American Redstarts have become a model species to study the ecology and evolution of migration, and other biological questions including communication, population limitation and structure, nesting ecology, and life-history traits. Their song and foraging behaviors are often conspicuous and structured; nests are often near enough to the ground to study nesting behavior and ecology relatively easily (at least in northern parts of breeding range); individuals are territorial or use localized home ranges much of the year, and are site-faithful between years, making it relatively easy to capture, mark, and follow individuals and populations in time and space; and they have been studied throughout the annual cycle, in part because of the advantages just listed, which has generated a variety of discoveries as well as new questions.
Recent reviews provide perspective on American Redstarts: Holmes 2007 reviewed long-term studies designed to understand population limitation both in a breeding (Hubbard Brook Experimental Forest, New Hampshire) and overwintering area (Jamaica, West Indies); and Holmes 2011 and Holmes and Likens 2016 focused on long-term studies of breeding forest birds in the Hubbard Brook site in an ecosystem context, with considerable information on American Redstarts as one of the most abundant breeding species. Faaborg et al. 2010breviewed both our current understanding and important knowledge gaps concerning New World migratory birds, including redstarts and many phylogenetically related species. More generally, Bowlin et al. 2010 addressed the ecology and evolution of migratory birds as a group, and call for “integrative research”, including using model species like redstarts to answer the grand questions about migratory animals, namely when, where, why, and how animals migrate, and how such animals in turn influence their ecosystems (see also Greenberg and Marra 2005). Research on redstarts has contributed considerably to addressing when and where birds migrate and what controls their populations, but less to why and how animals migrate. We focus below on a few specific aspects of redstart ecology and evolution that are particularly amenable to rapid advances.
Visual and auditory communication patterns have been investigated extensively in redstarts, although much remains to be learned. First, Game Theory should be profitable for interpreting redstart agonistic behaviors, which are relatively well described and defined. For example, redstart songs and plumages are all distinctive enough for individual recognition (see Sounds and Vocal Behavior: Vocalizations, and Appearance), and individuals interact with each other for up to several years at a time, so experienced males, for example, may be able to minimize territorial displays perhaps via reciprocal altruism and assess other individuals for a variety of purposes. Second, strong intraspecific territoriality in winter (Cooper et al. 2014, Cooper et al. 2015) suggests the opportunity to study agonistic behaviors in a context independent of sexual motivation. Third, the similar breeding-season displays of American Redstarts and related Setophaga species (Ficken and Ficken 1965, Lovette et al. 2010) could be analyzed in the context of emerging methods in comparative phylogenetic studies. Fourth, the reproductive advantage of song mimicry by younger males, leading to the formation of local song dialects, needs further study.
The recent discovery of frequent transient (non-territorial or floater) individuals in overwintering populations of redstarts (Peele et al. 2015; see also Lefebvre et al. 1994a, Lefebvre and Poulin 1996, Toms 2011, Ruiz-Gutierrez et al. 2016) focuses on the need to integrate transients into population processes. We do not know what determines the frequency of transient individuals, except their numbers fluctuate from year to year in Jamaican coastal mangrove and scrub habitats, and their frequency appeared to spike following a major hurricane that hit the southern coast of Jamaica (see Figure 5.1 in Peele 2015). Transients overlap territorial individuals to some extent (Peele et al. 2015), and likely impact territorial individuals via food competition, but the relative impacts of territorial individuals on transients and vice versa are not known. We also do not understand how well these winter-transient individuals ultimately survive and reproduce compared to those that are territorial, and thus it is difficult at present to integrate these transients into models assessing population limitation and regulation. An intriguing possibility is that Yellow Warblers, which are abundant in the coastal habitats where so many transients have been observed in Jamaica (Peele et al. 2015), may distract redstarts through frequent attacks and thus suppress the effectiveness of territorial redstarts in excluding transient individuals (e.g., see Cooper et al. 2015, L. Powell, personal communication). If this should happen, it would represent a novel interaction between interspecific competition and intraspecific competitive mechanisms. A variety of methods are available to study transients, including distinguishing transient from territorial individuals (Peele et al. 2015), how far they move using stable isotope ratios that differ among winter habitats (e.g., Studds and Marra 2005), and how well they survive compared to territorial individuals using telomeres and measures of body condition (Angelier et al. 2013).
We need more information for redstarts on questions as simple as where individuals travel throughout the annual cycle and not just within a season, as well as on the resulting structure of populations (i.e., connectivity; Marra et al. 2010, Bowlin et al. 2010). A variety of technologies are increasingly used to track and evaluate individuals and their condition, including stable-isotope ratios, light-level geolocators and GPS tracking devices, although the redstarts’ small size has thus far made such devices challenging to use. The devices that need to be retrieved to download data (e.g., geolocators) have their own biases that need to be addressed for some questions, e.g., the potential bias of only retrieving information from that subset of individuals that survived an annual cycle. Redstarts were one of the first migratory species used to study carry-over effects (e.g., Marra et al. 1998), but we know far more about individual level effects than those at a population level. The application of these techniques to determine natal dispersal, which is essentially unknown, would be useful.
Much of what we know about connectivity in redstarts and other migratory birds, and about carry-over effects and dispersal derives from use of stable isotope ratios. The validity of using this method was assessed and largely confirmed for redstarts by Langin et al. 2007 (see also Marra et al. 2006, Marra et al. 2010), who nonetheless urge care in interpreting these data, and argue for using broad latitudinal bands or regions when assigning individuals to breeding locations based on samples from individuals collected elsewhere. Moreover, these isotope ratios are sensitive to a number of factors (Langin et al. 2007), indicating a need for further research to address potentially confounding influences on ratios in various animal tissues. For example, stable-hydrogen isotope ratios are incorporated into feathers because of what birds eat where they molt following breeding. These ratios depend on precipitation amounts and distance from water source (ocean), because of how the heavier isotope (Deuterium) falls out of precipitation more readily than the lighter isotope, all of which change systematically with latitude, longitude, elevation (Meehan et al. 2004), and even year, e.g., in relation to El Niño and other weather fluctuations (Nordell et al. 2016). Thus, isotope-based inferences about where an individual bird actually molted a feather—retrieved at a later date—could confound latitudinal, elevational, and year effects, making it difficult to infer changes in response to any one of these factors such as latitude.
Redstart populations are increasing in some parts of their breeding range and decreasing in others (see Demography and Populations: Population Status: Trends; Sauer et al. 2014b). The ecological factors driving these dynamics are not well studied, in part because of the challenges of working at landscape and larger geographic scales. Different ecological factors influence populations in different parts of the geographic range, within summer, within the winter period, and during migration (see Demography and Populations: Range: Population Connectivity), which comprise temporal and spatial scales of factors just beginning to be studied and modeled in diverse ways (Hostetler et al. 2015) including network models (Taylor and Norris 2010). The relative importance of summer versus winter limitation in population dynamics is poorly known, although both are important (e.g., Sherry and Holmes 1995, Runge and Marra 2005, Holmes 2007, Sherry et al. 2015). These issues all need better understanding and integration into the annual cycle.
A strength of redstart research to date has been population-level studies throughout the annual cycle. This is also a weakness, insofar as very little research has been done to understand the underlying mechanisms involved, including the impact of predators (except during nesting), parasites, and competitor species. We know little about the mechanisms of breeding season habitat selection in redstarts, and surprisingly less than what is known about winter habitats. Interspecific competition has largely focused on single pairs of competitor species during both summer and winter, as reviewed above (see Demography and Populations: Causes of Mortality: Interspecific Competition), but the intriguing possibility of extensive diffuse competition involving redstarts (Sherry et al. 2016) suggests that other species of small insectivorous birds may affect food availability, and thus success of redstarts in winter, a topic largely unexplored to date.
Different nesting behaviors at different latitudes (nest heights, location in trees, cover, size and lining, phenology, and clutch size) vary as much in American Redstarts as among different species of birds (see Breeding: Nest Site: Site Characteristics), suggesting that at least some of these factors have a genetic basis that varies geographically due to local selection factors that may be strong enough to counteract dispersal. This suggests the need for comparative genomic studies such as demonstrated to be feasible in Wilson’s Warbler (Cardellina pusilla; e.g., Ruegg et al. 2014) that could detect both genetic population differentiation across redstarts’ geographic range and relevant selection pressures by linking genetic differentiation patterns to known gene functions and consequences. Although little geographic genetic variation was found using neutral genetic markers (mtDNA control region, and Amplified Fragment Length Polymorphisms; Colbeck et al. 2008), nuclear genetic loci relevant to local adaptations may show more geographic variation. However, the degree of genetic differentiation across species geographic ranges (particularly, local adaptation) is limited by the degree of natal dispersal and population connectivity (Marra et al. 2010). Causal relationships between local adaptation and population connectivity have not been studied in any migratory bird species to date, although they are suggested in Wilson’s Warbler (Ruegg et al. 2014) and in redstarts by geographic variation in nesting behavior (see Breeding: Nest Site: Site Characteristics).
A major conservation goal will be to understand and model how global change phenomena will impact future populations of animals like redstarts. Human-impacted habitats and landscapes, and directionally changing climate throughout redstarts’ geographic range will likely continue to impact population dynamics, as has occurred already (e.g., Wilson et al. 2011b). Understanding these complex and potentially interacting factors for redstarts and other migratory species will necessitate continental-scale models (like weather models) of population dynamics that integrate effects of all phases of the annual cycle and all geographic regions occupied, predation and diseases, habitat, population connectivity, geographically structured genetic differentiation, and interspecific competition, among other factors; and many of these factors are barely studied to date in redstarts. More studies incorporating effects of rainfall and temperature on annual survival, phenology, and reproductive success (e.g., Sherry et al. 2015), will be necessary to project global climate and other global change phenomena on populations. It also remains to be determined to what extent redstarts are behaviorally or phenotypically plastic enough to respond to global change phenomena, and whether or not evolutionary change can be rapid enough for populations to avoid extirpation (e.g., Bowlin et al. 2010, Lany et al. 2016).