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Northern Gannet
Morus bassanus
Order
SULIFORMES
– Family
SULIDAE
Authors: Mowbray, Thomas B.

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Behavior

Locomotion

Walking, Hopping, Climbing, Etc

Stance upright, slightly tilted backward; short tarsi make walking appear difficult; gait waddling. In nesting colonies on flat ground has to run and hop precariously between nests of other birds to gain sufficient speed to attain flight (Palmer 1976, Cramp and Simmons 1977).

Flight

Strong and direct; in light breezes alternates periods of deep, powerful beats and short glides, with occasional banking and soaring; in higher winds, banks and side-slips more, by shortening its wingspan and stroke (Cramp and Simmons 1977). Generally attains flight by launching from cliff ledge; however, capable of rising quickly into wind from water’s surface. Flight of adult Northern Gannets little affected by storms; some indication juveniles occasionally blown off-course by strong winds (Nelson 1978a). Normal flight 10–40 m above water; occasionally flies low over waves, also known to fly and soar at considerable altitudes; capable of flying at >60 km/h (Pennycuick 1969) and maintaining an average of 3.2 ± 0.2 SD (n = 7) wingbeats/s (Palmer 1976).

During nonbreeding season, Northern Gannet remains continuously at sea; may fly singly or in small, loose groups (5–10); occasionally in large flocks up to 100, and even up to 1,000, but large groups may comprise fishing flocks; may be patterns of groupings characteristic of motivation and direction; i.e., often occur in larger flocks when returning to breeding colonies in spring. During breeding season, often travel in distinctive skeins when returning to colonies laden with fish. Gannets tend to move mainly in the morning, though passage may continue all day (Maxwell 1972, Palmer 1976, Nelson 1978a).

Swimming And Diving

Swims well, with head up and tail held above water; roosts on water when on extended foraging trips and during nonbreeding season; often unable to take off when stomach full. Following natal dispersal from nest site, fledglings begin their first fall migration swimming; may swim extensive distances (>70 km) for several days before able to attain flight from water (see Migration: migratory behavior, above). A plunge-diver (see Food habits: feeding, above), but also dives from surface and pursues fish by swimming with strokes of half-opened wings (Garthe et al. 2000); swims for a brief period after catching and swallowing a fish (Cramp and Simmons 1977, Nelson 1978a).

Self-Maintenance

Preening, Head-Scratching, Stretching, Bathing, Anting, Etc

Descriptions of body-care activities follow Nelson 1978a, except where noted. Northern Gannets spend a great deal of time preening and prolonged bouts of preening often last up to 1 h, with same parts of body (i.e., wings, tail, and feathers of breast and belly) treated several times. Preens by nibbling feathers with mandible-tips, stropping bill sideways across feathers of underparts, and by drawing wing- and tail-feathers through bill; no specific sequence apparent, nor any seasonal trends; mutual preening, in which one partner preens the other, is not a form of feather care, but rather modified aggression; parents do, however, preen nestlings with their bills, even when so young as to still be largely naked. Intensive bouts of preening generally involve oiling, where gannet puts its head directly onto oil gland at base of tail and rolls its crown and sides of its head on the gland, then rubs its head onto feathers of wings and back, and spreads secretion onto feathers of wing and tail with its bill.

Northern Gannet practices another form of feather care, one that comes near to being a display, involving a complex plumage shaking and settling called the Wingflap-rotary Headshake (Nelson 1978a). Full sequence begins with body at 45°, a vigorous flapping of wings, that ruffle body-feathers, including head and neck, a rapid rotation of head through 90° or more and back again, a waggling of tail from side to side, and concluding with a re-settling of wings; full rotary headshake sequence beneficial for getting rid of loose feathers, and is commonly practiced during molt of body-feathers. A variant of this, in which wings are only partially loosened and not flapped, is called the “dogshake,” which is most common response to a sudden fouling of the feathers by a neighbor’s excreta or anything that may make birds anxious. Fully feathered young do not perform Wingflap-rotary Headshake; juveniles flap their wings incessantly, and headshake frequently, but not together in one sequence.

Northern Gannet bathes just offshore of colony by ducking and thrashing surface with partially open wings and occasionally rolling onto one side; oiling, rotary headshaking, and sideways headshaking with drinking common while bathing. When landing on water to bathe, or rest, commonly enters with a shallow dive, less often feet-first. Minor comfort behaviors include: wing-stretching or limbering, that is done one at a time and only downward over leg and foot, which may be raised, most commonly following preening; never arches both wings upward; may forward stretch with depressed hyoid without headshaking, yawning, with the buccal cavity enlarged by sideways and downward distension, and various heat mitigating behaviors such as panting, gaping slightly, and fluttering or resonating throat skin with head tilted upward and eyes almost closed. Not known to ant.

Sleeping, Roosting, Sunbathing

Sleeps standing or sitting, with head tucked into scapulars and bill snuggled into body-feathers. Spends about 50% of time at colony asleep. Before egg is laid, prefers to sleep standing; as laying time approaches, sleeps more in a sitting position. At breeding colony, established birds roost at nest or site, nonbreeders at club. Sleeps at intervals throughout day and almost totally inactive at night; birds arriving late from sea rarely, if ever, land at colony after dark, but sit on sea nearby until first light. During nonbreeding season, or temporarily away from colony, roosts on open sea (Cramp and Simmons 1977). Not known to sunbathe.

Daily Time Budget

Activity budgets differ between sexes and age classes and vary with season and habitat (Montevecchi and Porter 1980). Main activities throughout breeding season are resting, preening, incubating/brooding, and feeding.

Agonistic Behavior

Physical Interactions

Competition for and defense of socially adequate sites (near to or actually among existing breeders) source of often ferocious fighting, loud calling, and aggressive displays; defense shared by both sexes; females fight, threaten, and display just as do males, only difference being their aggressive displays slightly less intense and less frequent. Most intense fights occur when 2 males attempt to acquire legitimate and initially uncontested ownership of same site; clashes often prolonged and damaging; may result in death. Opponents lock bills and try to push each other off site, using their wings to brace forward thrusts, but never to strike; if one forced off ledge, may continue fighting in air or on sea. During fight, changes of grip among combatants may lacerate facial skin, and bill-tips may be forced far into mouth or even eye of opponent, but fights essentially involve fierce gripping and pushing rather than stabbing; most stabbing damage results from neighbors as combatants violate their territories. Another aggressive behavior important in keeping neighbors in their place and dispelling intruders, but not for settling serious disputes, is Jabbing (a vigorous forward lunge that makes contact with another bird, but does not result in holding, or if it does, only briefly), which can grade from silent, single, casual jabs, to vigorous and repeated strikes, accompanied by strident calling (Cramp and Simmons 1977, Nelson 1978a).

Communicative Interactions

Several well-differentiated signal behaviors for maintaining control in densely packed colonies; appear to be derived from aggressive behavior, and themselves verge on being aggressive. Common displays include: Threat Display, a simple modification of Jabbing, in which open bill is thrust toward intruder or neighbor, in a scooping movement, in silence with no contact made. A modified and more ritualized form of Threat Display, also likely derived from Jabbing, is Threat-gaping or Menacing, that involves a conspicuous sideways twist to the head and immediate withdrawal of the bill thrust-with-twist-and-withdrawal, thrust-with-twist-and-withdrawal, in a stereotyped sequence without any attempt to grip. Like Jabbing, it grades in intensity, and is one of most common behaviors in gannetry for preventing encroachment. A more highly ritualized display that signals site ownership and functions as a threat to repel intruders is the aggressively motivated display, Bowing. This display, commonly given during an intrusion, or even threat of an intrusion, begins with gannet shaking its head, with its bill held slightly below horizontal, and sweeping it forward and down while tilting body slightly forward, then raising head, shaking it from side to side, dipping forward again and so on, 2, 3, 4, or 5 times in quick succession, and finally terminating display by pressing bill-tip against upper breast in a pronounced “bill-tucking” or “pelican-posture,” that it holds for several seconds before relaxing. During first dip, wings opened and held up and out, but not waved, tail tips up as head tips down, and gannet calls loudly and repetitively throughout. Bowing always done in solo; most frequent early and late in season; more intense by male than female. A less ritualized form of this display is Nest-biting, in which site owner, with wings held out, dips its head and grips its nest or ground while calling loudly following expulsion of an intruder (Palmer 1976, Cramp and Simmons 1977, Nelson 1978a).

Appeasement displays include: “bill-tucking” or “pelican-posture,” in adults, and either “pelican-posture” or “bill-hiding” (settling down, belly to ground, hiding its bill, and remaining motionless) by nestlings (Palmer 1976, Nelson 1978a).

Spacing

Territoriality

Nests in dense colonies; territories established by males, usually in fourth or fifth year, and normally retained without breeding during first year, while attracting a mate and establishing pair bond; once established, nest-site defended as a territory by both sexes (see Agonistic behavior, above), but especially by male. Territory just large enough to accommodate both adults, including room for mutual display and a large chick; pairs spaced evenly within contact of neighbors at about 2–3/m2(average distance from nest center to nest center of about 80 cm; Poulin 1968); pair generally faithful to same site from year to year; may move a negligible distance after nest failure; female less site-tenacious than male. Territory used for resting, sleeping, preening, and feeding, as well as pair formation, pair interactions, copulation, incubation, and rearing of chick. Once fully established, after birds return to colony in spring, nest site rarely left unattended (Cramp and Simmons 1977, Nelson 1978a).

Upon returning to colony as 2- to 3-yr-olds, immatures join gatherings of other immature and nonbreeding adult-plumaged birds in areas separate from nesting birds, or if gannetry permits, on ledges near breeders, called “clubs.” In “clubs,” young gannets tend to assemble with similarly aged birds and interact with one another through low-intensity territorial and sexual behavior. During this time (which may be 2–3 yr), they learn behavioral skills necessary to acquire and maintain a territory and to cope with wind conditions around gannetry (landing and taking off from ledges), and learn location of adjacent fishing grounds. As nonbreeders, young males constantly prospect for nest sites, with a marked tendency to return to part of colony in which they were reared. Following frequent transient attachments to relatively imprecise sites, male gradually forms a hardened attachment for a specific, strongly defended area. Once acquired, male remains at site for several days, claiming ownership by his presence; if site had previously been claimed by another male who was away fishing, a serious fight to retain area may ensue upon initial claimant’s return (see Agonistic behavior, above). Once firmly established, male defends territory by ritualized displays (e.g., Bowing, Nest-biting) and sets about attracting a female.

Nonterritorial away from colony; may encroach on each other’s space when crowded on water during fishing, while gathering nest material, or when competing for trawler offal without any outward aggression (Nelson 1978a).

Individual Distance

See above. Highly gregarious during breeding season, nesting in dense colonies, with nest sites within contact of one another; usually gregarious at sea when plunge-diving while fishing; during nonbreeding season may be widely scattered in small, loose flocks; newly fledged juveniles relatively solitary until able to fly from surface of sea, then join small flocks of older birds or larger fishing flocks.

Sexual Behavior

Mating System And Sex Ratio

Monogamous for life. No information available on sex ratio for species or populations at North Atlantic breeding colonies.

Pair Bond

Pair formation occurs at breeding colony; site-owning males (usually from their third year) advertise status by Headshake-and-reach (exaggerated sideways Headshake with slight dips of head toward nest and reaching movements toward female). Females may visit several receptive males in quick succession before forming a bond; new pairs often break up after a few days or even weeks, but once fully formed, usually remain intact from year to year; bond reinforced each time pair reunites at site by Headshakes and Nape-bites by male, Facing-away by female, and Mutual Fencing (partners stand breast to breast, with wings out, shake heads from side to side, knocking bills together and interspersing forward and downward movements of head while calling loudly), followed by Allopreening of head and neck; may form new pair bond with loss of mate; pairs reoccupy same nest site from year to year (Cramp and Simmons 1977, Nelson 1978a, Montevecchi et al. 1980).

Copulation of mated pair occurs at nest site before egg-laying, often following a bout of Mutual Fencing; female Headshakes vigorously; male mounts female while gripping her nape, and with wings held out, patters webs of feet on her back; usually followed by brief Mutual Fencing and by female touching nest material; male then leaves to collect more material; no ritualized presentation (Cramp and Simmons 1977).

Extra-Pair Copulations

None observed.

Social And Interspecific Behavior

Degree Of Sociality

Colonial and gregarious during breeding season; very gregarious during bouts of fishing; from solitary to occurring in small, loose flocks during nonbreeding season. Both mates participate in all aspects of parental care; chick guarded throughout nestling period by 1 or both parents; chicks beg and receive food from parents until fledging; parents preen chick, chick also preens parents (Cramp and Simmons 1977, Nelson 1978a, Montevecchi and Porter 1980).

Play

Once chick can stand, and increasingly with age, it begins to “attack” parents in a playful manner; attacks boisterous with excited Yapping Call, jabbing, and contorted head movements, and may elicit Facing-away and avoiding movements from adults, which occasionally briefly retaliate. Chicks show early interest in nest material and at times add material to nest, or help adult do so; fully grown chicks occasionally perform short bouts of Mutual Fencing with parents. Interactions with neighboring chicks common, consisting of low-intensity aggression: gripping bills, jabbing, and menacing (Cramp and Simmons 1977).

Nonpredatory Interspecific Interactions

Little apparent interaction with other species; some aggressive interactions with Common Murres (Uria aalge) at colony on Funk I. and possibly elsewhere (W. A. Montevecchi pers. comm.).

Predation

Not a significant mortality factor in this species. Mammalian predation only a problem at mainland colonies (Cape St. Mary’s), or colonies on large islands (Anticosti and Baccalieu); avian predation a factor, albeit minimal, at all colonies (W. A. Montevecchi pers. comm.).

Kinds Of Predators And Manner Of Predation

Eggs. Occasional predators include Great Black-backed (Larus marinus) and Herring (L. argentatus) gulls, Common Ravens, red fox (Vulpes vulpes), and short-tailed weasel (Mustela erminea).

Young and Adults. Predators of nestlings include Great Black-backed and Herring gulls, Bald Eagle (Haliaeetus leucocephalus), red fox, and short-tailed weasel. Adults seldom attacked by predators on land, other than humans and occasionally Bald Eagles (W. A. Montevecchi pers. comm.); at sea, predation, other than by humans, insignificant except for an occasional gannet that may be snatched by a large fish or possibly a seal (Nelson 1978a).

Response To Predators

Normal response to avian predators such as gulls, is one of aggression; when a Bald Eagle flies over a colony, birds become very still, extremely quiet, and stare upward, closely following its every move (W. A. Montevecchi pers. comm.).