The white pelican (Pelecanus erythrorhynchos) is a large waterbird with a distinctive bump or bulge on the upper part of its long, straight bill. This bump is a unique feature of white pelicans and is not found on other pelican species. The white pelican uses this bump in a specialized feeding technique to catch fish, which forms the main part of its diet. In this article, we will explore the details of the white pelican’s unusual beak bump – its structure, how it forms and develops, its purpose and function, and how the pelican utilizes it to feed. Understanding the biology behind this peculiar bump provides fascinating insight into the adaptations and feeding behaviors of these iconic birds.
What is the bump on a white pelican’s beak?
The noticeable bump or protuberance on a white pelican’s upper bill is known as the gular pouch or gular sac. It is a thin-walled, expandable pouch of skin that bulges out from the top side of the lower mandible (lower half of the beak).
In adult white pelicans, the gular pouch is very large and prominent, usually around 12–15 cm long, 6–8 cm wide, and can hold up to 3 liters of liquid. When empty, it is collapsed against the sides of the beak. As the pelican fills it with water and fish, the elastic pouch balloons out conspicuously. The gular pouch does not contain any bones or cartilage and consists only of skin and subcutaneous tissue anchored to the underside of the mandible.
Anatomy of the White Pelican’s Bill and Gular Pouch
The white pelican has a long straight bill with a large hook at the end of the upper mandible. The lower mandible has flexible joints allowing it to bow outwards to accommodate the gular pouch.
Internally, the gular pouch has a branched network of blood vessels to regulate temperature. The inner walls contain touch and pressure sensory receptors to detect contents. It lacks external feathers but has small hair-like filoplumes projecting inwards.
Muscles, nerves and ligaments connect the pouch to the bill, allowing the pelican to rapidly open, close, expand, and contract it.
Development of the Gular Pouch
The gular pouch begins forming when pelican chicks are around 3 weeks old. At hatching, the mandibles are of equal length. As the pouch develops, the lower mandible grows longer than the upper one to accommodate the protruding sac.
Initially small, the gular pouch grows larger and more defined as the chick matures. In young fledglings, the pouch is still relatively small and shallow compared to the large bulge of adult pelicans. The pouch continues enlarging and reaching its full size after the birds reach sexual maturity at 2-3 years old.
Purpose and Function of the Gular Pouch
The white pelican’s gular pouch plays an important role in the bird’s feeding ecology.
White pelicans are piscivorous, feeding almost entirely on fish. They forage cooperatively, working together to corral fish into dense schools before scooping them up in their bills. The gular pouch allows pelicans to catch over 30 kg of fish and water in a single plunge.
As pelicans swim with mouths open, they use their pouches like dip nets, expanding the skin to engulf as many fish and water as possible. The pouch’s wrinkles and elasticity allow it to massively distend.
Once the pouch is full, the pelican closes its bill to contain the contents. The water drains out through the sides, while fish are retained inside. Back on land, the pelican tilts its bill downwards and squeezes the pouch to swallow its catch.
A few key benefits and functions of the gular pouch include:
- Increases amount of prey caught per dive by allowing pelicans to scoop up more fish and water.
- Functions like a built-in net for capturing fish.
- Wrinkled skin and flexible sac allows the pouch to hugely expand when filled.
- Elastic walls snap back after draining water to keep fish securely inside.
- Allows pelicans to temporarily store captured fish before swallowing.
- Draining the water filters and concentrates the fish into a swallowable bolus.
How Pelicans Use the Gular Pouch to Feed
White pelicans have perfected specialized feeding methods using their gular pouches to exploit seasonal fish spawns. Here is an overview of how they utilize their unique bills and pouches when feeding:
Fishing in Groups
White pelicans will gather in flocks and coordinate to herd schools of fish into compact masses close to the surface. Working as a group allows them to corral much denser shoals of fish.
Plunge-Diving
From a height of up to 20 m, pelicans will plunge dive beak-first into the water with their 9-ft wingspan folded back. Right before hitting the water, they point their bills downwards and open their mouth and pouch wide to take in as much water and fish as possible.
Scooping the Prey
As the submerged pelican swiftly chases the fish schools underwater, the expanded pouch billows out to engulf fish and water. The pelican may surface or continue diving to scoop more prey into its stretchable pouch.
Draining the Water
After filling its pouch, the pelican will close its bill and let the water drain through the sides back into the sea, while fish are retained inside the elastic sac. This concentrates the catch and makes it easier to swallow.
Swallowing the Fish
Once the water has drained, the pelican tips its bill downwards over its throat. Contracting muscles squeeze the pouch to force fish into its gullet in a swallowable clump. Pelicans may swallow over 30 medium fish in a single bill-load.
Unique Features of the Gular Pouch
The white pelican’s expansive gular pouch has several specialized structural features and adaptations that allow it to function effectively as a built-in dip net for catching fish, including:
Very Expandable
The pouch skin consists of highly elastic tissue allowing it to tremendously balloon out when filled with fish and water. Even deflated pouches can stretch to over 3 times their resting size.
Wrinkled Interior Surface
Many wrinkles and folds in the pouch’s interior surface allow it to massively expand outwards when engorged. The wrinkled skin unfurls as the pouch widens, providing extra stretch capacity.
Thin but Tough Walls
The pouch walls are relatively thin but very resilient. Their thinness maximizes interior capacity while their durability prevents ruptures from forceful use.
Waterproofing Feathers
Small feather-like filoplumes cover the pouch entrance, protecting it from water infiltration while diving and preventing prey escape.
Built-in Strainer
Draining the water out through the bag’s sides filters fish from water, allowing the pelican to swallow only its prey.
Sensory Receptors
Nerve endings in the pouch lining allow pelicans to carefully control pouch expansion and contraction when catching, holding, and swallowing fish.
Comparison to Other Birds
The white pelican is the only pelican species with a pronounced gular pouch. Other pelicans have only a slight or modest pouch compared to the white’s large sac. The adaptiveness of its huge gular pouch allows the white pelican to be an expert fish-scooper.
Some other waterbirds possess gular pouches, but none are as extensive as the white pelican’s. Close relatives like cormorants have small throat pouches for holding prey. Herons, spoonbills, and shoebills have slightly expandable pouches. But only the white pelican has evolved such an massive distensible pouch specialized for scooping fish underwater.
Conclusion
The unusual bump on a white pelican’s bill represents an intriguing adaptation. The flexible gular pouch allows pelicans to capture far more fish per dive than they could using their bill alone. By ballooning out this stretchy sac underwater, pelicans can engulf entire schools of fish in one gulp. The expandable pouch essentially functions as a built-in dip net, allowing pelicans to make the most of seasonal fish spawns. While many waterbirds have pouches or loose throat skin, the white pelican’s enormous highly expandable gular pouch is unique among birds and perfectly suited to their fish-scooping feeding strategy.