Movements of post-breeding royal terns (Thalasseus maximus) in Virginia, U.S.A

Introduction

Habitat use and movements between the end of breeding and departure for non-breeding areas are unknown for many avian species. Some bird species use staging locations to prepare for long migrations or stopovers for relatively shorter trips, with the length of stay and quality of the habitat being related to the type of migration (Warnock, 2010). The lack of a central place like a nest or a consistent roost location makes it difficult for researchers to study this period in the annual cycle.

The royal tern (Thalasseus maximus) is one of the largest terns in the world (Buckley, Buckley & Mlodinow, 2021). Royal terns nest in densely-packed colonies, typically lay a single egg, and the young form crèches soon after hatching (Buckley, Buckley & Mlodinow, 2021). Little is known about the post-breeding movements of royal terns. Banding records suggest that individuals from the mid-Atlantic coast of the United States tend to move north to stage before migrating southward to coastal wintering grounds as far south as Peru (Buckley, Buckley & Mlodinow, 2021; Gibson et al., 2023). In general, migration strategies in birds tend to minimize the amount of time spent migrating (Alerstam & Lindström, 1990), which appears to contradict with the northward movement of post-breeding royal terns. However, the pre-migratory period can be critical to survival during migration, and the strategies (e.g., time minimization, frequent stops, etc.) used to deal with this period vary among seabirds (Loring et al., 2017). It is possible that royal terns may capitalize on a resource or modify other behaviors in a way that would mitigate the overall cost of a delayed or counter-migration (Klaassen et al., 2012). Capitalizing on abundant food resources that occur opposite of the main direction of travel may allow individuals to adequately fuel for the journey (Lindström et al., 2011). It may also provide access to wind currents and more favorable migratory routes to the destination, or avoid migration routes of potential avian predators (Newton, 2010), such as peregrine falcons (Falco peregrinus).

Royal tern numbers declined throughout the mid-Atlantic in the 1990s and early 2000s (Brinker et al., 2007), as did survival rates of all age classes, although all age classes appear to be rebounding (Gibson et al., 2023). Commercial fishing, wind power development and climate change are of great concern for many seabird species (Croxall et al., 2012; Searle et al., 2023), including royal terns. Evidence from long-term banding of royal terns suggests that increasing temperatures, sea-levels, and continued fishing pressure may threaten the species (Gibson et al., 2023). Specifically, royal tern survival was negatively correlated with landings of Atlantic menhaden (Brevoortia tyrannus), a commercially and ecologically important fish species that also migrates along the Atlantic Coast and in the Chesapeake Bay (Anstead et al., 2021; Gibson et al., 2023; Liljestrand, Wilberg & Schueller, 2019). Additionally, survival of pre-breeding royal terns (age 0–2) has decreased 55% since the 1960’s, co-influenced by rising sea surface temperatures and increasing fisheries pressure (Gibson et al., 2023). Meanwhile, there is growing concern that maintaining sustainable menhaden harvest limits and whole-ecosystem sustainability may be impeded by climate-related changes (Anstead et al., 2021; Drew et al., 2021). If post-breeding royal terns from the mid-Atlantic Coast region depend on climatically driven fish resources, especially during the costly life stage of migration, then any sudden shifts in the distribution and availability of these food resources could be catastrophic, as it was for royal terns in California feeding on pacific anchovy (Engraulis mordax) (Buckley, Buckley & Mlodinow, 2021).

Interest and investment in offshore renewable energy sources has increased globally, although at a lesser rate along the Atlantic Coast of the United States (Musial et al., 2023; Sadorsky, 2021). There remains a lack of understanding how these offshore facilities, many with an operational life of several decades, may affect seabird populations in the long term, as their presence and operation may potentially influence the selection of migratory routes, introduce collision risk, and affect access to important resources (Loring et al., 2017; Searle et al., 2023). Recent evidence suggests that these facilities will attract fish (Knorrn et al., 2024; Wilber et al., 2022), which in turn could attract larger terns to the areas (Thaxter et al., 2024). This knowledge gap makes understanding species space use in an area of proposed offshore energy development paramount, and these data are more accessible given advances in GPS-tracking technology and other survey methods (Searle et al., 2023).

The goal of this study was to describe the post-breeding season movements of adult royal terns from the largest seabird colony in Virginia, U.S.A. Using tracking data, we estimated the time the birds spent in ‘resting’ (typically non-flight activity) vs. ‘exploratory’ (i.e., flying, foraging) states to understand behavior during this period. These are the first tracks of their kind for this species, and understanding these movements may determine what factors drive future population change (Brinker et al., 2007; Gibson et al., 2023).

Methods

Study area

We studied the post-incubation movements of adult royal terns between 16 June–01 December 2022 that were captured and deployed with tracking devices on Rip-Raps Island, the site of Fort Wool in coastal Virginia (Fig. 1). Rip-Raps Island was altered to create suitable nesting habitat for a seabird breeding colony displaced by a massive transportation project on the South Island of the Hampton Roads Bridge-Tunnel (Catlin et al., 2024). Rip-Raps Island is attached to the South Island by a rock jetty. The former South Island colony was the largest royal tern colony in Virginia, supporting several other seabirds including common terns (Sterna hirundo), sandwich terns (Thalasseus sandvicensis), gull-billed terns (Gelochelidon nilotica), black skimmers (Rynchops niger), laughing gulls (Leucophaeus atricilla), American herring gulls (Larus smithsonianus), and great black-backed gulls (Larus marinus) (Watts & Paxton, 2014; Watts et al., 2019).

Results

We captured and tagged 11 adult royal terns from 17 June–02 July, 2022. For one bird, the tag was removed the day after tagging as despite no obvious malfunction in the tagging process, the bird was not flying when we observed it the next day but did begin flying normally after the tag was removed. No other birds appeared affected by the transmitters. A second tagged bird was found depredated less than one month after tagging, after remaining in one location for five days. We could not determine the cause of the predation from remaining field evidence. We recovered the leg bands, small parts of the carcass, and the GPS unit under an avian predator perch. Thus, we analyze data from nine of 11 tagged birds.

From these nine birds, we collected 19,316 hourly locations. Individuals carried transmitters for 30–167 days, during which they traveled from 4,695–35,912 km (mean: 152–243 km d⁻¹; Table 1, Fig. 2). Individuals made 61–323 exploratory trips (defined as uninterrupted locations where the bird was designated as ‘exploratory’), averaging 1.31–2.55 trips d⁻¹ (Table 1). Birds were tracked as far north as Jamaica Bay in Long Island, NY (40.63885°, −73.84400°). Although we truncated our analysis to December 1, 2022, one bird was tracked until February 2023, traveling 51,072 km in 242 days. Its last known location was near Puerto Cortes, Honduras (15.81407°, −87.94353°; Fig. 3). We observed five royal terns return to the colony, and an additional two were observed elsewhere the following year. The average instaneous speed recorded was 3.1 ± 9.8 km/h for resting birds and 19.5 ± 19.5 km/h for exploratory birds. The model fit the data reasonably well (Fig. S1).

Royal terns spent a greater proportion of their time in the exploratory state (mean: 0.711, 95% CI [0.710–0.712]) than in the resting state (mean: 0.289, 95% CI [0.280–0.290]), and the proportion of locations designated exploratory increased through the study (0.49–1.00; Table 2), but the number of birds sampled was variable, and only a single bird comprised the tracks in the final month of the study (Table 2). The average length of exploratory trips, increased through the study, from 54 km/trip during the breeding season to 240 km/trip during migration (Table 2). Mean exploratory trip length, duration, and the distance from the colony increased through the study (Table 2), and mean latitude and longitude indicated that birds moved to the northeast until September or October, and moved to the southwest after (Fig. 1).

Average step length for individuals in the exploratory state (10.7 ± 8.1 km) was greater than for those at rest (0.08 ± 0.05 km; Fig. S2). Individuals in the exploratory state showed higher short-term persistence in the path of travel (concentration = 0.19 ± 0.01) than those in the resting state (<0.001, Fig. S3).

Discussion

Royal terns traveled long distances, became increasingly exploratory through the study (Table 2), and they moved north prior to migrating south. Banded individuals from this colony have been resighted or recovered from New England, U.S.A. to Peru during the winter (Gibson et al., 2023), and one of the birds we tracked spent the non-breeding season in Honduras, illustrating the lengthy journeys these birds undertake. Migration is one of the costliest periods in a bird’s life; as such, migratory behavior should reduce the time spent in migration (Klaassen et al., 2012). Thus, the northward post-breeding movements of royal terns that nested in Virginia must confer some benefit to individuals who employ this strategy. Here, royal terns did not begin to move south of the breeding colony until October, whereas birds banded in a Georgia, U.S.A. colony, did not move north prior to migrating south (D Catlin, 2019, unpublished data).

Prior work has demonstrated that royal terns forage primarily in shallow, inland waters up to 80 km from their breeding colony (Buckley & Buckley, 1972), and during the late incubation stage, individuals foraged <15 km offshore (Gatto et al., 2019; Rolland et al., 2020). During the late incubation and chick-rearing period (June), we found that on average individuals stayed within 54 km of the colony (Table 2). The average distances from the colony that we estimated were greater than the previously mentioned studies, which may indicate that royal terns nesting on Rip-Raps Island had to travel farther due to prey availability or quality (Rolland et al., 2020). Alternatively, the colony was on an engineered island, such that the birds were constrained to travel farther than they would from a natural site. Technology allowed us to track royal terns farther and longer than previous studies, so it is possible that distances increased as chicks grew larger and could survive longer periods without supervision (Buckley, Buckley & Mlodinow, 2021). Rolland et al. (2020) attempted to recapture and retrieve GPS trackers after three days and Gatto et al. (2019) used very high frequency (VHF) radio telemetry to track individuals for 11 days. Comparatively, we were able to track individuals for 30–169 days (Table 1). We also captured relatively late-nesting individuals, which is indicative of renests or young birds that may have lower hatching success (Buckley, Buckley & Mlodinow, 2021). Indeed, five of the nine individuals that we fitted with transmitters were first- or second-time breeders. Therefore, the individuals we captured late in incubation and subsequently tracked may never have hatched chicks, releasing them from the need to return to the colony as frequently.

Buckley & Buckley (1972) recorded royal terns traveling up to 65–80 km h⁻¹ parallel to their vehicle on a bridge over the Chesapeake Bay. Although our average recorded royal tern flight speeds were far slower, the transmitters did record birds traveling up to 100 km h⁻¹, albeit rarely. The average speeds that we recorded were less than half the speed of lesser black-backed gulls (Larus fuscus) during migration (38.6 km h⁻¹) (Klaassen et al., 2012) and for royal terns breeding in Louisiana (35.6 km h⁻¹) (Rolland et al., 2020). Both of those studies removed speeds of less than 10 km h⁻¹, but they used that speed to distinguish between resting and active states, rather than assigning states probabilistically, as in this study. Despite the relatively high instantaneous speeds, lesser black-backed gulls had relatively slow overall migration speeds that were attributed to frequent stops. Thus, lesser black-backed gulls, a relatively short-distance migrant, may have been employing an energy reduction strategy rather than a time-minimizing tactic (Klaassen et al., 2012). Royal terns in this study did not travel at faster instantaneous speeds during migration (Table 2), but, time in exploration and trip length increased.

Adult royal terns care for and feed their young throughout the day and night (Buckley, Buckley & Mlodinow, 2021), returning to feed every 2.5 h on average (Erwin, 1977), and royal tern chicks continue to beg and be fed through migration (Buckley, Buckley & Mlodinow, 2021). These extended efforts are reflected in other breeding strategies such as delayed age at first breeding and a single-egg clutch (Buckley, Buckley & Mlodinow, 2021). Moreover, fledged juvenile royal terns weigh less than adults captured during incubation (CE Weithman, 2022, pers. obs.), indicating that significant growth still is needed to achieve enough mass to survive migration. Given these somewhat strict constraints, royal terns ought to be capitalizing on a food resource (Thorup et al., 2010) that enables the birds to prepare for flights to wintering locations to the south (Warnock, 2010). Quillfeldt et al. (2010) found that thin-billed prions (Pachyptila belcheri) migrated south during the austral winter and that this behavior has increased in prevalence since the early twentieth century, presumably because of warming sea temperatures and in search of food. Such migrations (i.e., into colder latitudes/altitudes) are exceedingly rare (Newton, 2010), but they show the flexibility of migration strategies, and the capacity for a species to adjust quickly to changing conditions (Quillfeldt et al., 2010; Thorup et al., 2017). We speculate that the northward movement of royal terns that nest in the coastal mid-Atlantic region of the United States is a local adaptation to their food resource. Migration itself is a form of resource tracking that can span hemispheres and continents.

We suggest that adult royal terns were preparing for a potentially lengthy migration south by tracking the seasonal northward flux of fishes (Liljestrand, Wilberg & Schueller, 2019; Lucca & Warren, 2019; Nicholson, 1971; Nicholson, 1978). Atlantic menhaden are an important food source for royal terns in the mid-Atlantic region (Buckley, Buckley & Mlodinow, 2021). Gibson et al. (2023) found that the indirect impacts of menhaden fisheries drove adult survival lower as fish production decreased. As such, the spawning and distribution of menhaden may affect royal tern post-breeding distribution and movements. During the royal tern breeding season, Atlantic menhaden are distributed from Maine to Florida, but the largest menhaden are found farther north in the Atlantic and Chesapeake Bay (Ahrenholz, 1991; Liljestrand, Wilberg & Schueller, 2019; Lucca & Warren, 2019; Nicholson, 1971; Nicholson, 1978). This distribution remains stable until September when the fish begin to migrate south (Liljestrand, Wilberg & Schueller, 2019; Lucca & Warren, 2019; Nicholson, 1971; Nicholson, 1978). By December most schools are south of the Chesapeake Bay, and schools disperse by January (Ahrenholz, 1991; Nicholson, 1978). Thus, we suspect that royal terns in this study moved north to find larger or more available prey and then migrated with these schools as they moved south. Resource tracking by migratory animals is relatively common (Thorup et al., 2017). This pattern of large-scale northward migration was absent from a banding study of royal terns in coastal Georgia, but their movements and locations (D Catlin, 2019, unpublished data) were superficially similar to those of the Gulf menhaden (Brevoortia patronus;), who tend to migrate not latitudinally, but between off-shore and near-shore waters (Ahrenholz, 1991; Vaughan, Shertzer & Smith, 2007).

Conclusion

Little is known about the post-breeding movements of royal terns but understanding their resource needs and behavior at this critical period could vastly improve the success of any restoration efforts. Like many terns, the location and abundance of food resources can have an important effect on population persistence (Gibson et al., 2023). As pacific anchovy (Engraulis mordax) stocks declined, the Pacific breeding royal terns all but disappeared (Buckley, Buckley & Mlodinow, 2021), suggesting that the anchovies were critical to the presence of royal terns on the North American Pacific coast. Similarly, Gibson et al. (2023) found that survival of adult and pre-breeding (ages 0–2 years) were affected by climate and indirectly through decreasing Atlantic menhaden production. The dependence of royal terns on menhaden and the relationship between menhaden and sea temperatures may partially explain the negative correlation with sea surface temperatures and royal tern survival (Gibson et al., 2023).

Additionally, several offshore wind energy lease areas along the mid-Atlantic Bight (BOEM, 2025), where we recorded tracks of tagged royal terns, are moderate to high-importance habitat to Atlantic menhaden. It is unclear if construction activity in the lease areas will dispel (e.g., noise and disturbance during construction) and/or attract fish populations (e.g., accumulation of invertebrates on structures that create artificial reefs) (Friedland et al., 2021), but it seems likely that they will attract fish (Knorrn et al., 2024; Wilber et al., 2022). For example, GPS-tagged sandwich terns showed more complex interactions with bird and prey species’ space use in and around offshore wind farms than simple avoidance or range shifts (Thaxter et al., 2024). With the concern over Atlantic menhaden stock viability (Anstead et al., 2021; Drew et al., 2021) and other ecosystem interactions on the horizon, studies like these will be necessary to monitor and maintain viable royal tern populations in the face of uncertainty.

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