* Red-footed Booby (Sula sula)

rfbooby.jpg
Red-footed Booby. Photo: carla kishinami



SUMMARY INFORMATION

GENERAL ECOLOGY:

The Red-footed Booby is the most pelagic [7, 16] and smallest of the booby species [31, 35]. It has a pantropical distribution [19, 31] and breeds primarily on coral atolls and volcanic islands [35]. Frequently found breeding [6, 10, 35, 47, 48] and feeding [5, 8, 35] together with Masked S. dactylatra, Brown S. leucogaster Boobies and frigatebirds Fregata spp. throughout its breeding range. Colonies can vary greatly in size, ranging from tens to >100,000 pairs [9, 29]. It is primarily an arboreal nester, preferring to nest >1 m above the ground in shrubs and trees (e.g. Avicennia, Bruguiera, Cordia, Ficus, Pandanus, Rhizophora, Sauriana, Scaevola, Tournefortia) [10, 11, 29, 35, 39, 47], but will use almost any plant, readily nesting on grass if no bushes are present [35]. Occasionally will nest on man-made structures [31, 34, 35]. Does not appear to seek shade when selecting nest site, with most nests placed near tops of trees/shrubs or on edges of large trees [35, 40], but in some locations most nests were built on the leeward side of trees [11]. Nests are highly variable in size and construction, but most are circular, with rim around the edge, and cup/depression in middle for the egg, and are composed of larger sticks on bottom with smaller twigs on top [10, 29, 35]. The cup/nest bowl is frequently lined with grasses or other leafy matter [29, 35, 39]. Most colonies generally have a peak breeding season, but the season is often very extended [10, 23, 30, 47] and it is not uncommon for breeding to occur year round [3, 4, 30, 35, 36, 48]. Depending on the colony and temporal foraging conditions around the colony [31], chicks return to the nest for >1-4 months post-fledging, where adults continue to feed them [15, 30, 31, 35, 40]. During this time fledglings frequently will congregate into “gangs” of 5 to >100 individuals [29, 31] as they improve their ability to fly and forage for themselves [15, 31, 35]. Juveniles rarely leave to forage solely in the company of other juveniles, but almost always leave in the company of an adult [31]. By 10-12 months of age, most fledglings leave the colony [35]. Migration and non-breeding range are poorly understood, as little is known about movements outside of the breeding season and there is limited at-sea observations and banding data to determine this, although feeding flocks have been observed hundreds of miles from land [35]. Year round breeding [3, 4, 30, 35, 36, 48] and adults roosting at colonies year round [4, 10, 23, 47] further confuses the non-breeding movements and/or migrations of Red-footed Boobies. In the Pacific there is much movement between islands in the Hawaiian chain and banding records support a possible “migratory” movement between Johnston and Wake Islands to Kure Atoll [47]. There is some evidence that fledglings and adults disperse to different areas: adults are believed to remain close to breeding colonies [17], sometimes coming to roost [11], and juveniles, which may be more nomadic than adults [17, 47], are not found on many breeding islands [36]. Most birds found on non-natal islands are immature [35]. Will forage individually [31, 35], but also watches for other birds feeding and converges to form multi-species feeding flocks [1, 5, 31, 35], usually over schools of predatory fish and/or cetaceans [5, 20, 23, 31, 35, 42]. When deep water habitat is located close to land, will feed close to shore [2, 11, 24, 35], but usually forages >20 km from land [35]. Throughout its range, flying fish (Exocetidae) and squid (Ommastrphidae) are the most important prey items [3, 11, 12, 18, 36, 37, 46, 48].

FORAGING RANGE: Max 250 km, mean max 115 km, mean 71 km.

FORAGING DEPTH: Max 9.7 m, mean 1.63 m.

FORAGING TRIP DURATION: Mean 12.8 h (n=25).

KEY HABITATS: Generally deep oceanic waters with elevated productivity, probably near oceanographic features that enrich local waters like upwellings and convergences.

KEY PREY ITEMS: Flying fish (Exocetidae) and squid (Ommastrphidae).

KEY FORAGING ASSOCIATIONS: Often forages in mixed-species foraging flocks especially shearwaters, Sooty Terns Sterna fuscata, frigatebirds Fregata spp., Brown Noddies Anous stolidus, and Brown and Masked Boobies. Forage flocks almost always over schools of predatory fish and/or cetaceans that make smaller fish available at the water surface.

DETAILED INFORMATION

General feeding behaviour
Previous studies have suggested that Red-footed Boobies forage during both the day and night [31, 35, 38] but more recent studies using time depth and activity recorders indicate that they are strictly diurnal foragers [24, 26-28, 43, 44], leaving early in the morning and returning to the colony before or after dusk [40]. Feeds primarily by plunge diving vertically into the water [1], typically froPrevious studies have suggested that Red-footed Boobies forage during both the day and night [31, 35, 38] but more recent studies using time depth and activity recorders indicate that they are strictly diurnal foragers [24, 26-28, 44, 45], leaving early in the morning and returning to the colony before or after dusk [41]. Feeds primarily by plunge diving vertically into the water [1], typically from 4-8 m in the air [4, 11, 35], foraging within the top few meters of the water column [25, 26, 45]. Occasionally will make oblique, shallow surface dives, or sit on the water to grab prey items near the surface [11] and catch flying fish in the air [1, 4, 11, 41]. Typically forages out of site of land over deep oceanic waters [2, 24, 31, 35], but where deep water habitat is close will forage within site of land [2, 11, 24, 35]. Will forage individually finding fish on its own [31, 35], but typically is a collective forager and watches other birds of any species feeding and converges to form mixed-species feeding flocks [1, 5, 20, 31, 35]. Flocks are usually associated with schools of tuna and/or cetaceans [5, 23, 31, 35, 42], and can number into the hundreds [5, 35].

Power required for flight has been documented at one-third of predicted values [7], most likely due to complex set of behaviors that serve to take advantage of wind energy and substantially lower the their energetic cost of flight [7, 41, 44, 45].

Foraging habitat - breeding season

Best described as deep water feeders versus “pelagic” feeders with its implication to distance from land [11]. At islands where the deep water habitats are located close to land will forage within site of land [2, 11, 24, 35].

In the equatorial/southern tropical/subtropical (<5°N to 12°S; ESTS) region of the eastern Pacific boobies are typically found near islands and continental coasts. Boobies composed 42% of flocks in the warm (>25°C) low salinity (<34‰) waters of the northern tropical Pacific (5°N to 30°N and east of 125°W; NT) and 50% of flocks in the equatorial/southern tropical/subtropical (<5°N to 12°S; ESTS) region [5]. Red-footed Boobies at-sea in the eastern tropical Pacific were associated with surface waters >24°C that had surface salinities and chlorophyll values near 33‰ and 0.4 mg/m3, respectively [42]. Another study found that booby dominated foraging flocks in the eastern Pacific were seen more frequently over waters with a shallow thermocline (62.5 ± 1.4 m) and high chlorophyll (0.17 ± 0.01 mg/m3) concentrations, which are indicative of enhanced productivity [8]. During the summer, in the central north Pacific, boobies were associated with waters that had high mean temperatures (24.0°C, range 15.2-26.2°C) and salinities (34.9‰, range 33.9-35.8‰) when compared to other waters in the region [43]. Sea surface temperatures ranged between 25.5 and 29.0°C and chlorophyll a ranged from 0.1 to 0.2 mg/m3, and primary productivity was low compared to other Pacific waters during a study of Red-footed Booby foraging behavior on Palmyra Atoll, Overseas Territory, USA [48].

In the Indian Ocean Red-footed Boobies were seen predominately over low-salinity waters, although they were not exclusively found here [33]. Red-footed Boobies on Europa Island, French Overseas Territory, in the Mozambique Channel, take advantage of cool waters (relative to surrounding water masses), and presumably of a regional increase in primary productivity and tuna abundance, in relation to a seasonal shift of subtropical waters northward by timing their breeding season when sea-surface temperatures are at their lowest (~23.7°C), and the distance to the 23°C isotherm is at its least (<200 km) [23]. In 2003 boobies were found primarily to the west of Europa and south of Bassas da India, French Overseas Territory, almost exclusively where chlorophyll concentrations were 0.17 mg/m3, which was relatively high for the study period and area. Foraging was associated with sea surface temperatures <23.5°C and thermocline depths between 110 m and 130 m. Flying fish were abundant in these areas and pods of cetaceans were also present, suggesting that Red-footed Boobies search for the most productive waters to forage in [20]. Also in 2003, satellite telemetry data of Red-footed Booby foraging flights from Europa, found boobies foraged over deep and productive oceanic waters, preferentially going to zones of elevated primary productivity, south and north of Europa during the first half of the study and west of Europa during the second half of the study, where primary productivity was enhanced by a persistent field of anticyclonic gyres [24, 44].

In 1987 at-sea surveys in the Easter Indian Ocean, off the west and northwest coast of Australia coast, found Red-footed Boobies, were confined to the Christmas Island, Australia region, where they were abundant but patchily distributed. Boobies were most frequently observed over warm (26°C-27°C), low-salinity (34.3-34.5) waters of the South Equatorial Current; the southern boundary of which they encountered on their course between 13°40’S and 12°00’S and the northern boundary at 10°20’S [14].

Foraging habitat - migration and wintering period

At many breeding stations, Red-footed Boobies breed year round [3, 4, 30, 35, 36, 48] and at some colonies adults roost throughout the year [4, 23]. As such, foraging habitats for birds that are not actively breeding are likely similar to those used during breeding.

Important foraging associations

The Red-footed Booby is an important species in the formation of multi-species foraging flocks [1], most of which occur over schools of predatory fish and/or cetaceans [5, 20, 23, 31, 35, 42]. In the eastern Pacific Ocean, nearly 60% of all Spotted Dolphin Stenella attenuata schools and just over 50% of all Spinner Dolphin S. longirostris schools were associated with multi-species foraging flocks. These two dolphins frequently forage together and 96% of schools with both species were attended by multi-species foraging flocks, such that as the schools got bigger so did the associated flocks and as the flock size increased the number of individual birds of the different species increased. Red-footed and Masked Boobies were primarily responsible for the increase in Sula spp. Thirty-one percent of Common Dolphin schools Delphinu delphis were seen with flocks. Boobies composed 42% of flocks in the warm (>25°C) low salinity (<34‰) waters of the northern tropical Pacific (5°N to 30°N and east of 125°W; NT) and 50% of flocks in the from the equatorial/southern tropical/subtropical (<5°N to 12°S; ESTS) region. In the eastern portion of the NT Red-footed Boobies were positively associated with Wedge-tailed Shearwaters Puffinus pacificus and jaegers (Stercorariidae) in mixed-species foraging flocks and negatively associated with White-necked Pterodroma externa cervicalis/Juan Fernandez P. externa/ Dark-rumped P. phaeopygia Petrels. In the ESTS waters they were negatively associated with Sooty Terns. Flocks were much more numerous in the NT than the ESTS, where most schools of single or mixed dolphins were without flocks. This likely due to fact in the eastern NT Pacific, dolphins and birds are probably foraging in close association with surface-schooling Yellowfin Tuna Thunnus albacares, which is prevalent in this region [5]. Another study in the eastern tropical Pacific found several distinct types of foraging flocks, one of which was “Booby flocks” which were composed primarily of Red-footed and Masked Boobies, of which 24% were associated with marine mammals. Other important species in these flocks were Brown and Black Noddy A. minutus, Brown and Blue Footed Booby S. nebouxii, frigatebirds Fregata spp. and White Terns Gygis alba [8].

Sooty Terns acted as catalyst for foraging flocks in the Mozambique Channel, representing a good target for other foraging species, including Red-footed Boobies, during the 2003 breeding season. Frigatebirds favored flocks of Red-footed Boobies both of who were frequently in association with other marine predators [20]. Near Aldabra Atoll, Seychelles frequently seen foraging with frigatebirds and terns [11].

Diet

Red-footed Booby diets on Christmas Island, Republic of Kiribati (Pacific Ocean) in 1967 were composed exclusively of fish (75%) and squid (25%). Ninety-one percent of the fish were flying fish and the remaining 9% equally divided between Coryphaena sp. and Xanthichthys sp., both of which are found in more pelagic waters than the more inshore Arothron meleagris [37]. On Oahu, Hawaii, USA, booby diets contained 61% fish (Exocoetidae 38%, Gempylidae 9%, Coryphaenidae 5%, 10% unidentified) and 39% squid (Ommastraphidae) [3], and the main size classes of fish were between 8 and 20 cm, with half falling in the 8-12 cm range [4]. Diets samples collected from Red-footed Booby adults, juveniles, and chicks from 1978-1980, throughout the Northwest Hawaiian Island chain, revealed that 72% of the volume of the diet was fish and the rest squid. Fish came from 27 different families, squid from one family, and a lone copepod and insect were also recorded. Flying fishes were most important, then squid, and Carangidae, most of which were Decapterus sp. Diets varied temporally and spatially across the chain [18]. Diets of Red-footed Boobies on Palmyra Atoll, administered possession of the USA did not differ between sexes and by mass, were almost exclusively flying fish Exocoetidae (94%; 92%) with the remainder composed of squid, mostly Ommastrephidae. The average length of fish prey was 20.6 ± 4.0 cm, and the average mass was 134 ± 91 g [48].

In 2003 the diet of Red-footed Boobies breeding on Europa was predominately flying fishes (Exocetidae) and squid (Ommastrephidae). Males ate 58.6% Ommastrephidae and unidentified squid, 19.8% Exocetidae, 14.7% Hemiramphidae, and 6.9% unidentified fish, while females ate 54.8% Ommastrephidae and unidentified squid, 25% Exocetidae, 6.8% Hemiramphidae, 1.3% Carangidae, and 12.2% unidentified fish. The size and mass of prey items did not differ significantly between the sexes [46]. In 2005 brooding Red-footed Boobies at Tromelin Island, French Overseas Territory, Indian Ocean, consumed 55% fish and 45% squid, by mass. Ommastrephidae were the top prey item (53%) followed by Exocetidae (23%), unidentified squid (12%), unidentified fish (7%), and a mix of other fish families 4%. Prey items averaged 110.3 ± 57.7 mm [21]. At Aldabra Atoll, Red-footed Booby diets overlapped considerably with Greater Frigatebirds F. minor [12]. The main prey items by weight were flying fish (Exocoetidae). During the dry season (April-Oct) flying fish composed 98% of the diet and the rest was made up of flying squid (Ommastrephidae). In the wet season (Nov-March) flying fish composed 76% of the diet and squid 21% [11]. By number flying fish accounted for 72%, Ommastrephidae 25%, and Hemirhamphidae 3% [12].

Foraging range

Efficient flight behaviors [7, 45] allow Red-footed Boobies to forage far from their colonies. Typically regarded as long distance foragers with foraging trips averaging < 200 km but reaching up to a maximum of 300-340 km [31, 32], although at-sea observations in the Eastern Indian Ocean have recorded boobies between 800-840 km from the nearest breeding colony at Christmas Island [13]. Red-footed Boobies nesting on Johnston Atoll, administered by the USA, have been known to range over 250 km from the colony, yet adults return to feed their chicks everyday [7, 16].

Even though females are, larger and heavier than males [25, 27, 28, 37, 46] foraging trip duration of breeding Red-footed Boobies on Johnston Atoll did not differ between sexes in 2003 and ranged between 8 and 20 hours [25, 26]. At Palmyra Atoll booby foraging trips lasted on average 8.67 ± 5.30 h and birds foraged on average 67.52 ± 44.17 km from their breeding location, with a maximum distance of 112 km [48]. Juvenile Red-footed Boobies nesting on Dong Island, China had significantly shorter foraging trips 8.3 ± 2.5 h (n = 18) than their parents 11.0 ± 0.7 h (n = 18), shortly before they were fully independent [15].

Fifty percent of at-sea observations of Red-footed Boobies in the western Indian Ocean, were within 417 km of land and 90% were within 555 km of land [6]. Red-footed Boobies on Europa were strictly diurnal foragers [24, 27, 28] and never spent the night on the water, returning to land every night [24, 44, 45]. As a result, they could only forage for a maximum of roughly 12-13 h, due to the length of daylight [28, 44, 45]. Like Johnston Atoll, during the 1998 breeding season, the duration of booby foraging trips did not differ between sexes on Europa Island and averaged 11.2 ± 0.48 h during incubation and 7.5 ± 0.50 h after hatching [27]. Ship based surveys in the Mozambique Channel during the 2003 breeding season did not find Red-footed Boobies further than 160 km Europa [20]. In 2003 foraging trips during incubation averaged 67 ± 34 km (range: 12-148 km) from their colony and lasted up to 12 hours (mean: 10 ± 2 h) [24, 44, 45]. Male foraging trips lasted 10.7 ± 1.9 h (range: 5.8-12.2; n = 14) while females trips lasted 9.5 ± 2.9 h (range: 4.7-12.5 h; n = 30). Males traveled 50.2 ± 30.8 km (range: 22-111 km; n = 16) while females traveled significantly further than males: 85.3 ± 26.3 km (range: 54-148 km; n = 19) [46]. During the brooding period foraging trips decreased to 5.0 ± 2.0 h [44] (males: 5.0 ± 2.0 h, range: 1.2-7.2 h, n = 13; females: 4.0 ± 2.0 h, range: 1.1-7.3 h, n = 7) and birds did not travel as far (39.2 ± 28.5 km, range: 10-114 km) [44] and there were no differences between sexes (males: 40.1 ± 29.5 km, range: 9.8-87.4 km, n = 10; females: 30.5 ± 32.7 km, range: 20-114.4 km, n = 7) [46]. Based on average flight speed and length of foraging trips, the potential maximum range for boobies from Europa would be 237 km, but this would not include time to forage. Including the average time boobies spent on the water during a trip the maximum theoretical range would be 158 km, very close to the observed 148 km [44]. In 2005, boobies brooding downy chicks on Tromelin Island spent on average of 4.29 ± 2.45 h (0.69 – 5.06 h) on foraging trips, and travelled an average maximal range of 31.0 ± 21.7 km (7.59 – 71.6 km) [21]. Birds brooding chicks on Christmas Island, territory of Australia, Indian Ocean, had foraging trips that lasted an average of 12 h up to a maximum of 36 h. On average they foraged 155 km from the colony, and had a maximum distance between 200-250 km (J. Hennicke unpublished data).

Red-footedBoobyForagingTracks.jpg
Figure 1. Foraging trips of Red-footed Boobies tracked with GPS loggers during the brooding period at Tromelin Island (black star). Area restricted search zones are indicated by black circles. Source: Kappes et al. in press.

Red-footedBoobyTable.jpg
Table 1. Showing average value (km) of foraging range in different country/sea areas.


Red-footedBoobyForagingRange.jpg
Figure 2. Showing cumulative frequency (with standard deviation) and proportion of birds found foraging at different distances from the colony. Source: BirdLife Seabird Foraging Range Database.

Diving and depth association

Best described as deep water feeders versus “pelagic” feeders with its implication to distance from land [11]. At islands where the deep water habitats are located close to land will forage within site of land [2, 11, 24, 35].

Red-footed Boobies breeding on Johnston Atoll, USA in 2003 dove to a maximum of 6 meters below the surface when foraging [26], but dives for both sexes averaged only 0.8 m below the surface [25].

In 1994 a study using maximum depth recorders found Red-footed Boobies incubating eggs on Europa Island dove to an average of 4.9 ± 2.0 m (n = 22) and reached a maximum depth of 9.6 m [22]. A subsequent study on Europa, using time-depth recorders, found boobies dove to a maximum depth of 2.4 m, but on average dives were 0.87 ± 0.52 m and lasted between 1 to 8 s (mean: 1.67 ± 0.84 s) [45]. Maximum depths attained during dives were not significantly different between sexes with males averaging 1.9 ± 0.2 m (range: 1.6-2.4 m; n = 5) and females averaging 2.0 ± 0.11 m (range: 1.4-2.2 m; n = 8) [46]. Birds nesting on Christmas Island in the Indian Ocean dove on average to 0.7 m and had a maximum diving depth of 5.4 m (J. Hennicke unpublished data).

Red-footedBoobyDiveDepth.jpg
Figure 3. Showing cumulative frequency (with standard deviation) and proportion of birds found within foraging depths around the colony. Source: BirdLife Seabird Foraging Range Database.

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