Conservation and management of animal populations relies on our robust understanding of population processes. Yet, researchers are often missing demographic data and incompletely estimate population dynamics, which risks misinterpretation of critical processes. With the advent of Integrated Population Models (IPMs), researchers are able to combine data sources that directly and indirectly inform population dynamics. Using IPMs, we estimated the population dynamics of two long-distance migrant bird species with differing life histories, the Greenland white-fronted goose (Anser albifrons flavirostris) and tree swallow (Tachycineta bicolor).
It is commonly assumed that large, persistent aggregations of individuals must be sources (where births exceed deaths), but this ignores the possibility that they are sinks instead, buoyed demographically by immigration. We tested this assumption for Greenland white-fronted geese at their largest wintering site (Wexford, Ireland), combining capture-mark-recapture, census and recruitment data. Using results from IPMs, we parameterized an age-structured population projection matrix to determine the contribution of movement rates, recruitment and mortality to the dynamics of the Wexford subpopulation.
For tree swallows, we assessed whether continental-scale or local weather effects explained their population dynamics at sites in British Columbia, Saskatchewan and New York. Long-term regional population trends for tree swallows in British Columbia, Saskatchewan and New York suggest stable or slightly increasing populations (i.e., exhibiting synchrony). Therefore, we also utilized our IPMs to assess whether demographic rates were synchronous among sites.
The observed persistence of the Wexford subpopulation of Greenland white-fronted geese was only possible with high rates of immigration, which exceeded emigration in each year. Despite its apparent stability, Wexford has functioned as a sink over the entire study period. These results demonstrate that even large subpopulations can potentially be sinks, and that movement dynamics (e.g., immigration) among winters can dramatically obscure key processes driving subpopulation size.
For tree swallows, local weather conditions explained significant variation in adult survival in Saskatchewan and fledging success in New York, whereby adult survival and fledging success were greatest when birds experienced ‘good’ local weather conditions. Breeding and nonbreeding season continental-scale effects did not explain significant variation in tree swallow demographic rates. Correlations between common demographic rates among sites suggested little synchrony in population dynamics. Thus, the underlying demographic sources of synchrony in population trends among these sites are unclear. Broadly, our work with Greenland white-fronted geese and tree swallows illustrates the utility of IPMs to holistically estimate demographic rates and test hypothesized environmental drivers of dynamics.