In vertebrate ecology, a majority of studies focus on the breeding season of the study population (Marra et al. 2015). In avian ecology in particular, more is known about the ecology of birds during the nesting season than during any other portion of their life cycle (Harrison et al. 2011). The breeding season comprises only one small portion of an individual’s full annual cycle, which also includes the post-fledging period (from nesting to autumn migration), migration to wintering grounds, non-breeding season, and the migration back to the breeding grounds. These relatively understudied portions of the annual cycle have similar, or potentially greater, importance with regards to individual population-level ecology (Harrison et al. 2011). Even when events outside the nesting season do not contribute directly to mortality, they may influence individuals in a non-lethal manner during a subsequent portion of the annual cycle, a process known as carry-over effects (Norris 2005). In many migratory birds, there are few studies on migration and the non-breeding season, but for even more species there also remains a gap in knowledge about the post-fledging period. This period is arguably the relatively “low-hanging fruit” of the annual cycle outside of nesting ecology due to its importance in the annual cycle. In species that are of conservation concern, gaining knowledge about the full annual cycle could illuminate factors expediting population declines that would otherwise be unknowable.
Red-headed Woodpeckers (Melanerpes erythrocephalus) are charismatic cavity-nesters that breed in open forest systems across the eastern and midwestern United States and southern portions of central and eastern Canada. They are closely associated with oak savanna ecosystems throughout the midwestern United States because of the open canopies and low understories for the aerial foraging of insects, scavenging of tree mast, and abundance of dead limbs and trees for cavity-nesting (Brawn 2006, Frei et al. 2015). Once common throughout their range, a range-wide population decline of 70% has been seen in the past 50 years (Sauer et al. 2013). Many factors have been hypothesized to contribute to this decline including the loss of open forest systems and the removal of dead trees that could be potential nesting habitat (Smith 1986), increased rates of predation due to elevated Accipiter populations (Koenig et al. 2017), increased competition for nesting cavities from introduced species such as starlings (Graber et al. 1977), and mortalities resulting from vehicle collisions while aerially foraging (Eifrig 1937). A combination of some or all of these hypotheses is most likely responsible. Despite their charismatic appearance, Red-headed Woodpeckers are an under-studied species with a lack of understanding about several key stages of their full annual cycle including migration (Smith et al. 2000) and the post-fledging period (Frei et al. 2015).
Migration is a behavior found throughout the natural world that allows individuals or populations to avoid potential negative effects and take advantage of seasonal changes in resources. Migration comprises two portions of the annual cycle (Marra et al. 2015). Migration is typically divided into two categories: obligate migration, which occurs regularly and is considered “hard-wired” (Berthold 2001, Newton 2012); and facultative which occurs irregularly and is considered to be “optional” (Berthold 2001). This irregularity of facultative migration presents unique problems when attempting to study the behavior, and as such, little is known about the intrinsic and extrinsic factors controlling facultative migration (Berthold 2001).
Facultative migration has been observed most notably in avian species, including the Common Crane (Alonso et al. 1994), Yellow-rumped Warblers (Terrill & Ohmart 1984), and Great Bustards (Streichet al. 2006), but occurs in insects including melon aphids (Kring 1959) and cotton bullworms (Zheng & Zhang 2000). Current understanding of facultative migration suggests that variation in resource availability and seasonality are the cues (i.e., they underlie the triggering) for facultative migration. In Red-headed Woodpeckers, facultative migration has primarily been described anecdotally (Smith et al. 2000) with few banding records indicating that fall migration occurs at varying spatio-temporal scales (USGS 2019). Most empirical data regarding Red-headed Woodpecker migration has been a result of a single study in South Carolina, which used radio telemetry to track the migration routes of short distance migration (<30 km) and monitor the proportion of the breeding population that migrated each fall (Vukovich & Kilgo 2013). Otherwise, Red-headed Woodpecker migration has not been empirically studied. To better understand this unknown portion of the Red-headed Woodpecker’s annual cycle and add to the limited existing knowledge of facultative migration, we used GPS tags to study proportions of individuals migrating, migration routes, wintering sites, and the timing associated with migration in two breeding populations of Red-headed Woodpeckers in the Midwestern United States. The addition of migration knowledge in this species in a different part of its breeding distribution can help better understand its annual cycle and habitat requirements to inform conservation plans designed to prevent further population declines.
The post-fledging period of the breeding season is another understudied section of the full annual cycle in migratory birds (Streby et al. 2014). However, many bird populations, including those of waterfowl (Amundson et al. 2013) and songbirds (Streby & Andersen 2011), are more susceptible to variations in fledgling survival during the brood rearing phase rather than variation in nest success. Before recent technological advances of radio telemetry, fledgling survival was calculated using mark-recapture or re-sighting individual fledglings, which can be unreliable when attempting to determine the fate or movement of individual fledglings.
Radio telemetry has been used on adult Red-headed Woodpeckers (Vukovich & Kilgo 2009), but fledglings have not been tracked, and as such there is a lack of reliable fledgling survival rate estimates. Nest success and nest productivity has also been studied in Red-headed Woodpeckers (Frei et al.2015, Ingold 1989, Jackson 1976), but once nestlings fledge, little is known about their subsequent fate and movements. As with many other species, knowledge of the post-fledging period in Red-headed Woodpeckers is needed to better inform population models and to inform ongoing landscape management practices intended to benefit this declining species (Frei et al. 2015). To provide the first fledgling survival rate estimates for Red-headed Woodpeckers and quantify fledgling movements and cover-type associations, we used radio telemetry to track individual fledglings at two breeding populations of Red-headed Woodpeckers in the midwestern United States.
This thesis focuses on two aspects of the annual cycle of Red-headed Woodpeckers that are relatively understudied, facultative migration and post-fledging ecology. Knowledge of these two unique stages of the Red-headed Woodpecker’s lifecycle can inform management and conservation practices to better understand and address this species’ population declines. My research addresses (1) facultative migration timing, movements, survival, and proportions of populations migrating in adult Red-headed Woodpeckers and; (2) post-fledging survival, cover-type associations, and movements of fledgling Red-headed Woodpeckers.