Research Paper

INTRODUCTION

Seabird populations worldwide are experiencing significant declines due to a wide range of threats including introduced predators, urbanization, bycatch, overfishing, marine pollution, climate change, and disturbance at colonies (Croxall et al. 2012, Rodríguez et al. 2017, 2019, Dias et al. 2019). As a result, seabirds have become one of the most endangered bird groups globally, with 47% of species experiencing population declines and 84% at risk from at least one persistent threat (Dias et al. 2019). Colony creation projects (as opposed to in situ conservation measures within existing colonies) are becoming an increasingly popular way of attempting to counteract these declines. Colony creation projects utilize two primary methods: (1) social attraction of birds with auditory signals and sometimes decoys and (2) translocation of chicks from an existing colony to a new management area (Podolsky and Kress 1992, Gummer et al. 2003, Bell et al. 2005, Miskelly et al. 2009, Carlile et al. 2012, Jones and Kress 2012, Buxton and Jones 2012). These locations may be sites where the species bred historically and has been extirpated or sites where there is no evidence that they bred but where the habitat is deemed suitable for the species. Although costly and labour intensive (particularly if translocation is involved), colony creation projects can have significant conservation value as they create populations inside highly protected areas (typically surrounded by a predator proof fence to exclude mammalian predators or in a predator free area such as an island). They are also often located in more easily accessible areas than existing colonies, making management more practical and less costly than sites located in remote areas.

Whereas colony creation projects have been undertaken since at least the 1950s, there has been a rapid increase in these projects in the last two decades (Spatz et al. 2023). To date, at least 851 colony creation and restoration projects targeting 138 seabird species have been undertaken in 36 countries worldwide (Spatz et al. 2023). In the U.S. Tropical Pacific, colony creation projects are considered to be a vital conservation tool for seabirds (Raine et al. 2022a), particularly because climate change is predicted to cause widespread loss of existing seabird colonies in low-lying areas, potentially displacing millions of seabirds (Reynolds et al. 2012, 2015, Hatfield et al. 2012). In the Hawaiian Islands, multiple colony restoration projects have been initiated over the last decade (e.g., Young et al. 2018, VanderWerf et al. 2019, Penniman 2022). Although most of these projects have focused on albatross and non-endangered Procellariids, several have included two endangered Hawaiian species—the ʻuaʻu (Hawaiian Petrel, Pterodroma sandwichensis) and the ʻaʻo (Newell’s Shearwater, Puffinus newelli)—with the aim of creating new colonies in more manageable areas.

The island of Kauaʻi, the northernmost of the main Hawaiian Islands, is an important refuge for both the ʻuaʻu and the ʻaʻo, holding an estimated third of the world population of ʻuaʻu and 90% of the world population of ʻaʻo (Pyle and Pyle 2017). In recent decades, both species have experienced sharp population declines on Kauaʻi, with 78% for ʻuaʻu and 94% for ʻaʻo between 1993 and 2013 (Raine et al. 2017). Across their Hawaiian range, ʻaʻo and ʻuaʻu face numerous threats, including depredation by introduced species (such as cats, Felis catus, and pigs, Sus scrofa; Hu et al. 2001, Judge et al. 2012, Raine et al. 2020, Simons 1985), and Barn Owls Tyto alba (Raine et al. 2019, 2020), and burrow takeovers by feral honeybees (Apis mellifera; Raine et al. 2023a). Powerline collisions are also a major conservation issue causing thousands of fatalities a year (Travers 2023, Travers et al. 2021, 2023), as well as habitat modification within breeding colonies due to invasive plants (Raine et al. 2021, VanZandt et al. 2014). Additionally, the attraction of fledglings to artificial lights is a significant issue for the ʻaʻo (Reed et al. 1985, Telfer et al. 1987, Raine et al. 2017), which can also affect adults in certain scenarios (Raine et al. 2024b). Both species also undoubtedly face threats at sea that include marine pollution (Clark et al. 2023, Derraik 2002, Sileo et al. 1989), overfishing (Morra et al. 2019, Wiley et al. 2013), and the effects of climate change and bycatch (Gilman et al. 2008, Raine et al. 2023b). This combination of factors has led to the ʻaʻo and ʻuaʻu being listed under both the IUCN Red Data List (Birdlife International 2018) and the Endangered Species Act (USFWS 1983).

Breeding colonies of both the ʻaʻo and ʻuaʻu are now predominantly restricted to remote montane ranges in the interior of Kauaʻi, particularly in the northwest (Raine et al. 2021, Troy et al. 2017), where they breed in wet montane forests (both species) and the steep cliffs of the Na Pali coast and Waimea Canyon (ʻaʻo only). Although there are clear microhabitat differences in their selection of burrow sites (Raine et al. 2021b), there are some areas on Kauaʻi where the two species share the same nesting habitat (ʻÔhiʻa, Metrosideros polymorpha, dominated montane forests) and breed in close proximity to each other. Because of the range of threats already discussed, breeding density is very low in most of these areas and, therefore, good nesting habitat is widely available. However, in five management sites in the northwest of Kauaʻi, effective predator control actions have resulted in steady population increases over the last decade (Raine et al. 2020, 2024a). This increase in breeding density has the potential to lead to interspecific competition for nesting sites.

Interspecific competition occurs when two different species with similar needs and habits compete for the same resources (i.e., food or breeding areas; Crombie 1947). It typically occurs in areas where their respective populations reach a level where resources become limiting, such as when breeding density is high or when a new species moves into (or is introduced to) areas already occupied by a different species that is at or near carrying capacity. If the two species are not equally balanced (i.e., one is more dominant or aggressive than the other), then ultimately the dominant species will outcompete the other, a term known as competitive exclusion (Armstrong and McGhee 1980). Examples of interspecific competition and exclusion in seabird colonies have been recorded for a wide range of seabird species including auklets, murres, gulls, penguins, shearwaters, petrels, storm-petrels, cormorants and boobies (Spring 1971, Manuwal 1974, Burger 1979, Duffy 1983, Ramos et al. 1997, McClelland et al. 2008, Calabrese 2015, Fagundes et al. 2016).

When assessing the potential for interspecific competition for colony restoration projects targeting endangered seabirds in the Hawaiian Islands, there is a third burrow nesting Procellariid to consider: the Wedge-tailed Shearwater (Ardenna pacifica, known as the ʻuaʻu kani in Hawaiʻi). This seabird is the most common Procellariid breeding in the main Hawaiian Islands (Pyle and Pyle 2017) and is abundant throughout the tropical Pacific and Indian Oceans (Harrison 1990, Whittow 2020). It is a coastal nesting species that can occur in very dense breeding colonies. Although not endangered, it faces significant pressure from introduced predators and impacts related to urbanization (such as lights and powerlines) and loss of habitat (Smith et al. 2002, Lohr et al. 2013, Marie et al. 2014, Raine et al. 2021a, Urmston et al. 2022). It is also a species that aggressively evicts other burrow nesting seabirds (Harrison 1990, Whittow 1997), particularly smaller species such as the Tristram’s Storm-petrel Oceanodroma tristrami and Bonin Petrel Pterodroma hypoleuca, where it will often kill the chick while in the process of eviction (Warham 1990, McClelland et al. 2008). Whereas the ʻaʻo and ʻuaʻu are normally geographically separated from Wedge-tailed Shearwaters in Hawaiʻi, a cross-fostering project on Kauaʻi (where ʻaʻo eggs were taken from montane nesting colonies and incubated to fledging by Wedge-tailed Shearwaters; Byrd et al. 1984) has resulted in a scenario where ʻaʻo are now breeding in the same area as this coastal nesting seabird. This presents a unique opportunity to assess what interactions may be occurring between these two species.

Interspecific competition can have deleterious effects if one species is more dominant than the other, which can have important ramifications for conservation projects focused on these species. To assess to what extent this matters for Hawaiian colony creation and restoration projects involving the endangered ʻaʻo and ʻuaʻu, we analyzed data from all management sites to determine whether interspecific competition occurs between these three Hawaiian Procellariid species. This may be more likely for these three species because they are all similar sized—the ʻaʻo (426 ± 42g; A. F. Raine, unpublished data), ʻuaʻu (371 ± 48g; A. F. Raine unpublished data) and Wedge-tailed Shearwater (300–570g; Howell 2012). We then assessed how these interactions should be considered during the site selection phase of similar colony creation projects and seabird management projects Worldwide.

STUDY AREA

Monitoring work occurred at six seabird management sites on the island of Kauaʻi, where at least two of the three Procellariid species breed in close proximity to each other. These included five sites in the remote mountains of the northwest of the island and one on the coast. All monitoring work was carried out between 2014 and 2023 except for the coastal site, which was monitored between 2011 and 2020.

In the mountains, study sites were Upper Limahuli Preserve (a 153-ha area owned by the National Tropical Botanical Gardens, NTBG) and four sites in Hono O Nâ Pali Natural Area Reserve (a large 1,448-ha area owned by the State of Hawaiʻi); Pôhâkea, North Bog, Hanakâpîʻai, and Hanakoa (Fig. 1). All of these sites are located within the northwest of Kauaʻi, at an elevation of between 500 and 1300 meters above sea level. Habitat consists of intact wet montane forest, crisscrossed with deep drainages, narrow ridgelines, and steep valley walls, and dominated by native species such as ʻôhiʻa (Metrosideros polymorpha), lapalapa (Cheirodendron platyphyllum), and tree ferns (Cibotium spp.) in the canopy and large patches of uluhe fern (both Dicranopteris linearis and D. pinnatum) in the understory. The breeding population across all five mountain sites was estimated to be 1798—3362 breeding pairs of ʻuaʻu and 1149–1414 breeding pairs of ʻaʻo (Raine et al. 2024a).

The sixth study site, Kîlauea Point National Wildlife Refuge (KPNWR), was the only coastal site in our study and encompasses 82 hectares on the northeast coast of the island, a few kilometers north of the town of Kîlauea (Fig. 1). The refuge habitat is a coastal complex of steep cliffs abutting the ocean, with buildings and small lawns surrounded by dense Beach Naupaka (Scaevola sericea) and Hala (Pandanus tectorius) thickets and a few patches of Common Ironwood (Casuarina equisetifolia) and coconut trees (Cocos nucifera). This site holds a small population of ʻaʻo formed in the 1980s after the previously mentioned cross-fostering project, estimated at 4–15 breeding pairs in any given year. It also has a very dense breeding population of Wedge-tailed Shearwater, estimated at around 21,000 breeding pairs (Felis et al. 2020). ʻUaʻu did not breed in the refuge during our study period.

Site access was either by foot (KPNWR) or helicopter (all other sites). All sites in this study had active predator control operations in place, which reduced the chances of monitoring work in the colonies, increasing depredation risk for birds breeding in the area.

METHODS

Seabird monitoring was undertaken at all management sites each year using a combination of near-monthly burrow checks and motion-triggered cameras deployed at a subset of burrows throughout the breeding season. Burrows were located through a combination of nocturnal auditory surveys and dedicated ground searching. All burrows located within each colony were marked with a unique identification tag (colored and numbered cattle tags), and their locations were recorded using a handheld GPS (Garmin Rino530HCx, Rino650, or Rino750). Burrow checks started in mid-February before birds arrived and continued until December. During burrow checks, each burrow was inspected to assess breeding status. For deep burrows where direct visual inspection was not possible, a hand-held camera (Panasonic Lumix or Olympus Tough Stylus TG4/TG5/TG6) was used to take photos into the back of the burrow to assess burrow contents. A total of 1413 burrows (299 ʻaʻo and 1114 ʻuaʻu) were located in the montane sites and 26 ʻaʻo burrows in the coastal site over the course of the study. All active burrows at each site were monitored annually on this schedule.

A subset of these burrows was also monitored each year with cameras (mainly Reconyx Hyperfire PC900 and HP2X, although a small number of Reconyx Ultrafire XP9 were also used). Because of the disparity in total breeding population sizes of the target species between montane and coastal sites, the number of cameras deployed in these areas differed. An average of 177 cameras (minimum 157, maximum 196) were deployed on burrows in the montane sites annually, amounting to at least 30 camera-monitored burrows in each of the montane colonies. At the coastal site, there were very few ʻaʻo burrows with active ʻaʻo breeding pairs present each year—mainly because of the Wedge-tailed Shearwater evictions. Funding for cameras at the site also only became available in 2017. Burrows with breeding pairs were prioritized for camera monitoring, amounting to between two to four burrows annually. This meant that the majority of the breeding population of ʻaʻo at KPNWR were monitored with cameras in any given year.

Cameras were set with a high sensitivity motion trigger and took photographs whenever there was movement in front of the burrow, both during the day and at night. If the cameras were triggered at night, they used No-Glow High Output Covert IR (and no flash) so as not to disturb the birds. Cameras were also set to take five photos per trigger, with no quiet period, ensuring that activity and behavior around the burrow was adequately recorded. Cameras were mounted on poles located three to five feet away from the burrow entrance, with the camera pointed directly at the burrow mouth to catch all activity at the burrow. Burrows with a good field of view and only one entrance were preferentially chosen. Cameras were deployed at burrows in mid-February to mid-March, prior to adult arrival, and recovered in December, following fledging. If chicks were still in burrows during December, cameras were left in place until the following year to obtain data on late fledging events. If camera-monitored burrows became inactive during the breeding season (i.e., because of burrow failure) then the camera was moved to another burrow that was still active, thus ensuring that camera effort (no. camera/days) remained broadly standard each year.

All photographs taken by burrow cameras were individually viewed and digitally coded once teams were back in the office. When reviewing photographs from each site, we assessed whether there were any interspecific interactions at the burrows by looking for the appearance of a species that was not the same species as the burrow occupant. If a different species was seen, we collected data on the date and time of the event and categorized the interaction as one of the following: (1) walking past burrow, (2) sitting outside burrow, (3) investigating burrow (i.e., sticking head in burrow entrance but not fully entering it), (4) entering burrow, (5) occupant attacks visitor and (6) visitor attacks occupant, with categories (5) and (6) designated as aggressive interactions. We also categorized the outcome of aggressive interactions as (1) occupant chases off visitor and (2) visitor evicts occupant. Adults and chicks were differentiated by the presence of down, the state of the feathers if no down was visible (adults have clearly worn and sun-bleached feathers, whereas chicks have clean dark feathers), and behavior (chicks engaged in prolonged exercise bouts and spent a lot of time exploring their surroundings and adults were focused on returning to feed their chick and then immediately departing).

Data analysis

A series of chi square tests were conducted to compare differences in the timing of interspecific interactions within a 24-hour period and on an annual scale, as well as to assess whether instances of aggressive interactions occurred more frequently at ʻaʻo or ʻuaʻu burrows. Additionally, a t-test was conducted to consider interaction occurrence counts at individual burrows in relation to our relative burrow density overlap metric. Finally, a linear model was used to assess trends in interactions per camera day by year in the montane sites.

For ʻaʻo and ʻuaʻu at montane sites, a spatial analysis was also conducted to assess where interactions were occurring in the colonies relative to the distribution of the two species. Spatial overlap of burrows between the two species were considered at two sites only—Pôhâkea and Upper Limahuli. These sites were selected because they recorded the majority of interspecific interactions and had the highest populations of the two species. A burrow density surface was first interpolated spatially for each species at each site with burrow points as inputs using the Kernel Density function in ArcGIS Pro 3.4.2. These surfaces were then rescaled between 0–1 and multiplied across species on a per-site basis to result in a surface indicating relative overlap of burrow densities across both species. Relative overlap values were then considered to assess whether interspecific interactions occurred in areas with higher or lower overlaps. All statistics were carried out in R statistical software version 3.6.1 (R Core Team 2024).

RESULTS

Montane sites: interactions between ʻaʻo and ʻuaʻu

Over the course of the study, a total of 1413 burrows (299 ʻaʻo and 1114 ʻuaʻu) with confirmed breeding pairs were located and monitored. Of these, 10 (0.7%) switched species by the end of the study period with seven transferring from ʻaʻo to ʻuaʻu and three from ʻuaʻu to ʻaʻo. In six of these cases, the switch occurred at a burrow occupied in the previous year by a prospecting pair of the other species. In the other four cases, the switch occurred after the burrow became inactive for several years (in one case, when one of the original breeding pair was killed by a cat). There were no records of burrows switching species when the burrow was occupied by a confirmed breeding pair in that year or the preceding year, nor of burrows switching species after any interspecific interaction.

A total of 271 interspecific interactions at breeding burrows were recorded over the course of the study. These occurred at all five management sites, with Upper Limahuli Preserve and Pôhâkea (the two sites with the highest concentrations of both species) recording the majority (41.3% and 37.6%, respectively). Of monitored ʻaʻo burrows, 14.7% recorded an interaction over the study period, compared to 2.2% of ʻuaʻu burrows. The majority (62.3%) of burrows where interactions occurred recorded one or two interactions, with only 5.8% recording more than 10 interactions (range 1–41). Significantly more interactions occurred in June and July (Х² = 120.0, df = 8, p < 0.001; Fig. 2). Interspecific interactions were relatively rare, with an average of 5.6% of camera-monitored burrows recording an interaction each year (range 2.3%–8.7%) and at a frequency of 0.00067 interactions/camera day (range 0.0000086–0.0016). However, there was a significant increase (y = -0.24+0.00012x, r² = 0.58, p = 0.0062) in interactions over the study period (Fig. 3). Interactions included 91 instances where one species actively inspected the burrow entrance of the other species and 105 instances (38.7% of all interactions) where they fully entered the burrow. Interactions varied in length from less than one minute (median 16 minutes) to 480 minutes (an ʻaʻo that spent eight hours cleaning up vegetation around the front of an ʻuaʻu burrow, as well as regularly entering and exiting it). Interspecific interactions occurred at burrows throughout the night, although there were significantly more interactions in the first few hours after dark (between 20:00 and 22:59, Х² = 154.3, df = 13, p < 0.001).

The locations of burrows where interspecific interactions were recorded was considered at Pôhâkea and Upper Limahuli relative to the level of spatial overlap between the two species at each site. This was undertaken to assess whether interspecific interactions occurred randomly across the landscape or in areas where burrows of the two species were in closer proximity to each together, as breeding distribution of the two species was not uniform across the landscape (Fig. 4). Interspecific interactions were significantly higher at burrow locations where relative overlap values were higher (t = -3.0026, df = 26.793, p = 0.006), indicating the interactions were more likely to occur in areas where the two species were breeding close to each other.

Across all interspecific interactions recorded during the study, 31 were characterized as aggressive interactions, with all but two involving the resident bird attacking the visitor (e.g., Fig. 5). In the other two instances, a visiting ʻuaʻu attacked an ʻaʻo in its burrow. Aggressive interactions were recorded at 18 burrows (1.3% of total) and, on average, at 1.5% of all camera-monitored burrows in any given year (range 0.0–2.7%). In all cases, the occupant successfully chased the visitor off. In 29.0% of aggressive interactions the visitor was an ʻaʻo at an ʻuaʻu burrow, with the rest (70.1%) involving an ʻuaʻu at an ʻaʻo burrow. Aggressive interactions were more likely to occur at ʻaʻo burrows than ʻuaʻu burrows (Х² = 43.5, df = 1, p < 0.001), even though almost three-quarters (70.6%) of monitored burrows were ʻuaʻu. Aggressive interactions were recorded in all months except October to December, with significantly more occurring in June and July (Х² = 26.2, df = 8, p < 0.001). This coincided with incubation and early chick hatching periods for both species.

KPNWR (coastal site): interactions between ʻaʻo and Wedge-tailed Shearwater

Over the course of the study, a total of 18 ʻaʻo burrows were monitored at KPNWR, along with an additional eight burrows that were in the process of being created by new ʻaʻo pairs (prospectors). Of the 18 burrows with confirmed breeders, 10 (55.6%) were occupied by Wedge-tailed Shearwaters by the end of the study period. Cameras were present on three burrows where species occupancy changed in that particular year. In all cases, analysis of camera images revealed that occupancy changed because of the direct eviction of the ʻaʻo pair by Wedge-tailed Shearwaters. Of the eight ʻaʻo prospector burrows, seven (87.5%) were occupied by Wedge-tailed Shearwaters by the end of the study period (none of these had cameras on them in the year when occupancy changed).

A total of 703 interspecific interactions were recorded on burrow cameras during the study period. All camera monitored ʻaʻo burrows recorded multiple interactions with Wedge-tailed Shearwaters, with significantly more interactions occurring in July and August than any other month (Х² = 620.9, df = 8, p < 0.001; Fig. 6). The majority (88.9%) of burrows recorded 11 or more interactions (range 4–208). These included 122 instances (17.4% of interactions) where Wedge-tailed Shearwaters actively inspected the burrow entrance and 74 instances (10.5% of interactions) where they fully entered the burrow. Interactions varied in length from only one second (median 32 seconds) to 112 minutes (a Wedge-tailed Shearwater that sat in front of an ʻaʻo burrow, entered it, spent time in the burrow, exited, and sat outside again). On average, we observed 0.37 interactions/camera day, which is 520 times higher than the frequency of interactions recorded at burrows in montane sites between ʻaʻo and ʻuaʻu.

Within these interspecific interactions, 36 were characterized as aggressive interactions (e.g., Fig. 7), and these were recorded at 55.6% of all burrows monitored. These involved 13 (36.1%) instances where the resident ʻaʻo attacked a Wedge-tailed Shearwater attempting to enter its burrow and 23 (63.9%) instances where Wedge-tailed Shearwaters attacked the resident ʻaʻo. For the latter, this included an adult Wedge-tailed Shearwater that bit an ʻaʻo chick that was emerging to exercise and multiple instances where a pair of Wedge-tailed Shearwaters evicted an ʻaʻo pair and then prevented them from re-entering its burrow. Evictions were violent interactions that typically consisted of a pair of Wedge-tailed Shearwaters vigorously attacking the ʻaʻo pair and often involved a prolonged battle inside the burrow. Ultimately the ʻaʻo would be successfully evicted, and from that point onward the ʻaʻo would be prevented from re-entering its burrow for the rest of the season. Aggressive interactions were recorded in all months except November and December and significantly more occurred in June, July, and August (Х² = 26.8, df = 6, p < 0.001). This coincides with incubation and early chick rearing periods for both species.

Wedge-tailed Shearwater activity occurred at ʻaʻo burrows all night, although there was significantly more activity in the early morning (in the hourly periods between 03:00 and 06:00, Х² = 352.8, df = 12, p < 0.001). For ʻaʻo burrows where cameras were deployed prior to the start of the season (n = 6), Wedge-tailed Shearwaters arrived at the burrow before ʻaʻo 66.7% (n = 4) of the time, with Wedge-tailed Shearwaters arriving one and nine days after the ʻaʻo at the other two burrows. For burrows where Wedge-tailed Shearwaters appeared first, they were often present in and around the burrow for a lengthy time period (average 23.3 days, range 8–39 days) before the ʻaʻo pair appeared to start their breeding season.

DISCUSSION

Over the course of our study period, interspecific interactions were recorded at all study sites. Of all three species, the ʻaʻo fared the worst in interspecific interactions, being actively evicted from burrows by Wedge-tailed Shearwaters at the coastal site and being far more likely to be subjected to interactions with ʻuaʻu at their burrows in the mountains (Table 1). These results have implications for colony management and restoration projects in Hawaiʻi, where multiple projects across the island chain are focused on the ʻaʻo and ʻuaʻu because of their endangered status. They also have broad implications for projects worldwide that focus on multispecies assemblages of similar sized seabirds utilizing similar breeding habitats and strategies.

In montane colonies, although interspecific interactions occurred at a lower rate than that recorded at our coastal site, they significantly increased over the study period in tandem with observed breeding population increases. It is not clear whether this will be an emerging issue at management sites, where colonies are continuing to increase in size and prime breeding sites will thus become more limiting. If it does occur, it seems likely that this would be more problematic for the ʻaʻo than the ʻuaʻu. Although we did not record any evictions due to these interactions, we did record multiple aggressive encounters between the two species, including violent battles. Furthermore, two incidents at an ʻuaʻu management site on the island of Lânaʻi are worth considering. This site has also seen long-term population increases due to successful predator control operations with the result that the ʻuaʻu breeding there are often in very high density. In 2023, a burrow camera recorded a prospecting ʻuaʻu attacking and killing one individual of the established breeding pair of ʻuaʻu and successfully chasing its partner off (R. Sprague, personal communication). This intraspecific competition was recorded again in 2025, resulting in another mortality of a breeding bird. These were the first recordings of fatalities of this nature for this species and highlight the potential for significantly more serious interactions to occur as breeding density increases and habitat becomes more limited.

As our work at KPNWR demonstrated, Wedge-tailed Shearwaters are clearly a significant burrow competitor for ʻaʻo. The literature has shown this is also true for multiple other seabird species (e.g., Harrison 1990, Whittow 1997, McClelland et al. 2008) and has included regular cases of Wedge-tailed Shearwaters killing the chicks of smaller species (Warham 1990, McClelland et al. 2008). There is even a record of at least one (and probably multiple) case of infanticide by Wedge-tailed Shearwaters at a site on Oʻahu during burrow eviction events (Russell et al. 2011). This species was also identified during a survey of 75 regional seabird experts in the U.S. Tropical Pacific as being the main native seabird threat to proposed translocation and social attraction projects in the region because of its aggressive behavior (Raine et al. 2025a). It has even been suggested that climate change and the resultant warming of the seas may result in increased competition between Wedge-tailed Shearwaters and other species such as the Flesh-footed Shearwater as Wedge-tailed Shearwaters expand their breeding distribution (Lavers 2015).

Aside from their dominance in direct confrontations, Wedge-tailed Shearwaters are also better suited to carrying out evictions on ʻaʻo because of their breeding phenology and activity levels around burrows. The peak arrival period for Wedge-tailed Shearwaters at KPNWR is early March (Byrd et al. 1983), whereas for ʻaʻo, it is mid-April (Raine et al. 2022b). In our study, Wedge-tailed Shearwaters were often present at ʻaʻo burrows for extended periods before the ʻaʻo returned to initiate breeding for the year, allowing them to occupy the burrows for weeks prior to the ʻaʻo arrival. This made it much easier for them to ensure an eviction. They were also active outside the burrows all night and throughout the breeding season, again giving them a competitive advantage over the ʻaʻo who arrive and depart from burrows during very specific periods at the beginning and end of the night (Raine et al. 2022b). This behavior also has interesting implications for ʻuaʻu. We have not yet had the opportunity to observe interspecific interactions between Wedge-tailed Shearwater and ʻuaʻu, but ʻuaʻu arrive and depart far less frequently than ʻaʻo once the chick has hatched. Breeding pairs of ʻuaʻu arrive and depart from their burrows on average once every three days and stay in the burrow for only a short period of time (around half an hour) to feed the chick before departing again (Raine et al. 2025b). Although structurally they are a larger, bulkier bird with a thicker hooked beak (which might make them more capable of defending their burrows from Wedge-tailed Shearwaters), this visitation behavior could make them vulnerable to eviction simply because they are rarely at their burrows once the chick has hatched. This could become a management issue at Nihoku, where a small breeding population (four pairs) has been established because of a multiyear translocation project (Young et al. 2018)

Although introduced predators and other anthropogenic factors are clearly critical factors for the current restriction of the ʻaʻo and ʻuaʻu to remote montane areas, it is also possible that the dominant and aggressive nature of the Wedge-tailed Shearwater may have naturally resulted in the geographical separation of the three species in the Hawaiian Islands. Subfossil records do suggest that both ʻaʻo and ʻuaʻu were widely distributed on the landscape prior to the arrival of humans, and there are some localized lowland fossil deposits of both species (Hammat et al. 1981, Olson and James 1982, Burney 2001). However, this evidence suggests that the ʻaʻo would have bred in lowland hills rather than true coastal sites (Olson and James 1982, Hearty et al. 2005) whereas, for ʻuaʻu, the most compelling evidence of coastal breeding was in very localized karstic landscape dominated by sinkholes (Hammat et al. 1981) rather than the sandy, sparsely vegetated ocean-adjacent habitats preferentially selected by Wedge-tailed Shearwaters (Harrison 1990, Whittow 1997, 2020). It is therefore possible that the large coastal colonies of Wedge-tailed Shearwaters in the Hawaiian Islands may have naturally excluded both ʻaʻo and ʻuaʻu from many coastal areas, even prior to the arrival of introduced predators and other human related impacts. The present-day coastal population of ʻaʻo at Kilauea is entirely the product of human interventions.

Management implications

Our results have shown that interspecific interactions occur between the three Procellariid species in our study and that this can have important ramifications for breeding success and project site suitability through burrow competition. An assessment of existing or potential species assemblages in proposed colony restoration areas for ʻaʻo and ʻuaʻu should therefore be an important factor for project managers and decision makers to consider prior to initiating social attraction or translocation projects. Considering the fact that Wedge-tailed Shearwaters in particular were found to be significant nest competitors, areas with either existing or nearby colonies of Wedge-tailed Shearwaters (or coastal areas with suitable habitat for the species where recolonization after management is a possibility) should not be considered suitable for projects involving similar sized Procellariid species. The Kilauea egg-swap project should be considered as a cautionary tale—forty years after the initial egg swaps in 1978–1980, the site would by now be expected to hold hundreds of breeding pairs of ʻaʻo. Instead, only a few ʻaʻo pairs continue to breed there, and these birds are regularly harassed and evicted. In this respect, the egg swap project cannot be deemed a conservation success.

Additionally, although we did not record any evictions in our montane colonies, the increase in interactions (including aggressive interactions) as colonies have increased in size suggests that burrow competition could become a factor in the future. If this does occur, it seems likely that these interactions would be more detrimental to ʻaʻo than ʻuaʻu. Long-term monitoring at management sites is therefore critical to assess whether this becomes an issue. In the meantime, at social attraction sites focused on the ʻaʻo and ʻuaʻu, we would recommend that the broadcast calls focus on one species only per location, decreasing the chances of interspecific competition occurring within these project areas, particularly because these sites are typically small and contain limited breeding space. There are many successful colony restoration projects worldwide that focus on multispecies assemblages, and we are certainly not suggesting that all of these projects could face issues with interspecific competition. However, for projects focusing on attracting or translocating one species into an already established dense breeding colony of another species that is of a similar size and utilizes similar breeding habitats and strategies, this issue should be carefully considered prior to proceeding because newly arriving birds could have serious difficulty establishing a viable population if breeding space is limited.

Although careful site selection can help minimize interspecific interactions, in natural systems there are no guarantees. If new species unexpectedly naturally colonize colony creation sites, additional measures can be considered to protect focal species from being negatively affected. Methods that could be considered as ways to reduce interspecific burrow competition are as follows:

1. Habitat modification. This can be a useful tool if potentially competing species favor different microhabitat variables for burrow selection. For example, ʻaʻo prefer steeper slopes with denser ground cover than ʻuaʻu (Raine et al. 2021b), which is why colony restoration sites for ʻaʻo should not be selected if they are on flat or only gently sloping land. Altering habitat inside a social attraction area by manipulating ground cover and topography could attract different species to different portions of the site thus helping to minimize burrow competition. However, if one species is a generalist and the other a specialist, this would not be an effective strategy (e.g., Sullivan and Wilson 2001a) if the generalist is the dominant species because it would be able to utilize burrow sites whatever the terrain and vegetation.
2. Burrow modifications. Modifications can be included within the design of artificial burrows in an attempt to exclude burrow competitors. For example, in New Zealand, 56%–71% of Chatham Petrel Pterodroma axillaris burrows failed because of competition with Broad-billed Prions Pachyptila vittata, which regularly injured or killed Chatham Petrel chicks during burrow evictions (Gardner and Wilson 1999). Artificial burrows were designed to allow smaller Chatham Petrel to enter while excluding larger prions (Sullivan et al. 2000). Burrow entrance flaps were also added to Chatham Petrel burrows because this species is more likely to push its way through vegetation to get into burrows, and this was found to be at least partly effective (Sullivan and Wilson 2001b). Similarly, after it was found that White-tailed Tropicbirds Phaethon lepturus were regularly killing critically endangered Bermuda Petrel Pterodroma cahow chicks on small islets in Bermuda, wooden baffles were added to the entrances, allowing the smaller petrels to enter while keeping bulkier tropic birds out (Wingate 1978). For species that prefer shallow burrows, longer deeper tunnels could also be added to the front of artificial burrows to favor species that prefer deeper burrows. Unfortunately, in the case of the three species considered in our study, all are of a similar size and structure, so entrance size modifications would not help. However, because Wedge-tailed Shearwaters often nest in shallow burrows, it is possible that longer and more convoluted tunnels may help restrict them to some degree.
3. Direct exclusion. The entrances of artificial burrows can be blocked outside of the breeding season of the target species. Blocking the burrow entrance when the target species is not present will at least temporarily prevent other species from taking over burrows during the off season. In our study, we found that Wedge-tailed Shearwaters regularly used ʻaʻo burrows prior to the ʻaʻo arriving and again after they departed. This kind of behavior facilitates burrow evictions by allowing the Wedge-tailed Shearwaters to establish themselves in the burrow before the ʻaʻo returned. Blocking burrows off prior to the earliest arrival dates of ʻaʻo could help alleviate this issue. However, this technique has had mixed success. For example, it was not found to be effective for Chatham Petrels as Broad-billed Prions simply moved in as soon as the blockades were taken down at the start of the petrel season (Gardner and Wilson 1999).

Colony creation is a vital tool for seabird conservation in the U.S. Tropical Pacific and particularly the Hawaiian Islands (Spatz et al. 2023, Raine et al. 2025a). However, as this study shows, the presence of other breeding seabird species needs to be considered in the planning process if these projects are to succeed. This is particularly true if Wedge-tailed Shearwaters are present or could be present and/or if the proposed translocation or social attraction site already has a high density of a breeding species that is of a similar size and utilizes the same breeding habitat and breeding strategy. Ensuring that this variable is incorporated during the decision-making process will ensure that critical conservation dollars can be focused on colony creation projects that have the highest chance of success.

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