Animal Diversity Web

Di­ver­sity

There are six species of birds known as boo­bies in the genus Sula. These medium to large seabirds are found in ma­rine and coastal habi­tats in­clud­ing is­lands, coastal areas, and tem­per­ate, pan-trop­i­cal oceans (Lee and Haney, 1984). Boo­bies vary in size (1000 to 2500 grams), but gen­er­ally have long wings, a wedge-shaped tail, short and far-set legs, toti­palmate feet (four toes with web­bing), and a con­i­cal bill with a de­curved tip (Diaz, Al­fredo and Salazar Gomez, 2011). The species within Sula ex­hibit sex­ual di­mor­phism with fe­males often larger than males (Cor­nell Uni­ver­sity, 2015). Al­though phys­i­cally sim­i­lar, dif­fer­ent species of boo­bies ex­hibit unique be­hav­iors and other dif­fer­ences in their ecol­ogy. Plumage and bill color varies be­tween species, with some hav­ing brightly-col­ored feet used for mat­ing rit­u­als. Boo­bies feed pri­mar­ily on shoal­ing fish and use a stream­lined, pow­er­ful, head­first dive from up to 25 me­ters in the air into the water to catch prey (Na­tional Ge­o­graphic, 2018). They fold their long wings back to fur­ther stream­line the body for a more ef­fi­cient head­first dive (Diaz, Al­fredo and Salazar Gomez, 2011). Species in Sula form pairs dur­ing breed­ing sea­son through rit­u­al­ized mat­ing per­for­mances that show off their webbed feet, al­though in­fi­delity fre­quently oc­curs through­out the large nest­ing colonies that are formed (Nel­son, 1978). ("Blue-footed Booby", 2015; Diaz, et al., 2011; Lee and Haney, 1984; "Red-footed Booby", 2018; Nel­son, 1978)

Ge­o­graphic Range

Sula species live in tem­per­ate, pantrop­i­cal wa­ters across the world. Al­though non­mi­gra­tory, boo­bies are widely dis­trib­uted across ma­rine and coastal areas in the Gulf of Cal­i­for­nia, through­out the Caribbean Sea, along the Pa­cific coast to north­ern Peru, and west to the Gala­pa­gos Is­lands (Nel­son, 1978). The species of Sula are also found on small oceanic is­lands sur­rounded by tem­per­ate wa­ters in seas north of Aus­tralia and through­out the In­dian, Pa­cific, and At­lantic Oceans (Lee and Haney, 1984). On rare oc­ca­sions, blue-footed boo­bies (Sula nebouxii), masked boo­bies (Sula dacty­la­tra), brown boo­bies (Sula leuco­gaster), and red-footed boo­bies (Sula sula) have been ob­served off the Pa­cific and At­lantic coasts of the United States (Florida and Cal­i­for­nia) (Sib­ley, 2014). (Diaz, et al., 2011; Lee and Haney, 1984; Nel­son, 1978; Sib­ley, 2014)

• Biogeographic Regions
• nearctic
  • native
• neotropical
  • native
• australian
  • native
• oceanic islands
  • native
• indian ocean
  • native
• pacific ocean
  • native

Habi­tat

The ma­jor­ity of Sula species are soli­tary ma­rine for­agers that re­turn from sea and roost on coral atolls, vol­canic is­lands, coasts, and oceanic is­lands (Nel­son, 1978). Blue-footed boo­bies (Sula nebouxii) and red-footed boo­bies (Sula sula) are oc­ca­sion­ally seen for­ag­ing in small groups off of rocky islets and shore­lines near roosts (Cor­nell Uni­ver­sity, 2015). Each species has a unique habi­tat pref­er­ence for nest­ing. The ma­jor­ity of species, ex­clud­ing red-footed boo­bies, nest on open, min­i­mally shaded ground. Red-footed boo­bies are the only ar­bo­real nesters (Nel­son, 1969). Dif­fer­ent Sula species can be found roost­ing in the same areas al­though ter­ri­to­r­ial ag­gres­sion be­tween dif­fer­ent species oc­curs, re­sult­ing in dif­fer­ences in nest site se­lec­tion. For ex­am­ple, Blue-footed Boo­bies in the Gala­pa­gos Is­lands nest at coastal sites if Nazca boo­bies (Sula granti) are not pre­sent, but blue-footed boo­bies move fur­ther away from ideal nest­ing habi­tat when Nazca boo­bies are pre­sent (Townsend et al, 2002). ("Blue-footed Booby", 2015; Nel­son, 1969; Nel­son, 1978; Townsend, et al., 2002)

• Habitat Regions
• temperate
• tropical
• saltwater or marine

• Aquatic Biomes
• pelagic
• coastal

Sys­tem­atic and Tax­o­nomic His­tory

Sula was first named and pub­lished in lit­er­a­ture by Math­urin-Jacques Bris­son in 1760 ("Sula Bris­son, 1760", 2017). The tax­on­omy of boo­bies and gan­nets has been a source of dis­agree­ment among sci­en­tists since the early 1900s. Species in Sula are closely re­lated to gan­nets in Morus. A seg­ment of the mi­to­chon­dr­ial cy­tochromeb gene was se­quenced for all liv­ing Sulids, and the par­si­mony analy­ses placed gan­nets and boo­bies in sep­a­rate, mono­phyletic lin­eages (Friesen and An­der­son, 1997). Orig­i­nally, Sula and Morus made up the Sul­i­dae fam­ily (Diaz, Al­fredo and Salazar Gomez, 2011). Dorst and Mou­gin placed all species in Sula in 1930 and were com­bined back and forth until 1990 (Chaves-Cam­pos and Tor­res, 2002). In 1988, sci­en­tists pro­vided skele­tal ev­i­dence to sep­a­rate the gan­nets and boo­bies back into Sula and Morus (Diaz, Al­fredo and Salazar Gomez, 2011). Fur­ther mor­pho­log­i­cal ev­i­dence el­e­vated Ab­bott’s booby from Sula into their own genus, Pa­pa­sula (Chaves-Cam­pos and Tor­res, 2002). ("Sula Bris­son, 1760", 2017; Chaves-Cam­pos and Tor­res, 2002; Diaz, et al., 2011; Friesen and An­der­son, 1997)

Phys­i­cal De­scrip­tion

Sula species are medium to large birds (800 to 2400 g; 70 to 85 cm long) with wingspans up to five feet long (37.4 to 170 cm) (Nel­son, 1978). Boo­bies are sex­u­ally di­mor­phic, with fe­males gen­er­ally being larger and weigh­ing more than males. Only one species, the red-footed booby (Sula sula) ex­hibits color poly­mor­phism, with up to five dif­fer­ent color morphs (brown, white, black-tailed white, white-tailed brown, and white-headed brown) (Sib­ley, 2014). Sula species have a va­ri­ety of plumage col­ors rang­ing be­tween white, brown and black. Ju­ve­niles tend to have darker brown and black plumage but, as hatch­lings they are cov­ered in white down feath­ers. The often col­ored (red, blue, gray, yel­low, black) toti­palmate feet (four webbed toes) of Sula species are set far back on the body. Boo­bies have desmog­nathous palates with no ex­posed out­side nares, and nasal grooves run­ning down the stout, con­i­cal bill (Nel­son, 1978). Adult boo­bies have bare fa­cial and gular skin (Sib­ley, 2014). The feath­er­less skin, bill, and feet are col­ored dif­fer­ently based on the species, which can be red, blue, gray, yel­low, or black (Sib­ley, 2014). Boo­bies have binoc­u­lar vi­sion be­cause each eye is sit­u­ated on ei­ther side of the bill. The largest Sula species is the masked booby (Sula dacty­la­tra), with males rang­ing be­tween 1220 and 2211 grams and fe­males rang­ing be­tween 1470 and 2400 grams ("Sula dacty­la­tra (Masked Booby)", 2016). Red-footed boo­bies (Sula sula) are the small­est Sula species, with males be­tween 800 and 1160 grams and fe­males be­tween 850 and 1210 grams (Sib­ley, 2014 and Nel­son, 1978). Boo­bies are most closely re­lated to gan­nets (Morus), but have key dif­fer­ences be­tween them. Gan­nets are monomor­phic in size, are mi­gra­tory, and have feath­ers ex­tend­ing for­ward of the eyes, while boo­bies are sex­u­ally di­mor­phic, seden­tary, and have bare fa­cial skin for­ward from the eyes. ("Sula dacty­la­tra (Masked Booby)", 2016; Nel­son, 1978; Sib­ley, 2014)

• Other Physical Features
• endothermic
• bilateral symmetry
• polymorphic

• Sexual Dimorphism
• female larger

Re­pro­duc­tion

Sula species are monog­a­mous, but oc­ca­sion­ally par­take in in­fi­delity when their mate is out for­ag­ing for food on the ocean (Nel­son, 1978). As breed­ing sea­son ap­proaches, male boo­bies de­fend a ter­ri­tory and once a fe­male pairs with a male, they jointly choose a breed­ing site to de­fend within the male’s ter­ri­tory (Nel­son, 1978). Sula males and fe­males per­form a com­plex dis­play to find, at­tract, and bond with a mate. Sula courtship dis­plays in­volve ex­ag­ger­ated rais­ing and flaunt­ing of feet to their part­ner called “parad­ing” (Nel­son, 1978). To fe­males, the color of the feet is a good in­di­ca­tor of the male’s phys­i­cal and re­pro­duc­tive health. The brighter the feet are, the health­ier the mate is. Breed­ing pairs typ­i­cally prac­tice this courtship dis­play more than once over the breed­ing sea­son to pro­tect nest­ing ter­ri­tory and so­lid­ify their bond (Diaz, Al­fredo and Salazar Gomez, 2011). Both fe­male and male boo­bies prac­tice ex­tra-pair cop­u­la­tion dur­ing fer­tile pe­ri­ods. (Diaz, et al., 2011; Nel­son, 1978)

• Mating System
• monogamous

Boo­bies nest in colonies and their breed­ing sea­son can last up to 40 weeks (Nel­son, 1978). Breed­ing sea­son is di­vided into four main stages: pre-lay­ing, in­cu­ba­tion, chick care and post breed­ing (Nel­son, 1978). Pre-lay­ing is com­prised of de­fend­ing nest ter­ri­to­ries and es­tab­lish­ing ad­e­quate bond­ing pair re­la­tion­ships (Nel­son, 1978). Clutch sizes range from 1 to 3 eggs (blue to white in color) (Nel­son, 1978). Most boo­bies nest on the ground, ex­clud­ing red-footed boo­bies, who nest in trees (Nel­son, 1969). The in­cu­ba­tion pe­riod lasts around 50 days, with both males and fe­males al­ter­nat­ing egg in­cu­ba­tion (Nel­son, 1978). Boo­bies do not have brood­ing patches and use their toti­palmate feet to trans­fer heat to the eggs. g. Sula males that doubt the pa­ter­nity of their egg are known to de­stroy all of the eggs in the nest (Diaz, Al­fredo and Salazar Gomez, 2011).. g. Sula hatch­lings are feath­er­less and grad­u­ally gain white down feath­ers (Nel­son, 1978). As ju­ve­niles, the plumage is dark brown to black (Nel­son, 1978). Hatch­lings are al­tri­cial and re­quire male and fe­male care for an av­er­age of 100 days, dur­ing which hatch­lings are com­pletely de­pen­dent on their par­ents for food and pro­tec­tion (Nel­son, 1978). Both par­ents for­age for food in the ocean and re­gur­gi­tate semi-di­gested food into the hatch­ling’s mouth. Nestlings are vul­ner­a­ble to preda­tors like hawks and short-eared owls dur­ing the day when both par­ents are out for­ag­ing (Mellink, Dominguez and Lue­vano, 2000). In the Gala­pa­gos, adult Nazca boo­bies at­tack and in­jure blue-footed booby nestlings to pre­vent suc­cess­ful fledg­ing (Townsend et al, 2002). (Diaz, et al., 2011; Mellink, et al., 2000; Nel­son, 1969; Nel­son, 1978; Townsend, et al., 2002)

• Key Reproductive Features
• iteroparous
• seasonal breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual
• fertilization
• oviparous

Males and fe­males take turns in­cu­bat­ing the eggs by trans­fer­ring heat from their toti­palmate feet (Diaz, Al­fredo, and Salazar Gomez, 2011). Each par­ent guards the nest through the night if the coun­ter­part is out at sea for­ag­ing. Once the egg hatches, each par­ent for­ages for food for the nestlings (Nel­son, 1978). The nestlings are under parental care for ap­prox­i­mately 100 days, until they are ma­ture enough to for­age for them­selves (Nel­son, 1978). (Diaz, et al., 2011; Nel­son, 1978)

• Parental Investment
• altricial
• male parental care
• female parental care
• pre-fertilization
  • protecting
    • male
    • female
• pre-hatching/birth
  • provisioning
    • male
    • female
  • protecting
    • male
    • female
• pre-weaning/fledging
  • provisioning
    • male
    • female
  • protecting
    • male
    • female
• pre-independence
  • provisioning
    • male
    • female
  • protecting
    • male
    • female

Lifes­pan/Longevity

The typ­i­cal lifes­pan of boo­bies is 17 years to a max­i­mum 30 years (Nel­son, 1978). The old­est banded brown booby recorded was 27.2 years old (Longevity Records of North Amer­i­can Birds, 2017). Adult mor­tal­ity rate for boo­bies is 5-10% (Nel­son, 1978). Fac­tors that limit their lifes­pan in­clude pre­da­tion on land and sea, pres­sures from com­mer­cial fish­ing, coastal de­vel­op­ment, and sub­se­quent habi­tat de­struc­tion (Na­tional Ge­o­graphic, 2018). ("Red-footed Booby", 2018; Nel­son, 1978; "Longevity Records of North Amer­i­can Birds", 2017)

Be­hav­ior

Boo­bies are so­cial species that live and breed in colonies. Boo­bies are pis­ci­vores that for­age soli­tar­ily, in small groups, or in large groups, de­pend­ing on the species. Sula can dive up to 25 me­ters deep in the ocean for fish (Cor­nell Uni­ver­sity, 2015). Boo­bies pair up dur­ing mat­ing sea­son and per­form elab­o­rate rit­u­als show­cas­ing the feet, jab­bing bills, and nod­ding the head (Nel­son, 1978). Males of Sula de­fend small nest­ing areas and are ter­ri­to­r­ial dur­ing breed­ing sea­son (Diaz, Al­fredo and Salazar Gomez, 2011). Ag­o­nis­tic be­hav­ior oc­curs be­tween blue-footed booby sib­lings (Diaz, Al­fredo and Salazar Gomez, 2011). Larger off­spring will peck their smaller sib­lings, caus­ing par­ents to se­lec­tively feed the larger, more dom­i­nant (and healthy look­ing) off­spring. This can lead to star­va­tion and death of the smaller sib­ling (Diaz, Al­fredo and Salazar Gomez, 2011). Boo­bies use calls and body lan­guage to com­mu­ni­cate. ("Blue-footed Booby", 2015; Diaz, et al., 2011; Nel­son, 1978)

• Key Behaviors
• flies
• natatorial
• diurnal
• motile
• sedentary
• territorial
• colonial

Com­mu­ni­ca­tion and Per­cep­tion

Sula species are silent at sea. On breed­ing grounds, adult males whis­tle and bray, while fe­males and ju­ve­niles vo­cal­ize with a deep, duck-like note (Diaz, Al­fredo and Salazar Gomez, 2011). Po­ten­tial mates com­mu­ni­cate through elab­o­rate courtship dis­plays, char­ac­ter­ized by sky-point­ing with beaks, gaz­ing, mu­tual jab­bing, and parad­ing feet (Nel­son, 1978). Boo­bies ex­hibit ag­gres­sive be­hav­ior to es­tab­lish their ter­ri­tory. This in­cludes aer­ial at­tacks, wing-flail­ing, jab­bing bill to­ward their op­po­nent, and head bow­ing (Nel­son, 1969). Sula species do not ex­hibit so­cial or hi­er­ar­chi­cal rank­ing. (Diaz, et al., 2011; Nel­son, 1969; Nel­son, 1978)

• Communication Channels
• visual
• acoustic

• Other Communication Modes
• duets

• Perception Channels
• visual
• tactile
• acoustic
• chemical

Food Habits

Sula species are pis­ci­vores that feed on small, pelagic shoal­ing fishes like an­chovies (Ceten­graulis mys­tice­tyus and En­graulis mor­dax), sar­dines (Sardinops sagax), mack­erels, fly­ing fish (Ex­o­coeti­dae), and squid (Lolig­inidae) (Mellink, Dominguez and Lue­vano, 2000; and Diaz, Al­fredo, and Salazar Gomez, 2011). (Diaz, et al., 2011; Mellink, et al., 2000)

• Primary Diet
• carnivore
  • piscivore

Pre­da­tion

Sula species are preyed on by owls (St­rigi­formes), con­dors (Cathar­tidae), sharks, rats, hu­mans, cats, and gulls (Lar­i­dae) (Mellink, Dominguez and Lue­vano, 2000 and Nel­son, 1978). (Mellink, et al., 2000; Nel­son, 1978)

Ecosys­tem Roles

Sula species are prey to cats, rats, fish, and birds, and are preda­tors to shoal­ing fish like mack­erel, an­chovies, fly­ing fish, sar­dines, and squid (Mellink, Dominguez and Lue­vano, 2000). (Mellink, et al., 2000)

Eco­nomic Im­por­tance for Hu­mans: Pos­i­tive

Boo­bies are of eco­nomic im­por­tance to tourism in­dus­tries around the globe. For ex­am­ple, blue-footed boo­bies fre­quent the Gala­pa­gos and are of in­ter­est to tourists. Tourists also visit red-footed boo­bies that nest on Caribbean is­lands. Sci­en­tists are in­ter­ested in the ecol­ogy and ecosys­tem roles of boo­bies. These pelagic birds are in­di­ca­tor species of ocean and ecosys­tem health. Booby fecal mat­ter, or guano, has his­tor­i­cally been used as a crop fer­til­izer. The de­mand for guano has in­creased as a re­sult of the price in­crease of syn­thetic fer­til­iz­ers and up­ward trend fa­vor­ing or­ganic pro­duce (High­field, 2011). Guano is pri­mar­ily col­lected on is­lands off the coast of Peru, and is shipped as far as Eu­rope to fer­til­ize dif­fer­ent crops (High­field, 2011). (High­field, 2011)

• Positive Impacts
• ecotourism

Eco­nomic Im­por­tance for Hu­mans: Neg­a­tive

There are no known ad­verse ef­fects of Sula on hu­mans.

Con­ser­va­tion Sta­tus

Masked (Sula dacty­la­tra), blue-footed (Sula nebouxii), brown (Sula leuco­gaster), and red-footed boo­bies (Sula sula) are pro­tected species under the Mi­gra­tory Bird Treaty Act ("Mi­gra­tory Bird Treaty Act", 2013). The IUCN Red List clas­si­fies all species in Sula as species of least con­cern ("Sula dacty­la­tra (Masked Booby)", 2016 and "Sula var­ie­gata (Pe­ru­vian Booby)", 2016). The pop­u­la­tion num­bers of masked, brown, red-footed, and Nazca (Sula granti) boo­bies are de­creas­ing ac­cord­ing to the IUCN Red List. Ab­bott’s booby (Pa­pa­sula ab­botti), a species in the the sis­ter genus of Sula, Pa­pa­sula, is listed on CITES, but no Sula species are listed ("UNEP", 2018). De­clines in pop­u­la­tions are at­trib­uted to food scarcity caused by over­fish­ing, and coastal habi­tat frag­men­ta­tion and loss (Na­tional Ge­o­graphic, 2018). ("Mi­gra­tory Bird Treaty Act", 2013; "Sula dacty­la­tra (Masked Booby)", 2016; "Sula var­ie­gata (Pe­ru­vian Booby)", 2016; "UNEP", 2018; "Red-footed Booby", 2018)

• IUCN Red List [Link]

  Not Evaluated

Other Com­ments

Species in Sula are thought to get their com­mon name, “booby” from the Span­ish word “bobo”. The term trans­lates to “stu­pid” in Eng­lish and is thought to be rep­re­sen­ta­tive of the clumsy, goofy walk the booby ex­hibits on land (Na­tional Ge­o­graphic, 2018). ("Red-footed Booby", 2018)

Con­trib­u­tors

Makenna Spencer (au­thor), Col­orado State Uni­ver­sity, Tanya Dewey (ed­i­tor), Uni­ver­sity of Michi­gan-Ann Arbor.

Ref­er­ences

Cor­nell Uni­ver­sity. 2015. "Blue-footed Booby" (On-line). The Cor­nell Lab of Or­nithol­ogy All About Birds. Ac­cessed Jan­u­ary 31, 2018 at https://​www.​allaboutbirds.​org/​guide/​Blue-footed_​Booby/​id.

USGS. 2017. "Longevity Records of North Amer­i­can Birds" (On-line). Ac­cessed Feb­ru­ary 26, 2018 at https://​www.​pwrc.​usgs.​gov/​bbl/​longevity/​Longevity_​main.​cfm.

2013. "Mi­gra­tory Bird Treaty Act" (On-line). U.S. Fish and Wildlife Ser­vice. Ac­cessed Feb­ru­ary 28, 2018 at https://​www.​fws.​gov/​birds/​management/​managed-species/​migratory-bird-treaty-act-protected-species.​php.

Na­tional Ge­o­graphic. 2018. "Red-footed Booby" (On-line). Ac­cessed Feb­ru­ary 26, 2018 at https://​www.​nationalgeographic.​com/​animals/​birds/​r/​red-footed-booby/​.

2017. "Sula Bris­son, 1760" (On-line). In­te­grated Tax­o­nomic In­for­ma­tion Sys­tem (ITIS). Ac­cessed Feb­ru­ary 11, 2018 at https://​www.​itis.​gov/​servlet/​SingleRpt/​SingleRpt?​search_​topic=TSN&​anchorLocation=SubordinateTaxa&​credibilitySort=Subordinate%20Taxa&​rankName=Subspecies&​search_​value=174697&​print_​version=SCR&​source=from_​print#​SubordinateTaxa.

2016. "Sula dacty­la­tra (Masked Booby)" (On-line). The IUCN Red List of En­dan­gered Species. Ac­cessed Jan­u­ary 31, 2018 at http://​www.​iucnredlist.​org/​details/​22736173/​0.

2016. "Sula var­ie­gata (Pe­ru­vian Booby)" (On-line). The IUCN Red List of En­dan­gered Species. Ac­cessed Jan­u­ary 31, 2018 at http://​www.​iucnredlist.​org/​details/​22696686/​0.

2018. "UNEP" (On-line). The Species+ Web­site. Ac­cessed Feb­ru­ary 28, 2018 at https://​speciesplus.​net/#/​taxon_​concepts/​6600/​references.

Chaves-Cam­pos, J., J. Tor­res. 2002. Dis­tri­b­u­tion of Nests of the Brown Booby (Sula Leuco­gaster) in Re­la­tion to the In­cli­na­tion of Ter­rain. Or­ni­tolo­gia Neotrop­i­cal, 13: 205-208. Ac­cessed Jan­u­ary 31, 2018 at http://​www.​avibirds.​com/​pdf/​B/​Bruine%20Gent5.​pdf.

Diaz, H., J. Al­fredo, E. Salazar Gomez. 2011. "Neotrop­i­cal Birds On­line" (On-line). Blue-footed Booby (Sula nebouxii). Ac­cessed Feb­ru­ary 11, 2018 at https://​neotropical.​birds.​cornell.​edu/​Species-Account/​nb/​species/​bfoboo/​overview.

Friesen, V., D. An­der­son. 1997. Phy­logeny and Evo­lu­tion of the Sul­i­dae (Aves: Pele­can­i­formes): A Test of Al­ter­na­tive Modes of Spe­ci­a­tion. El­se­vier, Vol­ume 7, Issue 2: 252-260. Ac­cessed Jan­u­ary 31, 2018 at https://​www-sciencedirect-com.​ezproxy2.​library.​colostate.​edu/​science/​article/​pii/​S1055790396903978?_​rdoc=1&_​fmt=high&_​origin=gateway&_​docanchor=&​md5=b8429449ccfc9c30159a5f9aeaa92ffb&​ccp=y.

High­field, R. 2011. The New Guano Era. New Sci­en­tist, Vol. 212 Issue 2842: 26-27.

Lee, D., J. Haney. 1984. The Genus Sula in the Car­oli­nas: An Overview of the Phe­nol­ogy and Dis­tri­b­u­tion of Gan­nets and Boo­bies in the South At­lantic Bight. Chat, Vol. 41, No. 2: 29-45. Ac­cessed Jan­u­ary 31, 2018 at https://​www.​carolinabirdclub.​org/​chat/​issues/​1984/​v48n2sula.​pdf.

Mellink, E., J. Dominguez, J. Lue­vano. 2000. Diet of East­ern Pa­cific Brown Boo­bies Sula Leuco­gaster Brew­steri on Isla San Jorge, North-east­ern Gulf of Cal­i­for­nia, and an April Com­par­i­son with Diets in the Mid­dle Gulf of Cal­i­for­nia. Ma­rine Or­nithol­ogy, 29: 29-38. Ac­cessed March 01, 2018 at http://​citeseerx.​ist.​psu.​edu/​viewdoc/​download?​doi=10.​1.​1.​669.​9553&​rep=rep1&​type=pdf.

Nel­son, B. 1978. The Sul­i­dae: Gan­nets and Boo­bies. Ab­erdeen, UK: Uni­ver­sity of Ab­erdeen.

Nel­son, J. 1969. The Breed­ing Ecol­ogy of the Red-footed Booby in the Gala­pa­gos. Jour­nal of An­i­mal Ecol­ogy, Vol. 38, No. 1: 181-198. Ac­cessed Jan­u­ary 31, 2018 at http://​www.​jstor.​org.​ezproxy2.​library.​colostate.​edu/​stable/​2745?​sid=primo&​origin=crossref&​seq=1#​page_​scan_​tab_​contents.

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