Our results reveal similar patterns of at-sea behaviour between two segregated colonies of brown boobies in the Chagos Archipelago. Despite differences in colony location, monsoon period, year, and breeding stage during tracking, trip metrics did not differ significantly between colonies. Foraging similarities could arise because of shared environmental preferences for shelf edge habitats close to the colony. Both colonies foraged in distinct at-sea areas inside the MPA.
Brown boobies used comparable foraging strategies at both tracked colonies in the Chagos Archipelago with similar trip lengths of <12.7 hours duration (Figure 1), foraged at distal locations (Figure S7) up to a maximum of 111.5 km from the colony, and spent very little time resting at sea (Figure 3). Nevertheless, we note that the number of tracked individuals is quite small (DI n=9, NI n=4) and while similarities should be treated with caution, all tracked birds had more than one trip (Table 1). Trip durations and distances were similar to those reported for this species in other oceans (Table 3), further highlighting that brown boobies are generally neritic foragers (Weimerskirch et al. 2009; Young et al. 2015; Miller et al. 2018; Correia et al. 2021; Austin et al. 2021). However, given that temporal changes in the environment can influence foraging strategies (Soanes et al. 2015), and seabirds often forage further during incubation (Sommerfeld and Hennicke 2010), it is interesting that variability in foraging effort was not observed between study colonies, perhaps because of shared environmental preferences.
Table 3. Comparison of trip metrics for breeding brown booby colonies in the Pacific Ocean, Gulf of California, Caribbean Sea from prior studies, and in Indian Ocean from this study. Trip metrics were rounded to one decimal place and presented as reported by the studies, with sexes separated or pooled together. Colony size refers to individuals unless otherwise stated. Values in parenthesis indicate sample sizes of studies, or number of loggers retrieved ‘loggers’ where sample sizes were not specified.
Colony
|
Colony size (individuals)
|
Stage of breeding
|
Mean trip duration (h)
|
Mean max distance from colony (km)
|
Mean total distance
(km)
|
± trip metrics
Indicating
|
Johnston Atoll,
North Pacific Ocean A
|
450
|
incubating eggs
|
6.7 ± 3.8
(23)
|
-
|
-
|
±sd
|
Isla San Ildefonso, Mexico B
|
2000–4000
|
Provisioning chicks
|
2.0 ± 1.0 ♂ (14)
3.0 ± 1.3 ♀ (17)
|
50.9 ± 31.5 ♂ (8)
105.6 ± 45.9 ♀ (11)
|
16.6 ± 14.3 ♂ (9)
39.2 ± 16.6 ♀ (11)
|
±sd
|
Dog Island, Anguilla C
|
2462
|
Provisioning chicks
|
5.6 ± 2
(19)
|
48.4 ± 18.9
(19)
|
125.3 ± 54.4
(19)
|
±sd
|
Swain Reefs,
Great Barrier Reef D
|
18–158
|
Provisioning chicks
|
6.8 ± 2.8
(5)
|
14.5 ± 10.1
(5)
|
49.5 ± 30.3
(5)
|
±sd
|
Palmyra Atoll,
North Pacific Ocean E
|
200–600
|
6 incubating eggs
3 provisioning chicks
|
3.7 ± 2.4
(9)
|
37.3 ± 19.6
(9)
|
98.5 ± 52.9
(9)
|
Not specified
|
Sombrero, Anguilla F
|
724
|
incubating/ provisioning chicks
|
4.7 ± 0.3
(21 loggers)
|
28.5 ± 6.1
(21 loggers)
|
78.3 ± 2.4
(21 loggers)
|
±se
|
Prickly Pear West, Anguilla (2013) F
|
185
|
Incubating/ provisioning chicks
|
5.5 ± 0.5
(32 loggers)
|
46.2 ± 1.6
(32 loggers)
|
105.6 ± 3.9
(32 loggers)
|
±se
|
Prickly Pear West, Anguilla (2014) F
|
520
|
Incubating/ provisioning chicks
|
4.9 ± 0.4
(11 loggers)
|
30.3 ± 2.9
(11 loggers)
|
72.5 ± 7.1
(11 loggers)
|
±se
|
Raine Island,
Great Barrier Reef G
|
2642
(1994–2003)
|
Provisioning chicks
|
5.4 b2 ± 2.06
(19)
|
57 ± 22
(19)
|
150 ± 59
(19)
|
±sd
|
Tinhosas Islands, Gulf Of Guinea H
|
738 pairs
|
Provisioning chicks
|
11.4 ± 7.1 ♀ (49)
9.5 ± 8.2 ♂ (78)
|
92.7 ± 45.4 ♀ (49)
64.4 ± 39.1 ♂ (78)
|
217.9 ±105.7 ♀ (49)
153.1± 99.0 ♂ (78)
|
±sd
|
Cayman Brac, Cayman Islands I
|
146 breeding adults (2017)
|
Provisioning chicks
|
3.0 ± 0.3 ♀ (27)
4.4 ± 0.4♂ (31)
|
14.8 ± 1.4 ♀ (27)
23.5 ± 1.7 ♂ (31)
|
49.9 ± 3.9 ♀ (27)
77.2 ± 5.5 ♂ (31)
|
±se
|
Nelson’s Island,
Chagos Archipelago
(This Study)
|
12
|
Incubating eggs
|
3.7 ± 0.5
(4)
|
23.9 ± 3.7
(4)
|
63.0 ± 9.0
(4)
|
±se
|
Danger Island,
Chagos Archipelago
(This Study)
|
35
|
Provisioning chicks
|
3.8 ± 0.4
(9)
|
35.6 ± 3.8
(9)
|
88 ± 9.0
(9)
|
±se
|
A Lewis et al. 2004; B Weimerskirch et al. 2009b; C Soanes et al. 2015; D Bunce 2015; E Young et al. 2015; F Soanes et al. 2016; G Miller et al. 2018; H Correia et al. 2021, I Austin et al. 2021
Brown boobies are one of three sulids breeding in the Chagos Archipelago, alongside red-footed boobies, Sula sula (total 22,871 pairs),and masked boobies, Sula dactylatra (total 164 pairs) (Carr et al. 2021b) . Brown boobies forage much closer to the islands than the smaller, more pelagic red-footed boobies (mean maximum distance: 112.9 ± 3.7 in Chagos Archipelago, (Trevail et al. 2023), that are resident in the archipelago year-round (Votier et al. 2023). Masked booby at-sea distributions in the Chagos Archipelago are unknown, however population sizes are smaller, so intraspecific competition is less likely.
The habitat selection analyses demonstrated that remaining close to the colony was the most important environmental predictor across both populations, followed by sea-level height anomaly and sea-surface temperature. Although the range of these dynamic variables differed across the two tracking periods, birds at both colonies foraged in areas with higher sea-level height anomaly than available, indicative of eddy circulation and potential nutrient entrainment (Hyrenbach et al. 2006). Sea-surface temperature was also an important driver of habitat selection at both colonies, with birds at Nelson’s Island selecting for waters between 27.5 and 28°C, and birds at Danger Island selecting warmer waters >29.5°C (Fig 4). Both of these temperature ranges correspond with favourable feeding habitats of skipjack tuna (Druon et al. 2017), so could suggest preference for facultative feeding opportunities, although further investigation into foraging modality is necessary (Dunn et al. In review). At both colonies, individuals foraged over steeper slopes, away from the central Great Chagos Bank, similar to brown boobies in the Great Barrier Reef that also forage on the reef edge (Miller et al. 2018). Bathymetric slopes are associated with greater productivity as currents and topography can drive prey closer to the surface (Piatt et al. 2006; Reisinger et al. 2018), thereby attracting foraging seabirds (Piatt et al. 2006; McDuie et al. 2018; De Pascalis et al. 2020).
There were significant differences in the proportion of time spent in foraging and transiting states between the colonies. Differences may be explained by trip distances (Fig. S3); birds on Danger Island travelled marginally greater distances than birds from Nelson’s Island, and therefore may have incurred greater transit distances (Oppel et al. 2015). Alternatively, birds at Nelson’s Island experienced greater wind speeds (Fig 4), potentially facilitating lower transit times. Female brown boobies sometimes travel further than males during foraging trips (Lewis et al. 2005; Weimerskirch et al. 2009; Soanes et al. 2015; Bunce 2015; Miller et al. 2018), although the reverse may also occur (Austin et al. 2021). Sample sizes here precluded formal testing, although we note no obvious sex differences in descriptive statistics (Fig S 5).
Conservation implications
Foraging effort and at-sea distribution models are both used to help designate MPAs for seabirds (Lascelles et al. 2012; Le Corre et al. 2012). Our results demonstrate the value of the Chagos Archipelago large scale MPA for encompassing the foraging area and habitat preferences of brown boobies during breeding. This is encouraging for seabirds that appear to be increasing following the long-term impacts of invasive rats and vegetation (Carr et al. 2021a, b) Nevertheless, distribution of brown boobies within the Chagos Archipelago MPA does not imply complete protection of foraging resources, as illegal fishing remains a concern within the MPA (Collins et al. 2021).
Study limitations
Here, we attempted to understand variability in foraging strategies of two colonies of brown boobies in the Chagos Archipelago. As outlined previously, limited access to field sites meant that individuals between the two colonies were tracked during different years, monsoon periods, and breeding stages, and that sample sizes of both sexes were relatively low. Whilst similarities in foraging strategies are therefore noteworthy, future studies in the region could focus on filling these data gaps and providing greater understanding of intra-colony variability in this species.
Because of low sample sizes, at-sea distributions (Fig. 2) may under-represent true population kernel densities (Soanes et al. 2013). Despite the limited number of individuals with retrieved tracking data, incorporating multiple trips per individual undoubtedly improves estimation of area use (Soanes et al. 2013). Understanding the proportion of general and core foraging areas represented by tracked individuals is dependent on multiple factors, including year, and sex. Therefore, drawing conclusions on the effectiveness of the Chagos Archipelago MPA surrounding Danger Island and Nelson’s Island using brown boobies could be assessed in future studies with a particular emphasis on sample sizes.