Protecting Stopover Habitat for Cerulean Warbler

The primary purpose of this project is to determine the status of the Cerulean Warbler (Dendroica cerulea) and other Neotropical migrant birds during spring migration through lower montane forest on the Caribbean slope of Northern Central America. The project is structured to test the hypotheses that Cerulean Warblers use lower montane forest habitat in mountains facing the Caribbean coast as a staging area to complete their migration between wintering grounds in northern South America and breeding grounds in North America. We hope to gain insights into stopover distribution and habitat associations of this high priority species and enhance long-term local capacity for independent monitoring of Neotropical migrants through training.

The Cerulean Warbler Technical Group, a broad coalition of private, industry, and governmental organizations whose focus is the long-term conservation of the Cerulean Warbler across its range, has identified an urgent need to gain information on the warbler in its non-breeding range, especially during migration. This information is critical for the development of a comprehensive conservation strategy for this species. The goal for the 2007 field season is to more accurately determine the distribution and habitat associations of this species by using data gathered from 2004 – 2006 to build and field-test a model that will predict potential Cerulean Warbler stopover habitat.

Project Need

One of the highest priority needs identified during the Second Cerulean Warbler Summit held December 2002 in Shepherdstown, West Virginia, was basic natural history information about Cerulean Warblers in the non-breeding season. The Cerulean Warbler Atlas Project of the Cornell Lab of Ornithology (Rosenberg 2000) has provided important breeding range information and El Grupo Cerúleo, a group of concerned biologists and land managers from outside the U.S. has compiled valuable winter distribution information. However, virtually nothing was known regarding Cerulean Warbler migratory routes and stopover or staging locations (Hamel 2000) until the initiation of this project in 2004. Very few records even existed for Cerulean Warblers during their migration from the northern Andes until they arrived in the southeastern United States in spring, and vice versa in fall (Robbins et al. 1992, D. Pashley unpubl. data, P. B. Hamel 2004 pers. comm.). Previously, the only report of a large concentration of Cerulean Warblers during migration was that of Ted Parker in 1992 from the Maya Mountains of Belize (Parker 1994).

If, as Parker speculated, the entire world population of Cerulean Warblers is concentrated in wet forests at 500-1500m in a very small geographic area in northern Central America during late March and early April, “…then destruction of these forests may well represent the greatest threat to the long-term survival of this species. This revelation underscores the urgent need to determine the migratory routes and staging areas of Neotropical migrant landbirds in Middle America” (Parker 1994).

Project Description

This research project to identify Cerulean Warbler stopover habitat was initiated in April 2004 with an expedition to the site where Parker made his observations in 1992, as well as two additional potential stopover sites in Belize. Despite the devastating effects of Hurricane Iris in 2001 on much of the surrounding forest Cerulean Warblers were still found using Parker’s Union Camp site. Sixteen individuals were observed during 24 hours of surveys. Interestingly, it was the most common Neotropical migrant recorded. No Ceruleans were detected at the other two potential stopover sites.

In 2005 and 2006 teams of foreign and local biologists conducted a total of 78 surveys in seven regions of Honduras and Guatemala (Fig. 1). Sites were selected based on criteria suggested by Parker, historic records, and interviews with ornithologists familiar with Honduras and Guatemala. A total of 77 Ceruleans were observed during these surveys in the first two weeks of April.

The 2004 – 2006 survey results generally confirm Parker’s 1994 hypothesis. However, in contrast to Parker’s observations of Cerulean Warblers between 600 and 750 m in Belize, in this project Cerulean Warblers were found from 100 to over 1,000 m with a majority of sightings below 500 m. The most interesting finding, however, was that the frequency of encountering Cerulean Warblers was much higher in Guatemala and Belize compared with Honduras. This suggests that rather than being evenly distributed in spring migration between southern Belize, eastern Guatemala, and northern Honduras, Ceruleans may instead be stopping disproportionately in the northern portion of this narrow arc.

A better understanding of the species’ distribution and abundance is needed to develop a Cerulean Warbler stopover habitat conservation strategy. Using the 90 known occurrences of Ceruleans collected during this project, in combination with vegetation, topographic, and other remotely sensed environmental data it may be possible to model the spatial distribution of potential stopover habitat. This has recently been done for Cerulean Warblers on a portion of the breeding range (Buehler et al. in press) and for the wintering range in South America (Hamel et al. in prep). Rigorous field-testing of this model could lead to the identification of critical stopover locations and help prioritize future on-the-ground conservation actions.

Project Objectives

  1. Ground-truth the Cerulean Warbler stopover habitat model with several field teams simultaneously conducting surveys in areas predicted as potential Cerulean Warbler stopover habitat from Chiapas, Mexico to Nicaragua during the first two weeks of April.
  2. Enhance long-term local capacity for independent monitoring of Neotropical migrants by providing training in survey protocol, bird identification, and by providing field experience.

Project Components

1. Cerulean Warbler Survey Methodology

Survey locations will be selected based on the results of Cerulean Warbler stopover habitat modeling. A survey team will consist of at least one local biologist and one biologist familiar with Neotropical bird identification. Routes of 1.5 to 2.5 km long will be identified and continuous transect surveys conducted between 1 to 18 April. Transects will be located on trails in contiguous forest habitat and will be conducted between 0630 – 1130 and 1430 – 1730. Observers will walk slowly scanning mid-story, upper-story, and emergent trees for mixed species flocks and single foraging birds. When a flock is encountered all individuals present will be recorded. A CD with Cerulean Warbler songs will be played for one minute if either a Cerulean Warbler or one of 6 common Cerulean flock-associates (Black-and-white Warbler, Mniotilta varia; Blackburnian Warbler, Dendroica fusca; Magnolia Warbler, Dendroica magnolia; Lesser Greenlet, Hylophilus decurtatus; American Redstart. Setophaga ruticilla; Golden-winged Warbler, Vermivora chrysoptera) is detected. When a Cerulean Warbler is observed additional data will be recorded including gender, GPS location, distance from observer, foraging height, tree height, and habitat description.

Expected survey locations include: Reserva Biosfera los Montes Azules, Chiapas, Mexico, Parque Nacional Sierra del Lacandon, Peten Province, Guatemala, Western Maya Mountains, Peten Province, Guatemala, Sierra Santa Cruz, Izabal Province, Guatemala, and rustic coffee plantations near Copán, Copán Department, Honduras.

2.   Workshop on survey techniques and field identification of Neotropical migrants.

To enhance the capacity of local biologists to conduct surveys for Neotropical migrants and other birds, a workshop on survey techniques and field identification of Neotropical migrant landbirds will be held prior to conducting Cerulean surveys in the offices of Defensores de la Naturaleza in Guatemala City.