Guillemots, also known as murres, are seabirds in the auk family. They are medium-sized black and white birds with long, thin bills. Guillemots nest on cliff ledges in huge breeding colonies and their eggs have a very unique shape – they are conical with a pointed end.
The egg shape allows them to roll in a circle rather than off the cliff
Guillemots lay their eggs directly on bare rock ledges without building a nest to hold them in place. This means the egg is at risk of rolling off the narrow ledge and falling into the sea below. The pointed shape of guillemot eggs allows them to roll in a tight circle if disturbed rather than falling off the cliff edge. The cone shape causes the egg to roll back towards the parent birds.
Guillemot eggs evolved to be this tapered, pyriform (pear-shaped) form over many generations. Birds with rounder eggs would have had less breeding success as their eggs rolled off the cliffs. The guillemots with pointed eggs were more likely to survive and pass on their conical egg genes.
The shape helps deter predators
The pointed shape also makes the eggs difficult for predators to eat. Many predators find rounded eggs much easier to crack open and access the insides. The tapered shape of guillemot eggs makes them awkward for predators to handle and less likely to be stolen.
Some of the main predators of guillemot eggs include gulls, ravens, foxes and mink. These predators often patrol the breeding cliffs looking for an easy meal. The pointed shape makes guillemot eggs less appealing and more effort to eat compared to eggs with more rounded shapes.
The eggs are stronger structurally
Research has found the elongated, conical shape also makes guillemot eggs stronger structurally. The tapered shape distributes mechanical stress more evenly across the shell making them less likely to crack. This is important for eggs laid on exposed cliffs without nest padding.
Studies have shown guillemot eggs can withstand much stronger compression and puncture forces compared to eggs of other shapes. This structural strength is essential for their cliff nesting behavior. The hard shell also protects the developing chicks inside.
The shape maximizes internal volume
Another advantage of the tapered egg shape is that it provides maximum internal capacity relative to the shell surface area. This allows the egg to hold more yolk and nutrition for the growing chicks inside without being excessively heavy for the parent birds.
The conical shape means guillemot eggs have among the highest internal egg contents compared to shell thickness of any bird species. This maximizes nutrition for the chicks while minimizing egg weight for the parents who must incubate and transport them.
The shape likely evolved alongside cliff nesting behavior
Experts believe the unusual pointed eggs of guillemots evolved in tandem with their habit of nesting on narrow cliff ledges. The conical shape emerged as an optimal solution to keep the eggs safely in place on precarious breeding sites high above the ocean.
Species of auks that nest on flatter surfaces away from cliff edges tend to have more rounded or elliptical egg shapes. This includes the Razorbill and Atlantic Puffin which often breed on grassy slopes and rocky crevices rather than steep, exposed cliffs.
The pointed egg shape of guillemots is a specialized adaptation for their specific nesting behavior and habitat. It provides multiple benefits that improve their breeding success in challenging cliff locations.
Comparison of guillemot eggs to other bird eggs
Here is a table comparing features of guillemot eggs to other bird egg shapes:
Bird Species | Egg Shape | Nest Location | Rolls in circle | Hard for predators | Strength | Internal capacity |
---|---|---|---|---|---|---|
Guillemots | Conical pointed | Exposed cliffs | Yes | Yes | Very high | Very high |
Gulls | Rounded | Rocky outcrops | No | No | Moderate | Moderate |
Puffins | Elliptical | Grassy slopes, crevices | No | Somewhat | High | High |
Chickens | Oval | Nests | No | No | Low | Moderate |
As the table shows, guillemot eggs differ from the rounded or oval shapes of most birds by being elongated and tapered for specialization in their cliff habitat.
How the eggs develop inside the mother
The distinctive shape of guillemot eggs originates within the mother’s reproductive system. As the shell forms around the yolk, it is molded into an elongated, asymmetric cone shape rather than a smooth oval.
The shell gland in the guillemot oviduct secretes the shell in this unique form. The exact biochemical mechanisms are not fully understood but likely involve differences in calcium transport and deposition as the egg passes through the oviduct.
The shell needs to be thin but also rigid and strong to protect the developing embryo inside. The female’s body has evolved to produce the ideal cone architecture to serve as both a protective casing and balance optimal weight and nutrition.
How the chicks hatch out of pointed eggs
Guillemot chicks manage to hatch out of their pointed eggs through a combination of their egg tooth, wiggling, and pipping. The egg tooth is a small protuberance on their beak which provides the first break in the shell.
The chicks then wiggle and rotate inside the pointed egg which applies pressure on the inside walls, causing fractures that radiate out from the initial pip hole. They use their egg tooth and beak to gradually enlarge the hole until they can fully emerge.
It takes guillemot chicks 24-72 hours to fully hatch out of their conical eggs. Their tapered shape likely adds some challenge compared to rounder eggs. But the chicks’ persistence allows them to successfully emerge from their strong, pointed shells.
Conclusion
In summary, guillemot eggs have a very unique pointed, pyriform shape that provides multiple advantages for cliff nesting auks. The tapered end allows the eggs to roll in a tight circle rather than off the cliffs. Their shape makes them difficult for predators to eat and also structurally strong to withstand their precarious breeding sites. Guillemot eggs maximize internal volume to nourish growing chicks. This exceptional shape likely co-evolved with the guillemots’ habit of nesting on narrow ledges high above the ocean. Their pointed eggs are a specialized adaptation that improves breeding success in the challenging habitat where they raise their young.