Demography and Populations

Age At First Breeding; Intervals Between Breeding

First breed after male acquires a nest site, sometime in fourth, fifth, or sixth year (i.e., when they are 3, 4, or 5 yr old); in extreme cases, a 3-yr-old may acquire a nest site, attract a mate, and breed that season, in which case he would be in his fifth year (4 yr old) by the time chick hatched; most males, however, are 5–6 yr old before first breeding; where exact ages of paired gannets known, males generally 1 yr older than females. Pairs retain same nest site each year and breed annually (Cramp and Simmons 1977, Nelson 1978a).

Clutch

One egg/clutch, 1 clutch/breeding season. Can successfully hatch >1 egg, and raise >1 chick to fledging (Nelson 1964, Wanless 1984).

Annual And Lifetime Reproductive Success

Most comprehensive data on breeding success in North America comes from Bonaventure I., Québec, where hatching success (chicks hatched from eggs laid) has varied considerably since 1960s; between 1966 and 1970, hatching success on island was 36–40%, after which it increased to 58% in 1974 and then to 78–89% during period 1976–1984, where it has remained until at least late 1990s (86.2% in 1999); increase in breeding performance coincided with a significant decrease in DDT and Dieldrin residues in eggs, suggesting low hatching success was due to toxic chemical contamination (see Conservation and management: effects of human activity, below; Poulin 1968, Chapdelaine et al. 1987, G. Chapdelaine unpubl.). At Cape St. Mary’s, Newfoundland, hatching success about 80% (W. A. Montevecchi unpubl.). For e. Atlantic, most complete data comes from Bass Rock, e. Scotland, where, between 1961 and 1973, percentage of chicks fledged from eggs laid was 77.7% (range 73–85; Nelson 1978a). Hatching success in Northern Gannets generally >80%, and in some groups as high as 95%; at Bass Rock, of 500 eggs laid, 82% (range 74–87) hatched (Nelson 1978a), and Ailsa Craig, w. Scotland 81% (range 76–86; Nelson 1978a).

Natural egg loss mainly result of inadequate parental care, particularly by inexperienced adults; at Bass Rock, inexperienced pairs hatched 62.5% of eggs laid compared with 86% for experienced breeders in same group; a few eggs also lost as a result of predation or being knocked out of nest (gannets cannot roll eggs and never pick them up in their mandibles); some eggs may also be lost to chilling during extremely cold conditions (Nelson 1978a).

Lose fewer chicks than eggs; at Cape St. Mary’s, Newfoundland, about 90% of eggs that hatch fledge young (W. A. Montevecchi unpubl.); at Bonaventure I., 1984–1999, 89.6% (range 85.6–95.9) of eggs hatched fledged young; during same period, net productivity (young fledged from eggs laid) averaged 73.5% (range 71.7–75.1), well above threshold level of 65.0% necessary to sustain population (G. Chapdelaine and J.-F. Rail unpubl.); at Bass Rock, 92.3% (range 89–94) of hatched eggs fledged young, and at Ailsa Craig, 92% (range 91–93; Nelson 1978a). Main causes of chick loss include: inexperienced parental care (mandibulating or moving chicks with exaggerated actions over nest rim), weather (high winds with prolonged rain or sleet and low temperatures), attacks by neighbors (neighbors may attack and kill unguarded chicks), falling (from nest sites on steep cliffs while wing exercising), and accidents during fledging descent (failing to make it beyond rocks and scree to open water). At Cape St. Mary’s, 1976–1982, of 236 fledging departures observed, 205 (86.9%) were successful; flight success varied significantly between cliff (93.3%) and plateau (70.2%) nest sites (p < 0.001), and cliff (93.3%) and slope (76.6%) nest sites (p < 0.01); main cause of reduced departure success appears to be greater number of adult attacks on fledging young at plateau and slope nest sites as they make their way to departure site; no significant difference in flight success related to wind direction or time of departure (Montevecchi and Wells 1984).

Using overall breeding success figure of 75% for Bass Rock, productivity of 22 pairs of individually identifiable gannets that remained together for >5 yr was 92.7% (127 chicks in 137 pair-years), or 0.92 chicks/pair/yr (Nelson 1978a). During 11 yr gannets were observed, 1 pair remained together for 11 yr and fledged 11 young, 2 pairs remained together for 9 yr and each fledged 9 young, and 1 pair fledged 8 young in 8 yr they remained together. Using average life expectancy of 16.2 yr after death rate becomes independent of age (see Life span and survivorship, below), lifetime productivity of each pair on average 16.2 × 0.75 = 12.2 chicks (Nelson 1978a).

Artificial twinning experiments indicate hatching success as high for 2 eggs as 1 egg, but fledging success somewhat lower at Bass Rock (83%; Nelson 1964) and Ailsa Craig (60%; Wanless 1984); pairs with twins, however, still reared more young (1.2 young/pair) than normal pairs (0.92 chicks/pair); no indication of survival rate once leaving nest (Nelson 1964, Wanless 1984).

Number Of Broods Normally Reared Per Season

One brood/season; will lay a replacement egg if first egg lost <25 d after initial laying (see Breeding: egg-laying, above; Poulin 1968, Nelson 1978a).

Proportion Of Total Females That Rear At Least One Brood To Nest-Leaving Or Independence

No specific figure available, but likely very high.

Oldest reported individual in North America, 20 yr 5 mo (Klimkiewicz and Futcher 1989); in e. North Atlantic at least 21 yr (Nelson 1978a).

Based on band-recovery data, 1960–1975, adult mortality rates at Bass Rock <6%/yr, yielding an average life expectancy for Northern Gannets of 16.2 yr; mortality highest during first year (65%), and thereafter relatively proportional. Using these mortality values along with average clutch size of 1 egg/pair/yr and a fledging rate of 75%/pair/yr, a life table for Northern Gannet can be calculated (Fig. 5).

Diseases

Disease a minor mortality factor; no widespread occurrences found among Northern Gannets, though salmonella (Salmonella typhimurium) has been isolated from wild birds and Newcastle disease virus from a single dead (wild) gannet. Fungal disease aspergillosis (Aspergillus fumigatus) has been found in Cape Gannet and more than likely occurs in Northern Gannet (Nelson 1978a).

Body Parasites

The mite Neottialges evansi likely present in gannets in subcutaneous form (Nelson 1978a). Nematode and tapeworm infections common in Northern Gannets killed in other ways and are probably present to some extent in all birds, usually without becoming a significant hazard. Two species of trematodes, Cryptocotyle lingua and Diplostomum spathaceum, reported in e. Atlantic (Nelson 1978a), and 2 species of diplostomes, Bursatintinnabulus bassanus and Bursacetabulus morus, reported in birds along Texas coast (Tehrany et al. 1999).

Exposure

Although Northern Gannets breed in very cold conditions in the w. North Atlantic, exposure does not appear to be a major mortality factor; some egg loss by inexperienced breeders attributable to chilling as a result of inadequate parental care, especially in very severe snowy weather. Unusual weather conditions, such as severe winds, can also cause some egg loss due to exposure when gannets are blown off their nests (Nelson 1978a).

Loss of chicks to exposure minimal; critical period first 19–24 d, until chick acquires thermal independence, during which brooding continuous; loss during this period attributable to inadequate parental care (Nelson 1978a, Kirkham and Montevecchi 1982, Montevecchi and Vaughan 1989).

Predation

See Behavior: predation, above, and Conservation and management: effects of human activity, below.

Competition With Other Species

No documentation of mortality directly from interspecific competition.

Initial Dispersal From Breeding Site And Colony

Dispersal from breeding site and colony occurs with fledging; fledgling flies from cliff edge at or near nest site 250–800 m into sea, and swims south-ward away from colony; initially unable to fly from water, attains flight within a week, and continues dispersal/migration southward along Atlantic coast (Nelson 1978a).

Natal Philopatry

Site fidelity very strong; at Bass Rock, 95% of color-banded chicks returned to same group in which they were reared, and in a few cases to same ledge; males slightly more philopatric than females; some interchange between colonies, especially in the case of rapidly expanding groups, which may take in large numbers of recruits (Nelson 1978a).

Fidelity To Breeding Site

Pair strongly philopatric to breeding site; at Bass Rock, 1961–1976, of 149 marked pairs, 94% of males and 88% of females either returned to nest site or presumably died the following year; the following year, both members of pair returned to same site and used same nest in 64% of cases (Nelson 1978a).

Fidelity To Winter Range

Little information; observations suggest winter range varies with age, breeding status, weather conditions, and availability of food.

Home Range

During breeding season, adult non-breeders and breeders range extensively (>120 km) within waters of North Atlantic, having been reported from Bay of Fundy (Nelson 1978a), off Labrador coast (Todd 1963), sw. Greenland (Salomonsen 1967, Nelson 1978a), regularly in Strait of Belle Isle (Kirkham et al. 1985), and throughout Gulf of St. Lawrence (Chapdelaine 1996).

Estimates Of Density At Different Colonies

Since 1984, counts of total number of site-holding birds at each of the 6 North American colonies made every 5 yr, by aerial survey and photography, during breeding season (see Table 1). Aerial photo inventories in 1999 indicate a total North American population of 72,298 site-holding pairs (144,596 individuals), of which 69% occur in Gulf of St. Lawrence and 31% in the Atlantic region (Chardine 2000). Largest breeding colony in North America is Bonaventure I., Québec, with 36,936 pairs (73,872 individuals) in 1999 (G. Chapdelaine unpubl.).

Trends

All Northern Gannet populations in North America increased between 1994 and 1999: at Bonaventure I., Québec 42.9%; Anticosti I., Québec, 23.6%; Bird Rocks, Québec, 61.0%; Funk I., Newfoundland, 38.2%; Cape St. Mary’s, Newfoundland, 24.0%; and Baccalieu I., Newfoundland, 60.5% (Chardine 2000, G. Chapdelaine unpubl.). Between 1972–1973 and 1984, total North American population of Northern Gannets increased about 22% (Nettleship and Chapdelaine 1988); most recent estimates of population (see above) indicate a 51.6% increase between 1984 and 1999 (Nettleship and Chapdelaine 1988, Chardine 2000, G. Chapdelaine unpubl.). Between 1984 and 1999, no difference in proportion of North American population in Gulf of St. Lawrence and Atlantic colonies: 69.0 and 31.0%, respectively.

Population increases at colonies in Gulf of St. Lawrence attributed to improved breeding success following ban on DDT in early 1970s and reduced use of PCBs (see Conservation and management: effects of human activity, below; Chapdelaine et al. 1987). Overall increase in Northern Gannet populations since early 1900s the result of Migratory Birds Convention between Britain (on behalf of Canada) and U.S. signed in 1916 and the 2 national enabling acts (Migratory Birds Convention Act of 1917 in Canada and Migratory Bird Treaty Act of 1918 in U.S.); also declaration of colonies as migratory bird sanctuaries, minimal human disturbance at colonies, favorable foraging conditions, and 2 decades of relatively mild winters.

Nest-site availability may limit breeding opportunities in some areas. Evidence to date, however, suggests that density-dependent factors do not limit size of populations at individual colonies, or that Northern Gannet populations are less productive when their numbers are high. Within colonies, however, reproductive success can vary among microhabitats. At Bonaventure I., 1984–1999, reproductive success differed significantly between birds nesting on cliff and upper plateau habitats. During the period, hatching success, fledging success, and net productivity were significantly greater for birds nesting on plateau than on cliffs (G. Chapdelaine unpubl.). At Bonaventure I., recruitment into breeding population can be estimated by the simple model R_t+k= N_t× b_t× S_j× (S_a) ^k-1× S_a, where: R_t+k= size of a cohort of potential recruits available in a particular year, N_t= number of breeding pairs k yr previously, b_t= number of chicks fledged per pair k yr previously, S_j= first-year survival, (S_a) ^k-1= annual survival rate from then until recruitment k–1 yr later, k = age at recruitment, S_a= annual adult survival rate; and, percent population growth (r) by the equation: r = R_t-k-([1-S_a] 2N_t)/2N_t× 100 (G. Chapdelaine unpubl.). Based on the model, the threshold net productivity necessary to maintain a stable or growing population is ≥65%, and given the current productivity of 73%, the population is growing 3.6%/yr. Weather conditions and availability of food during laying, incubation, and brooding appear to have minimal effect on mortality, and thus do not influence breeding success; but they could affect mortality during first year, and thus subsequent recruitment. Little information available on factors affecting mortality of juveniles during their first winter.