Barnacle geese (Branta leucopsis) are fascinating migratory birds that breed in the Arctic but spend their winters in Northern Europe. As their name suggests, they like to nest on cliff ledges and feed on barnacles and other marine invertebrates at low tide. However, once the female starts incubating eggs, neither parent can leave the nest to feed. This raises the question: why can’t barnacle geese feed while in the nest?
The Importance of Incubation
Incubation is a critical period for barnacle geese, as the eggs and hatchlings require constant warmth and protection. If the adults leave the nest unattended, the eggs can quickly get cold, fail to develop properly, or get eaten by predators. Barnacle geese have adapted to maximize incubation:
- The female lays a clutch of 3-5 eggs and starts incubating as soon as the first egg is laid.
- Incubation lasts 24-28 days until hatching.
- The male stands guard near the nest site while the female incubates.
- Both parents lose 20-25% of their body weight during incubation due to fasting.
Given the importance of continuous incubation, it is clear why barnacle geese are unable to leave the nest to feed – even for short periods. Interrupted incubation would jeopardize egg viability and offspring survival.
The Challenges of the Nest Site
Barnacle geese nest in precarious locations on rocky cliffs and steep slopes, often hundreds of feet above the ocean. The adults must remain still to avoid eggs rolling out of the shallow scrape nests. While these spots provide safety from Arctic foxes, they make feeding impossible:
- The cliffsides offer no vegetation for foraging.
- Flying down to the waterline exposes the nest to predators.
- Abandoning the site even briefly risks temperature fluctuations.
- Incubating birds are constrained to the small nest itself.
With their mobility extremely limited, there is simply no way for barnacle geese to access food while incubating. The precipitous nests that keep eggs safe also prevent the parents from feeding until hatching.
Physiological Adaptations for Fasting
To cope with fasting for up to 28 days, barnacle geese have evolved several physiological and behavioral adaptations:
- They build up fat reserves by gorging on vegetation before migrating to breeding sites.
- Their metabolism slows down during incubation.
- Protein is spared by recycling nitrogenous waste like uric acid.
- Both parents lose significant weight by the time eggs hatch.
- The male stands guard close to the nest so the female can take short breaks.
These adaptations allow barnacle geese to survive prolonged fasting during incubation. However, they are unable to restore nutrients or feed until the hatchlings arrive. The parents face a trade-off between self-preservation and ensuring offspring survival.
Reliance on Endogenous Reserves
With no incoming nutrition during incubation, barnacle geese rely on endogenous reserves built up beforehand:
- Fat deposits provide energy. Barnacle geese may store up to 500g of fat before nesting.
- Protein from breast muscles helps maintain function during wasting.
- Minerals like calcium are needed for egg shell formation.
- Water balance is maintained through metabolic water and precipitation.
By emptying these reserves, the fasting adults can nurture the developing clutch. However, the supply is finite and not replenished until they can feed after hatching. This underlies the imperative to incubate without interruption.
What Do Barnacle Geese Eat When Not Nesting?
Barnacle geese have varied diets depending on the time of year:
Wintering Grounds
In coastal wintering areas like Scotland, Ireland, and the Netherlands, barnacle geese forage on:
- Grass – prefer short, high-quality shoots.
- Rhizomes – underground storage organs of plants.
- Tubers – swollen stems like small potatoes.
- Roots – fleshy roots of grasses and herbs.
- Barnacles – arthropod crustaceans attached to rocks.
They maximize intake of carbohydrates, proteins, and minerals to build up endogenous reserves for breeding.
Breeding Grounds
On Arctic breeding grounds, barnacle geese switch to foods that provide rapid energy and nutrients for egg-laying:
- Sedges – grasslike Arctic plants with edible stems and roots.
- Shoreline vegetation – high in nitrogen and phosphorus.
- Aquatic plants – pondweeds, crowfoot, starworts.
- Algae – procured by dip-feeding underwater.
- Invertebrates – insects, crustaceans, mollusks.
They feed intensively to accumulate reserves before incubating when feeding stops completely.
Migration Stops
On migration stopovers between breeding and wintering grounds, barnacle geese refuel on:
- Cereal grains – wheat, barley, oats.
- Potatoes – an important source of carbohydrates.
- Grass shoots – gleaned from pastures and fields.
- Herbaceous plants – dandelions, clovers, alfalfa.
These foods rapidly restore fat and protein after migratory flights.
How Do Barnacle Geese Feed?
Barnacle geese have evolved specialized beak and tongue structures to feed on different sources:
Beak Adaptations
- Strong muscular beak to bite off vegetation.
- Serrated tomia (cutting edges) to efficiently shear tough stems.
- Ability to open beak underwater to feed on algae and aquatic plants.
- A morphology ideal for grasping and pulling up buried roots and rhizomes.
Tongue Adaptations
- Hard tip to scrape algae off rocks or dig up underground foods.
- Spiny papillae point backwards to maneuver food.
- Ability to protrude tongue underwater while tip remains sealed to vacuum up prey.
Combined with their powerful jaw muscles, these features allow barnacle geese to exploit diverse food sources across oceanic and freshwater environments.
What Are Some Interesting Facts About Barnacle Geese?
Beyond their fascinating nesting behavior, barnacle geese exhibit other intriguing biology and natural history:
Migration
- Migrate 2,000-3,000 miles between Arctic breeding grounds and temperate wintering areas.
- Make nonstop flights of 18-24 hours during migration.
- Navigate using mental maps, celestial cues, and geomagnetic senses.
- Males migrate ahead of females and juveniles to secure nest sites.
Physiology
- Weigh 3-4.5 lbs with a wingspan around 50 inches.
- Have modified feathers on bellies that store endogenous protein.
- Concentrate urine into semisolid paste to conserve water and electrolytes.
- Heart rate slows and body temperature drops during incubation.
Social Behavior
- Lifelong monogamous pairs stay together year-round.
- Nest in dense colonies with territorial nest defense.
- Male assiduously guards the nest while female incubates.
- Young leave nest within 24 hours and are led to feeding areas.
Population Trends
- Populations declined in 20th century but have rebounded since 1990s.
- 500,000 birds in Europe increasing 5-10% per year.
- No longer endangered but considered a species of least concern.
- Expanding range and wintering further north in Europe.
Conclusion
In summary, barnacle geese are unable to feed while incubating eggs due to:
- The need for constant incubation to ensure hatching success.
- Inaccessible cliffside nest locations that preclude foraging.
- Physiological and behavioral adaptations to survive prolonged fasting.
- Reliance on finite endogenous reserves during incubation.
Their specialized beak and tongue allow them to exploit diverse foods during other phases of breeding, migration, and wintering. Barnacle geese remain fascinating models of resilience and adaptation in extreme environments. Ongoing research continues revealing new insights into their unique reproductive strategies.
