Colony Island Network Design and Implementation (CINDI) to Recover Waterbirds in the Gulf of Mexico: a Pilot Study
Funded by the Knobloch Family Foundation
Harte Research Institute for Gulf of Mexico Studies at Texas A&M University Corpus Christi, Caesar Kleberg Wildlife Research Institute at Texas A&M University Kingsville, and Coastal Bend Bays & Estuaries Program
The NOAA-funded Colony Island Network Design and Implementation Project leverages information collected from a pilot study conducted between 2021 and 2023. On this page, you will find information and results from the pilot study.
Background
Colonial-nesting waterbirds, including wading birds, gulls, terns, and pelicans, concentrate annually to breed on small coastal islands. Over $24 million has been invested in colony island projects along the Gulf Coast to address threats facing existing nesting habitats and current island creation and rehabilitation projects often rely on educated guesses regarding potential bird populations and are decided by engineering feasibility and available funding rather than strategic planning.
There is a clear need for a prioritization tool to identify which islands can most effectively support waterbird nesting, taking into account biological, geographical, and economic factors. This pilot study, funded by the Knobloch Family Foundation, created the initial framework for a tool that will enable stakeholders to focus conservation efforts on the best outcomes for colonial-nesting waterbirds along the Texas coast.
The project focuses on five representative focal species: Reddish Egret (Egretta rufescens), Great Egret (Ardea alba), Tricolored Heron (Egretta tricolor), Black Skimmer (Rhynchops niger), and Caspian Tern (Hydroprogne caspia), each of which has different habitat and foraging strategies. The Reddish Egret, Tricolored Heron, and Black Skimmer are identified as high priority conservation species, while the Great Egret serves as an important indicator of the Gulf's health.
Methods
Screening Model - We assembled traits associated with colony persistence and size from ~45 years of data for the Texas coast combined with expert knowledge to produce a screening model that prioritizes existing islands or candidate sites for enhancement. Data associated with this screening model included:
Waterbird population data from 19 breeding seasons (2003-2021) on 162 rookery islands between the Lower Laguna Madre and Redfish Bay
Area of vegetated and bare substrate on 224 islands from four years of NAIP imagery between 1995 and 2018 delineated in GIS software to calculate the change in area of available nesting habitat and island size over time.
An index of human disturbance on rookery islands based on proximity to busy boat ramps
An index of predation on rookery islands based on proximity to land
A Texas Parks and Wildlife Department fish and marine arthropod dataset used to estimate prey densities surrounding each island
An index of any given island’s potential benefit for waterbird populations based on various characteristics compared between restored and unrestored scenarios
Foraging Habitat - We fitted GPS/GSM tracking devices to 32 birds in our study area and leveraged existing movement data for Reddish Egret and Black Skimmer to identify distances that focal species will fly to access foraging habitat and to determine core foraging areas. These data were implemented into a bioenergetic-based model that established the carrying capacity of the region.
Productivity - We surveyed 6 colonies weekly throughout the breeding seasons of 2021 - 2023 with a small Unmanned Aerial System (sUAS) and high-resolution camera operated at an altitude of 50m to determine nest survival and number of fledglings produced.
Prioritization Tool - The above datasets were used to develop a GIS-based prioritization model that describes the relative suitability of 162 rookery islands for island rehabilitation on a 90-mile portion of the Texas coast from Mission-Aransas Bay through the upper Laguna Madre.
Key Findings
The key findings for the pilot study can be located in the Executive Summary below.
Preliminary Outputs
Click the button below to navigate to the Mapping Application. Notable preliminary outputs from the screening model, foraging habitat, and productivity components can be found below.
Screening Model Outputs
Our screening model assessed population trends for our study region (between the Lower Laguna Madre and Redfish Bay) by calculating a population index from 162 colony islands between 2003-2021.
The output of our model indicated that Black Skimmer and Tricolored Heron experienced decline in this region during this period.
The screening model also incorporated the change in the area of available nesting substrate over time derived from the 224 islands that we delineated. We found that on average, rookery islands lost 2% ± 2% of potential nesting habitat per year and it is projected that most small islands will be completely submerged by 2065.
We assessed human disturbance by assigning the highest index scores to colonies that are nearest to busy boat ramps and modeling bird responses based on waterbird populations. The screening model output suggested that colonies near heavy recreational boating activity were more likely to be abandoned and the number of breeding pairs of waterbirds decreased closer to busy boat ramps for all species except Great Egret. In the first graph below, probability of persistence on the y-axis refers to the probability that birds will continue to return to a colony over a period of time.
We modeled the effect of predation by determining the colonization rates on islands based on their proximity to shore. The screening model output suggests that colonization rates are higher on islands further from shore, where mammalian predation is less common.
The output of the screening model is used to direct the prioritization tool. The screening model predicts the difference in the amount of available habitat and numbers of breeding pairs of waterbirds under restored and unrestored scenarios. The difference between these two scenarios becomes an index of the potential benefit of restoring an island and is used to inform the prioritization tool.
Foraging Habitat Output
Our datasets from the birds we fitted with GPS/GSM tracking devices identified important foraging areas and revealed the following average distances each species was likely to travel from the colony for foraging:
Reddish Egret
15.26 km ± 11.65 km
Tricolored Heron
9.92 km ± 11.65 km
Black Skimmer
6.49 km ± 5.70 km
Great Egret
6.06 km ± 2.84 km
Caspian Tern
10.11 km ± 5.11 km
Productivity Output
Our drone surveys revealed that overall nest survival rate was high for our three wading birds species, but varied by site for Black Skimmer, with some islands experiencing high nest survival and others failing due to episodic weather events such as storms and colony disturbance.
Prioritization Tool
The screening model, foraging habitat outputs, and nest productivity outputs were used to develop a prioritization tool that ranks island based on their restoration potential. The greatest restoration potential was in the southern Upper Laguna Madre and the Lower Laguna Madre.
