to higher elevations in order to breed.

Altitudinal migration by birds: a review of the literature and a comprehensive list of species

“…, Green et al. ). Thus, bird altitudinal migration may be a response to both reproductive needs and survival needs, where birds either migrate to higher elevations where reproductive success is likely to be higher or to lower elevations where reproductive success is likely lower, but where mortality rates are lower or there is less competition for resources.…”

The rapid return of marine-derived nutrients to a freshwater food web following dam removal

“…What remains to be seen is the time frame over which species and ecosystem respond to returning subsidies. For instance, dippers adopt life history strategies with greater lifetime reproductive success in areas with intact salmon migrations or greater abundance of salmon (Gillis et al, 2008;Green et al, 2015;Tonra et al in review). Continued monitoring will reveal the rate at which these strategies are adopted by individuals in areas with returning MDN.…”

Altitudinal migration: ecological drivers, knowledge gaps, and conservation implications

“…An example of social dominance and competition explaining partial migration is in American dippers in southeast British Columbia where migrants and residents overwintered together at low elevation, but migrants moved to higher elevation sites during the breeding season while residents remained at low elevations (Morrissey, ; Gillis et al ., ). Migrants of this partially migratory population had lower reproductive success than residents, implying that migrants were prevented from using the best breeding habitats due to competitive exclusion (Gillis et al ., ; Mackas et al ., ; Green et al ., ). High‐elevation rivers in this area did not provide good breeding sites for the dippers during the summer because the snow/ice melt occurred later compared to low elevations, resulting in lower food abundance and a shorter breeding season.…”