Overwintering strategies of migratory birds: A novel approach for making inferences on movement patterns of residents and transients
Patterns of movement in wildlife populations make up the core foundation of much of our current ecological knowledge. Direct measures of movement can be made using marked individuals, but this requires a spatial allocation of capture effort that is not feasible for most studies. Capture effort is most often restricted to a single site, limiting our ability to make inferences on the timing and nature of how individuals are associated with a given site. We demonstrate how movements can be monitored indirectly by accounting for permanent and temporary emigration, as well as transience. We apply a multistate open robust design with state uncertainty model (MSORD-SU) to account and estimate transience, to infer movement patterns of overwintering migrants. We treat resident and transient as two phenotypic states, and use the timing and estimates of probabilities of entry of each state as indirect measures of movement. We applied the MSORD-SU to eight species of Neotropical migrants, using a data set of captures from 14 countries collected in 2002-2011. We estimated within-season dynamics of probabilities of entry for residents and transients, the probability of persisting at a site for residents, the ratio of residents to transients using the study area, and residence time.
Our results identified overwintering movement patterns which were largely independent of prior categorization of territoriality for each species. The timing of when transients enter a site varied widely between species, and did not correlate with when residents entered a site. Residents of all species were likely to enter a site early in the season, but on average, individuals did not remain at sites for the full duration of the overwintering period. The MSORD-SU model is a useful tool to correct for and estimate dynamics of both residents and transients, in the absence of recaptures of individuals at multiple sites. The MSORD-SU also allows us to estimate the proportion of individuals that reside in the study area relative to the total number of individuals that use the area (residents + transients), which is a useful metric for determining the ecological importance or conservation value of an area.