3 research outputs found
Effects of Life History Strategies on Annual Events and Processes in the Lives of Tidal Marsh Sparrows
I investigated ways in which events and processes within the annual cycle of migratory birds differ between the sexes of individual species, and between two closely-related species with different breeding systems. Specifically, I compared male and female Saltmarsh Sparrows (Ammodramus caudacutus), which are highly promiscuous, non-territorial, and have female-only care, and Seaside Sparrows (A. maritimus), which are socially monogamous, territorial, and have bi-parental care, to investigate how differential life histories influence: 1) the timing of migration and molt events; 2) feather quality and condition throughout the year; 3) within-season body condition and survival during summer and winter; and 4) migration patterns. The most significant conclusions were: 1) early male arrival to the breeding grounds may occur for different reasons in different species; 2) reproductive investment can delay pre-basic molt and migratory departure and reduce molt rate; 3) most feather damage occurs in the breeding season and can be influenced by a bird’s reproductive status and investment; 4) reproductive investment can lead to poor body condition that carries over into the winter, but condition differences do not necessarily translate into survival differences; 5) early male arrival may not correspond with sex-based latitudinal segregation on the wintering grounds, and 6) extensive banding efforts imply low connectivity between breeding and wintering sites in Saltmarsh Sparrows. Together, these results suggest that breeding strategies influence the performance of individuals throughout the annual cycle, and, while parental care can be costly, indirect forms of reproductive investment are costly too
Demographic analysis demonstrates systematic but independent spatial variation in abiotic and biotic stressors across 59 percent of a global species range
Data from: Quantifying the importance of geographic replication and representativeness when estimating demographic rates, using a coastal species as a case study
Demographic rates are rarely estimated over an entire species range, limiting empirical tests of ecological patterns and theories, and raising questions about the representativeness of studies that use data from a small part of a range. The uncertainty that results from using demographic rates from just a few sites is especially pervasive in population projections, which are critical for a wide range of questions in ecology and conservation. We developed a simple simulation to quantify how this lack of geographic representativeness can affect inferences about the global mean and variance of growth rates, which has implications for the robust design of a wide range of population studies. Using a coastal songbird, saltmarsh sparrow (Ammodramus caudacutus), as a case study, we first estimated survival, fecundity, and population growth rates at 21 sites distributed across much of their breeding range. We then subsampled this large, representative dataset according to five sampling scenarios in order to simulate a variety of geographic biases in study design. We found spatial variation in demographic rates, but no large systematic patterns. Estimating the global mean and variance of growth rates using subsets of the data suggested that at least 10-15 sites were required for reasonably unbiased estimates, highlighting how relying on demographic data from just a few sites can lead to biased results when extrapolating across a species range. Sampling at the full 21 sites, however, offered diminishing returns, raising the possibility that for some species accepting some geographical bias in sampling can still allow for robust range-wide inferences. The sub-sampling approach presented here, while conceptually simple, could be used with both new and existing data to encourage efficiency in the design of long-term or large-scale ecological studies