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Technical Summary

STUDY TITLE: Habitat Affinities and At-Sea Ranging Behaviors among Main Hawaiian Island Seabirds

REPORT TITLE: Habitat Affinities and At-Sea Ranging Behaviors among Main Hawaiian Island Seabirds: 

Breeding Seabird Telemetry, 2013–2016

OCS STUDY NUMBER: BOEM 2020-006

NUMBER OF PAGES: 111

CONTRACT NUMBER: M13PG00022

SPONSORING OCS REGION: Pacific

APPLICABLE PLANNING AREAS: Hawaii

FISCAL YEARS OF PROJECT FUNDING: FY 2013: $400,000; FY 2014: $500,000; FY 2015: $0; 

FY 2016: $0; FY 2017: $0; FY 2018: $0; FY 2019: $0; FY 2020: $0

CUMULATIVE PROJECT COST: $900,000

COMPLETION DATE OF REPORT: January 2020

PROJECT MANAGER: Josh Adams

AFFILIATION OF PROJECT MANAGER: U.S. Geological Survey, Western Ecological Research Center

ADDRESS: U.S. Geological Survey, Western Ecological Research Center Headquarters, 3020 State 

University Drive East, Modoc Hall, Room 3006, Sacramento, CA 95819

PRINCIPAL INVESTIGATOR: Josh Adams

KEY WORDS: seabird, telemetry, bio-logging, GPS tag, accelerometry, Hawaii, foraging behavior, diving 

behavior, foraging range, flight height

BRIEF ABSTRACT: Understanding seabird movements at sea is a complex problem because most 

seabirds tend to be far-ranging and cover long distances during foraging trips. Traditional methods 

quantifying distributions and abundances at sea among seabirds involve environmentally and diurnally

constrained ship-, aircraft-, or land-based visual surveys. Since the late 1970s, technology-aided 

observations of seabirds have increased rapidly. Miniaturization and technological improvements now 

allow even the smallest seabirds (<200 g) to be tracked at sea. Individual seabirds can now be equipped 

with a wide-range of data loggers and location-tracking devices to provide much more detailed 

information not only on movement patterns, but also various environmental conditions that influence 

movements and habitat use. In this study, we used GPS loggers and temperature-depth-recorders 

(TDRs) to examine the at-sea distributions and ranging behaviors of five abundantly breeding species in 

the main Hawaiian Islands (MHI): Red-tailed Tropicbird, Laysan Albatross, Wedge-tailed Shearwater,

Brown Booby, and Red-footed Booby. We tracked these breeding seabirds from 14 different sites 

throughout the MHI during 2014–2016. We used the Residence in Space and Time (RST) algorithm to 

classify behavior into resting, transiting, and searching/foraging (Torres et al. 2017). We used GPS 

altitude measurements to examine species-specific flight height and to estimate time spent flying in the 

RSZ. We mapped interpolated tracking data for seabirds at each study colony according to behavioral 

class and trip type (when applicable) using kernel density estimates.
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BACKGROUND: Recent Hawaiʻi state clean energy policy mandates and federal interest in developing 

offshore renewable energy resources have prompted unsolicited lease requests for offshore wind energy 

infrastructure (OWEI) to be located in ocean waters off Hawaiʻi. The MHI supports important seabird 

breeding populations and individual seabirds can now be equipped with a wide-variety of data loggers 

and location tracking devices that can provide complex, high-resolution information on movement 

patterns at sea.

OBJECTIVES: Our study objectives were: (1) synthesize previously existing bio-logging and other 

relevant data for the five study species, (2) conduct multi-species and multi-scale quantifications of at-sea 

habitat utilization, (3) evaluate at-sea ranging behaviors and flight heights in relation to potential OWEI, 

and (4) quantify and map species-specific and multi-species hotspot areas at sea off MHI. Data 

generated are intended to inform resource managers concerned with evaluation of proposed renewable 

energy sites and environmental review of specific renewable energy project proposals received by 

BOEM. In addition, new data regarding colony-based seabird ranging distributions at sea throughout the 

MHI will enhance incipient efforts to apply predictive modeling for species distributions and applied 

vulnerability evaluations for MHI seabirds that could overlap with offshore renewable energy 

infrastructure.

METHODS: We deployed two types of Global Positioning System (GPS) loggers on seabirds: modified 

i-gotU GT-120 (Mobile Action Technologies, New Taipei City, Taiwan) and e-obs GPS tags (Bird Solar 30 g,

Bird Solar 15 g, and Bird Battery 30 g; e-obs GmbH, Gruenwald, Germany). We tracked Laysan 

Albatrosses (all years) and Red-footed Boobies (2016) with e-obs GPS/accelerometer tags. We deployed 

miniature Temperature-Depth Recorders (TDRs; Cefas G5, 3 g, Cefas Technology Ltd., Lowestoft, UK) to 

measure dive depths and wet/dry condition (depending on TDR model year). TDRs were deployed on a 

subset of GPS-tagged birds (Red-tailed Tropicbirds, Brown Boobies, Red-footed Boobies) or on 

additional non-GPS-tagged birds (Wedge-tailed Shearwaters). We captured birds at marked nests 

containing eggs (Brown Booby, Red-footed Booby) or chicks (all species). Red-tailed Tropicbirds, Laysan 

Albatrosses, Wedge-tailed Shearwaters, and Red-footed Boobies were captured by hand. Brown Boobies 

were captured using a telescoping noose pole. Non-banded birds were fitted with an appropriately sized 

hard-metal leg-band for identification (USGS Bird Banding Lab). Morphometric measurements and 

blood/feather samples (for DNA-sexing and stable isotope analysis) were collected from most individuals 

at deployment (e-obs GPS tags) or at recapture for tag recovery (i-gotU GT-120 GPS and TDR archival 

tags). Downloaded GPS tag data were clipped to deployment and recovery times and then filtered using a 

speed-distance-angle filter to remove erroneous locations and individual trips were visually screened to 

flag trips that were potentially problematic for analyses. Before summarizing trips, we rediscretized the 

sequence of GPS track points for each trip using linear interpolation after choosing the longest sampling 

interval per species (2–15 min). Summary statistics of foraging trip measurements (e.g., trip duration, 

distance traveled, and maximum range) were calculated overall, and also by trip type, island, colony, and 

year, depending on the species. We annotated the rediscretized location points for each trip with 

behavioral state and day phase. We used the RST algorithm to classify behavior into resting, transiting, 

and searching/foraging. We used GPS altitude measurements to examine species-specific flight-height 

and estimate the percentage of time spent flying at altitudes equivalent to an expected rotor-swept-zone 

(RSZ; 30–194 m) for an offshore wind turbine. We mapped tracking data for each colony by behavioral 

class and trip type (when applicable) using kernel density estimates.

RESULTS: We obtained GPS and TDR data from 59 and 34 Red-tailed Tropicbirds, respectively. Taken 

together, individuals revealed a bimodal (short- [~3 h, <100 km range] and long- [>3 d, >800 km range]) 

trip foraging strategy. While ranging at sea, we estimated that Red-tailed Tropicbirds spend 70.6% (95% 

confidence interval [CI] 70.1–71.0%) of flight time in the RSZ. TDR data for tropicbirds was noisy and we 

could not reliably identify dives. During 2014 and 2016, we obtained GPS data from 35 Laysan

Albatrosses nesting on Kauaʻi and Oʻahu. Individuals during the mid- to late-chick rearing period engaged 

in a bimodal short- (<6 d, <400 km range) and long- (>6 d, >2,000 km range) trip foraging strategy. While 

ranging at sea, we estimated that Laysan Albatrosses spend 2.57% (95% CI 2.50–2.64%) of flight time in 

the RSZ. During 2013–2015, we obtained GPS and TDR data from 313 and 55 Wedge-tailed 

Shearwaters, respectively. Considering all the data together, individuals revealed a multi-modal trip 

duration foraging strategy consisting of intra-day (<24 h, <100 km range), short (<4 d, ~200 km range), 
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and long (>4 d, ~100–400 km range) trips. While ranging at sea, we estimated that Wedge-tailed 

Shearwaters spend 5.20% (95% CI 5.13–5.27%) of flight time in the RSZ. Wedge-tailed Shearwaters 

dove to a mean (± SD) depth of 1.78 ± 1.35 m (median = 1.38 m); the deepest dive recorded was to 

10.06 m. The mean dive duration for Wedge-tailed Shearwaters was 3.12 ± 3.44 s (median = 1.80 s). 

During 2014–2015, we obtained GPS and TDR data from 42 and 37 Brown Boobies, respectively. Almost 

all foraging trips (97%) were single-day trips and we did not detect any bimodality in the distribution of 

single-day trip durations. Brown Boobies foraged relatively close to their colony (<60 km range) and 

focused their at-sea use in nearshore, coastal waters off Kauaʻi and Niʻihau. While foraging at sea, we 

estimated that Brown Boobies spend 3.41% (95% CI 3.16–3.67%) of flight time in the RSZ. Brown 

Boobies dove to a mean depth of 1.42 ± 0.75 m (median = 1.25 m); the deepest dive recorded was to 

8.19 m. During 2014–2016, we obtained GPS and TDR data from 164 and 108 Red-footed Boobies, 

respectively. Considering all the data together, individuals revealed generally a bimodal foraging trip 

strategy (short-duration trips lasted <12 hrs and long-duration trips lasted 12–24 h); 53 individuals 

performed longer multiday trips. Short trips were <70 km in range and long trips were <130 km in range; 

the less frequent multi-day trips ranged ~120–250 km from the colony. While ranging at sea, we 

estimated that Red-footed Boobies spend 2.74% (95% CI 2.63–2.84%) of flight time in the RSZ. Redfooted Boobies dove to a mean depth of 1.12 ± 0.53 m (median = 1.00 m); the deepest dive recorded was 

to 7.57 m.

CONCLUSIONS: These data contribute to a growing amount of seabird telemetry data from the 

northwestern Hawaiian Islands (mostly Laysan Albatross) and provide more recent information regarding 

the variability in the distribution, habitats, and behaviors at sea among the seabird community that exists 

within the central and northeastern subtropical Pacific Ocean. Oceanic variability can affect trophic 

conditions, breeding propensity, reproductive success, and ranging behaviors at sea. More studies would 

be required to fully evaluate variability in the at-sea behaviors among MHI seabirds associated with 

interannual environmental variability. From a phenological perspective (timing of events during the 

breeding season), our studies focused for the most part on breeding adults that were engaged in 

provisioning their chicks. Additional studies that included early-breeding-season movements would be 

useful for evaluating seabird vulnerability to OWEI off Hawaiʻi. Integrating telemetry data with revised 

population counts and MHI colony distributions will be particularly important for estimating potential 

colony-specific foraging areas that may overlap with planned infrastructure at sea. Ultimately, speciesand colony-specific data for MHI seabirds will support better evaluations of risk and options for mitigation 

strategies that allow diverse populations that characterize the Hawaiian seabird avifauna to co-exist with 

a changing ocean environment.

STUDY PRODUCTS:

1. BOEM final report: Adams, J, Felis, JJ, Czapanskiy, MF. 2020. Habitat Affinities and At-Sea 

Ranging Behaviors among Main Hawaiian Island Seabirds: Breeding Seabird Telemetry, 2013–

2016. Camarillo (CA): U.S. Department of the Interior, Bureau of Ocean Energy Management, 

Pacific OCS Region. OCS Study BOEM 2020-006. 111 p.

2. USGS Data Release: Felis, JJ, Czapanskiy, MF, and Adams, J. 2019. At-sea ranging behavior of 

seabirds breeding in the main Hawaiian Islands: Bio-logger data release: U.S. Geological Survey 

data release, https://doi.org/10.5066/P9NTEXM6.
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MAP SHOWING AREA OF STUDY

Study sites in the Main Hawaiian Islands where marine birds were tracked, 2013–2016. (Figure 1 of 

report OCS Study 2020-006).