The Cape gannet (Morus capensis) and the Northern gannet (Morus bassanus) are two closely related seabird species in the genus Morus. While they share some similarities, there are also some key differences between the Cape gannet and the Northern gannet in terms of their physical characteristics, breeding behaviors, habitats, and conservation status.
Quick Facts
Here is a quick overview of the key differences between Cape gannets and Northern gannets:
| Characteristic | Cape Gannet | Northern Gannet |
|---|---|---|
| Scientific name | Morus capensis | Morus bassanus |
| Size | Length: 84 cm Wingspan: 175-195 cm Weight: 2.1-3.2 kg |
Length: 89-100 cm Wingspan: 175-195 cm Weight: 2.6-3.6 kg |
| Plumage | Mostly white with black wingtips and yellowish forehead | Mostly white with black wingtips and yellowish head |
| Range | Coastal southern Africa | North Atlantic and associated coastlines |
| Habitat | Subtropical and tropical coasts | Temperate and subarctic coasts |
| Diet | Anchovies, sardines, other small fish | Herring, mackerel, sand eels, other medium fish |
| Conservation status | Near Threatened | Least Concern |
Physical Characteristics
The Cape gannet and Northern gannet share the typical gannet body form with long, pointed wings suited to diving for fish from the air. However, there are some subtle differences in their physical characteristics:
Size
On average, Northern gannets are slightly larger than Cape gannets. Cape gannets have an average body length of 84 cm, wingspan of 175-195 cm, and weight of 2.1-3.2 kg. Northern gannets average 89-100 cm long, with the same wingspan of 175-195 cm, but are heavier at 2.6-3.6 kg.
Plumage
Both species have predominantly white plumage with black flight feathers on the wingtips. However, Cape gannets have a yellowish wash on the crown and nape, whereas Northern gannets have a yellow crown and hindneck that fades to white on the nape and throat.
Bill
The bills of both species are long, straight, and pointed to aid in plunge diving for fish. Cape gannets have black bills with some light blue markings inside the mouth. Northern gannets have larger, grayish-blue bills with dark gray culmens and pale blue mouth linings.
Eyes and legs
Cape gannets have dark brown eyes and pale bluish-gray legs and feet. Northern gannets also have dark brown eyes but their legs and feet are blackish-blue with dark webbing between the toes.
Breeding and Nesting
Both Cape gannets and Northern gannets breed in dense colonies. However, there are some differences in when breeding occurs and details of nest construction.
Breeding season
Cape gannets nest during the southern hemisphere spring and summer between August and March. Northern gannets nest earlier in the northern summer between April and September.
Mating
Cape gannets form monogamous pairs while breeding but may mate with new partners in subsequent seasons. Northern gannets are mostly monogamous within each breeding season but some males may mate with two females. In both species, pairs engage in ritualized displays including sky-pointing, head-shaking, and fencing with their bills prior to mating.
Nest
The nests of both species are made mainly of seaweed, feathers, and guano scraped together into a mound. However, Cape gannets also incorporate some soil into nests whereas Northern gannets use only organic materials. Cape gannet nests tend to be sturdier with a donut-like central hollow. Northern gannet nests are flatter platforms of debris.
Eggs
A single chalky blue egg with a thin red-brown outer coating is laid per breeding attempt for both species. Cape gannet eggs average 71 x 47 mm in size, slightly smaller than the average Northern gannet egg at 78 x 51 mm.
Chicks
Chicks of both species hatch after 42-46 days of incubation. They have light gray down initially, darkening as they grow. Cape gannet chicks fledge at 100-110 days old. Northern gannets fledge at 85-87 days on average.
Habitat and Range
Cape gannets and Northern gannets inhabit different coastal regions due to their preferences for tropical versus temperate climate zones.
Cape gannet
Cape gannets are found along the coasts of southern Africa from southern Angola to eastern South Africa. Their breeding colonies are located on offshore islands and headlands along the Namibian and South African coastlines.
They favor warm temperate and subtropical climates and avoid areas where cold, nutrient-rich water upwellings occur. Their range shifts slightly north or south depending on water temperatures and food availability.
Northern gannet
Northern gannets inhabit both sides of the north Atlantic and associated coastlines. Breeding colonies are found on islands, sea stacks, and cliffs in Canada, the northeastern United States, Iceland, Ireland, Great Britain, Norway, and France.
Northern gannets prefer cool, temperate climate zones near nutrient-rich upwelling areas where prey fish are abundant. They winter further south than their breeding range along the Atlantic coast.
Diet and Feeding
Both Cape gannets and Northern gannets feed on fish and marine invertebrates. However, the types of prey taken differ between the species based on the fish communities present within their respective ranges.
Cape gannet
Cape gannets primarily eat anchovies (Engraulis encrasicolus), sardines (Sardinops sagax), and saury (Scomberesox saurus). They occasionally take other small forage fish such as herring and pilchards when available.
Cape gannets forage over shelf waters generally less than 100 km from breeding colonies. They dive from heights of 4-40 m to depths of 4-10 m to catch prey.
Northern gannet
Northern gannets mainly eat larger shoaling fish found in cooler waters, especially herring (Clupea harengus), mackerel (Scomber scombrus), and sandeels (Ammodytes spp.). They also take some squid and crustaceans.
Northern gannets routinely forage 100-600 km offshore from breeding sites. They plunge dive from heights up to 40 m and descend 15-25 m underwater to catch prey.
Conservation Status
Despite their close taxonomic relationship, Cape gannets and Northern gannets differ in their conservation status due to varying threats to their populations.
Cape gannet
Cape gannets are currently listed as Near Threatened by the IUCN Red List. Their global population is estimated at 180,000-200,000 breeding pairs as of 2018. numbers have declined by 30-49% since the 1950s.
Threats to Cape gannets include competition with commercial fisheries, oil spills, pollution, disturbance of nesting sites, and reductions in prey availability due to climate change and overfishing.
Northern gannet
In contrast, Northern gannets are listed as Least Concern and have a stable, increasing global population estimated at over 1 million breeding pairs as of 2015. They are still common and widespread throughout their range.
Northern gannets faced historical threats from hunting, egg collection, and habitat loss. They have rebounded under improved protection and management. However, they remain vulnerable to oil spills, fishery interactions, climate change, and pollution in parts of their range.
Similarities
Despite their differences, Cape gannets and Northern gannets exhibit many similarities as members of the Morus genus:
- Large seabirds with long, narrow wings adapted for plunge diving
- Bright white plumage with black wingtips
- Long, pointed bills ideally shaped for catching fish
- Forage exclusively on live fish and marine invertebrates
- Nest colonially in high densities
- Slow, steady reproductive strategy with one egg per season
- Cover chicks soon after hatching to protect from heat and rain
- Adults have no natural predators; longevity over 15 years
Conclusion
In summary, the Cape gannet and Northern gannet exhibit similarities in morphology, behavior, and ecology as congeners occupying comparable marine niches. However, differences in size, plumage details, habitat preferences, diet, and conservation status distinguish the two species. Their similarities reflect their evolutionary relationship while their differences reflect adaptations to their respective environments over time.
Understanding these similarities and differences helps shed light on gannet taxonomy, biogeography, and conservation. It also illustrates broader patterns in how morphology, physiology, and life history evolve in relation to climate, habitat, and ecological interactions.
