Ecology of Hawaiian Waterbirds

Science Center Objects

Loss of wetlands, invasive plants, and non-native predators pushed Hawaii’s waterbirds to the brink of extinction by the early 1900s, although population numbers have improved somewhat in recent decades with conservation efforts. Nonetheless, all Hawaiian waterbirds have global population sizes estimated around or below below 2,000 individuals, making them still highly vulnerable to extinction. Increasing our understanding of how waterbirds move among wetlands, refining our ability to estimate population sizes and trends, and optimizing management strategies are key research needs to ensure these species remain as part of Hawaii’s native landscape.

Observers counting waterbirds

Trained observers count waterbirds on O‘ahu Island, Hawai‘i. Photo: R. Camp

Overview:

The Hawaiian Islands historically supported a diverse group of waterbirds (at least 30 species), but today only 6 species persist, all of which are listed as endangered species. The Laysan Duck (Anas laysanensis) occurs only on the remote Northwestern Hawaiian Islands, and the Hawaiian Goose (Nēnē, Branta sandvicensis) largely depends on upland habitats. The remaining four species of waterbirds are non-migratory species dependent on wetlands and endemic to the main Hawaiian Islands: the Hawaiian stilt (Ae`o; Himantopus mexicanus knudseni), Hawaiian coot (`alae ke`oke`o; Fulica alai), Hawaiian Gallinule (ʻAlaeʻula, Gallinula galeata sandvicensis), and Hawaiian duck (Koloa maoli, Anas wyvilliana).  Habitat loss, predation, and hunting pressures brought these species to numbers below 1000 in the 1900s.  With protection, population sizes for both species have recovered somewhat and are estimated to be stable to slowly increasing although populations are still small (~1,484 stilts, ~2,000 coots, ~287 gallinules, and ~2,200 ducks; USFWS Waterbird Recovery plan 2011).  Stilts, coots, and ducks are found on all the large Hawaiian Islands (Hawaii, Maui, Oahu, and Kauai), while gallinules are limited to Kauai and Oahu

Population monitoring for Hawaiian waterbirds consist of an area search and simple count of species conducted biannually by the State of Hawaiʽi Division of Forestry and Wildlife (DOFAW) and consists of visits to wetlands on a single day each winter and summer. This sampling approach provides only indices of species’ presence and relative abundance and is not robust compared to other census techniques widely used. Specifically, population counts from area searches and direct counts do not account for imperfect detection, with simple counts widely known to underestimate relative abundances by some unknown but estimable amount. Further, while conducting the counts all on one day minimizes double counting birds moving among wetlands, waterbirds may seasonally have significant movements among wetlands, which can be important to understand their metapopulation dynamics. Thus, the current count methodology for Hawai‘i waterbirds do not provide information managers need, including accurate population counts and statistical analysis of trends, metrics which are important for downlisting or delisting, and recovery actions. 

Project Objectives:

Improve understanding of the ecology of Hawaiian waterbirds, and provide guidance to managers.  Specifically:

  1. Document movement among wetlands to understand seasonal changes in distribution, metapopulation dynamics, and resource needs and threats.
  2. Development of a sampling protocol to provide a statistically robust estimate of the true number of birds
  3. Understand how distribution, abundance, and metapopulation dynamics are related to habitat quality, predator presence, and geographic size and isolation of wetlands

Highlights and Key Findings:

Wetlands on O‘ahu Island, Hawai‘i

Wetlands on O‘ahu Island, Hawai‘i. Photo: R. Camp

We established a network of automated radio telemetry tracking stations across the Pearl Harbor area, at each major wetland, and at key wetland locations on the Northshore and windward sides of the island. Fitting Hawaiian stilts, Hawaiian coots, and Hawaiian gallinules with long-lived (~ 2 year lifespan) coded radio telemetry tags, we were able to track individuals within and across wetlands. We found a high degree of movement by Hawaiian stilts within the Pearl Harbor area, and to a lesser extent coots and gallinules. The tracking network was operated for 4 years, ending in 2018.

We are linking studies of distribution and movement to understand how waterbirds are using key wetlands, and better understand the meta-population of waterbirds in the multiple wetlands.  Starting in early 2015, we established a network of automated tracking towers in the Pearl Harbor area of Oahu Island to track Hawaiian Stilts, Hawaiian Coots, and Hawaiian Gallinules. The information from this study will help us understand important wetlands, main corridors for movement among the wetlands, and the degree to which waterbirds are dependent on multiple land owners, which would require collaborative conservation approaches.

Progress:

The multi-year, multi-site tracking network provides the information necessary to understand how wetlands are connected by the movement of waterbirds, and the degree that they are a shared resource across wetlands and land manager jurisdictions. Using network analysis approaches, we are exploring how these interconnections among the wetlands change daily and seasonally.

We are also working with wetland and waterbird managers to evaluate current survey approaches and develop new methods that will account for issues of detectability and thus provide more robust estimates of population sizes and trends.

An evaluation of monitoring methods for Hawaiian waterbirds was completed and is available.

Planned Work:

We are currently analyzing the movement data from the automated tracking network, both to understand how wetlands are interconnected via movement of waterbirds, and to understand factors that drive and thus help explain movement patterns. In addition, we are analyzing the most recent waterbird survey data to estimate current population sizes and trends over the past several decades.