Recruitment patterns and processes in Canadian parkland mallards
An improved ability to assess whether individuals have been added through immigration or natality and lost through emigration or mortality could alleviate several problems in population ecology. Fortunately, advances in stable isotope techniques now allow the movements of individuals to be retraced from tissue values and provide an opportunity to link information about the origins of individuals with demographic rates so that questions about the significance of dispersal can be assessed. I used such an approach by combining feather isotope information with demographic rates derived from capture-mark-recapture of individual mallards (Anas platyrhynchos) breeding in the Canadian aspen parklands, at multiple spatiotemporal scales, to answer questions about population persistence, settling patterns by dispersers, and the fitness of immigrant birds relative to residents. Feather isotope (ä34S, äD, ä15N, and ä13C) values from an independent sample of flightless mallard ducklings sampled from across the mid-continent breeding range was used to validate an existing model used for origin assignments. Spatial resolution analysis within the mid-continent mallard breeding range generally showed a loss in prediction when attempting to assign individuals to more narrowly separated geographic origins among boreal, aspen parkland and prairie regions. For feather äD, spatial resolution may be limited by temporal patterns of local climatic events that produce variability in consumer tissue values. Thus, the use of multiple feather isotope signals would provide more reliable information about the origin of individuals for addressing questions about long-distance dispersal in yearling mallards. Demographic rescue in an apparent population “sink” near Minnedosa, Manitoba, Canada, was due to elevated survival rates from a highly productive group of nesting female mallards using nest tunnels (i.e., an artificial nesting structure) and recruitment of yearling females having natal origins within the aspen parklands. There was little evidence that immigration by yearling females dispersing long-distances was important to annual population growth rates. Consistently high annual survival rates of adult females using nest tunnels lowered the recruitment rates needed for population stability. While tunnel-origin and within-region recruitment of yearling females were nearly equally important to local population growth rate, fine-scale limitations of isotopic origin assignments prevented further assessment of where recruits originated from within the aspen parkland region. Factors related to breeding area settling patterns of yearling females are not well understood despite implications to local population dynamics. The likelihood that immigrant yearling females would settle in a parkland breeding area was positively correlated with local breeding-pair density and the amount of perennial nest cover, but was negatively correlated with the amount of wetlands. Although these relationships were not well estimated, they are most consistent a hypothesis that females were attracted to breeding sites by conspecific cues rather than avoidance. Immigrants comprised an average of 9% (range: 0 – 39% over 22 sites) of yearling recruits; most had natal origins in the U.S. prairie pothole region but a non-trivial number originated from the boreal forest, indicating a high degree of connectedness among breeding regions resulting from long-distance natal dispersal. One of the most frequent explanations for strong site fidelity in breeding female ducks is that females benefit from site familiarity. However, evidence for differential reproductive success between immigrant and resident yearling females was weak, On sites with favourable wetland conditions and low breeding-pair densities immigrant females were more likely to breed and nest successfully than were residents whereas under opposite wetland and pair conditions, resident females were favoured. Thus, the costs and benefits of a natal dispersal decision seemed to vary with social context and environmental conditions, and further work is needed to clarify these processes.
dispersal, immigration, mallard, recruitment, stable isotope
Doctor of Philosophy (Ph.D.)