INTRODUCTION

Over two-thirds of the birds that breed in eastern North America engage in long-distance, intercontinental migration between their temperate breeding grounds and their tropical overwintering grounds in Mexico, Central and South America, and the islands of the Caribbean. Although landbird migrants are capable of making long-distance, non-stop flights over ecological barriers such as the Saharan Desert, the eastern Atlantic Ocean, and the Gulf of Mexico, these migrants do not necessarily fly non-stop between destinations. Rather, they stop to rest and refuel in environs, both temperate and tropical, that are likely to be unfamiliar. Coastal areas in the temperate zone are known to be important stopover areas for migratory birds. Unfortunately, the loss of coastal habitat suitable for migrants is accelerating due to the extensive development of coastal regions. Tropical coastal regions are likewise experiencing habitat loss. The importance of these tropical coastal areas for migratory birds is little known, however.

Populations of several species of Neartic-Neotropical migratory songbirds have been shown to be declining (Sauer et al. 2004). Both the en route period and the overwintering period have been implicated as population limitants for migratory birds. As these birds may have different habitat needs during the various phases of the migratory cycle, an understanding of habitat use during each phase is necessary for effective conservation. Our understanding of the stopover biology of landbird migrants is decidedly biased by the predominance of work in the temperate zone, however. We thus know little about the en route behavior and ecology of migratory songbirds in the New World tropics. Consequently, we do not know if the results of stopover studies in the temperate zone are applicable to areas in the Neotropical zone as well.

We know even less about the factors influencing habitat use by temperate breeding migrants arriving on their tropical overwintering grounds. Recent studies have demonstrated that the phases of the annual migratory cycle (breeding, migration, overwintering) are interrelated and that phenomena occurring during one phase of the cycle can have consequences in subsequent phases. We refer to these phenomena connecting the phases as “cross-seasonal factors”. For example, several studies have focused on the relationship between the overwintering and spring migration periods. Research by Bearhop et al. (2004) suggests that overwinter habitat use by Black-throated Blue Warblers (Dendroica caerulescens) influences spring migratory body condition. Marra (2000) found that social interactions influence habitat use in American Redstarts (Setophaga ruticilla). Marra and Holmes (2001) furthermore demonstrated that a redstart’s overwinter body condition is influenced by that same habitat use and may subsequently influence departure times for spring breeding grounds. Additionally, Smith and Moore (2003) investigated the transition between spring migration and breeding and showed that redstarts arriving early upon their breeding grounds and with larger fat stores experience reproductive advantages. Heise and Moore (2003) examined the post-breeding dispersal – autumn migration connection and found that post-fledging Gray Catbirds (Dumetella carolinensis) deposited less fat less quickly than adult catbirds and suggested that juveniles likely depart for migration with less fat stores than adults. Several studies have proposed the idea that events occurring during the overwintering period are linked to the previous autumn migration. The connection between these specific phases of the migratory cycle has rarely been explicitly tested, however.

RESEARCH QUESTIONS

My research focuses on 1) the stopover biology of migratory passerines in the tropical zone and 2) the cross-seasonal factors connecting autumn migration with the overwintering period and the influence of these on subsequent habitat use.

My hypotheses revolve around the following questions:

• Does energetic condition of migratory birds vary among habitats in the tropical zone?
• Energetic condition can be age-dependent during stopover in the temperate zone. Does the same pattern exist during stopover in the tropical zone?
• Many species of Nearctic-Neotropical migratory passerines exhibit territoriality and habitat segregation on the overwintering grounds. The presence of territoriality during this period suggests that competition for resources is strong. Several studies propose, analogous to the breeding season, that the timing of arrival of territorial birds on their overwintering grounds may be an important factor in the ability to acquire an overwinter territory. I want to examine how the cross-seasonal factors of A) arrival timing, as well as B) physiological condition, and C) social dominance interact and influence habitat use during the initial territory establishment period.

STUDY SPECIES

Hooded Warblers (Wilsonia citrina) are a migratory passerine species that exhibit territoriality and habitat segregation on their overwintering grounds in Central America. Field observations and some limited experimental data suggest that males and females may exhibit habitat segregation based on different innate habitat preferences with females generally preferring short-statured secondary habitat and males generally preferring forested habitat (Lynch et al. 1985, Morton 1990). However, a removal experiment on the overwintering grounds found that the sex of the original territory owner did not predict the sex of the replacement bird on the same territory (Stutchbury 1994). These phenomena make the Hooded Warbler an ideal study species to test hypotheses related to arrival on the overwintering grounds and consequential habitat use using an analysis of cross-seasonal factors.

STUDY AREA

My research takes place in Honduras, Central America.

In 2005, I conducted preliminary stopover fieldwork on the island of Utila in the Bay Islands of Honduras. This fieldwork served to provide initial evidence of the importance of coastal areas of Honduras to migratory birds. Interestingly, the banding data from this year showed initial evidence of age-related differences in both arrival timing and energetic condition for several species of migratory passerines including Hooded Warblers.

Click here for photos from Utila, 2005.

Provided that funding is available, future field seasons will continue in Utila as well as in Pico Bonito National Park on the northern coast of Honduras.

LITERATURE CITED

Bearhop, S., G. M. Hilton, S. C. Votier, and S. Waldron. 2004. Stable isotope ratios indicate that body condition in migrating passerines is influenced by winter habitat. Proceedings: Royal Society London B 0310313.S2.
Heise, C. D. and F. R. Moore. 2003. Age-related differences in foraging efficiency, molt, and fat deposition of Gray Catbirds prior to autumn migration. Condor 105:496-504.
Lynch, J. F., E. S. Morton, and M. van der Voort. 1985. Habitat segregation between the sexes of wintering Hooded Warblers. Auk 102:714-721.
Marra, P.P. 2000. The role of behavioral dominance in structuring patterns of habitat occupancy in a migrant bird during the nonbreeding season. Behavioral Ecology 11:299-309.
Marra, P. P. and R. T. Holmes. 2001. Consequences of dominance-mediated habitat segregation in a migrant passerine bird during the non-breeding season. Auk 118: 92-104.
Morton, E. S. 1990. Habitat segregation by sex in the Hooded Warbler: experiments on proximate causation and discussion of its evolution. American Naturalist 135:319-333.
Sauer, J. R., J. E. Hines, and J. Fallon. 2004. The North American Breeding Bird Survey, Results and Analysis 1966 - 2003. Version 2004.1. USGS Pautuxent Wildlife Research Center, Laurel, MD
Smith, R. J. and F. R. Moore. 2003. Arrival fat and reproductive performance in a long-distance passerine migrant. Oecologia 134:325-331.
Stutchbury, B. J. 1994. Competition for winter territories in a Neotropical migrant: the role of sex and age. Auk 111:63-69.