1. How Big and Small Can Bird Eggs Be? From Coffee-Bean Size to the Ostrich Egg
The largest egg among living birds is the ostrich egg. Encyclopedias list it with a long axis of about 15 cm (about 6 inches), a short axis of about 13 cm (about 5 inches), and a weight of about 1.6 kg (about 3.5 pounds). Field guides show a range of 1.3 to 1.9 kg (about 2.9 to 4.2 pounds), so the numbers vary by source [7]. Imagine holding an egg heavier than a 1 kg (about 2.2 pounds) bag of sugar; it seems difficult to handle. A small bird example is the bee hummingbird, found only in Cuba. As one of the world's smallest birds, its egg is described as being the size of a coffee bean [9].
The record for the largest egg belongs to an extinct bird. Guinness World Records lists the egg of the Aepyornis, which lived in Madagascar, as the largest, with a maximum length of 33 cm (about 13 inches) and a capacity of 8.5 liters (about 2.2 gallons), which is about 7 ostrich eggs [8]. That is more than eight 1-liter milk cartons. This figure is the maximum value within a large estimated range, so not every egg was this size. Even just in size, there is a huge difference. However, bird eggs are not uniform in shape either.
2. What Did a Study of 50,000 Eggs Show About Egg Shape and Flight?
In this study, egg shape was expressed using two measures: the degree to which one end is more pointed than the other (asymmetry) and how elongated it is (ellipticity). This allows eggs from round to pointed to be placed on a single scale [1]. The shapes were compared with the birds' food and nest locations. The factor that showed a correlation across a wide range of birds was flight power. The results indicated that birds better at flying tend to lay more asymmetrical or elongated eggs [2].
Japanese media reports often divide egg shapes into four types: chicken-like, pear-shaped (for seabirds), circular (for owls), and elliptical (for ostriches). It is explained that birds with pointed wingtips and high flight power are more likely to lay elongated, pointed eggs, while gliding birds like owls are more likely to lay round eggs [3]. However, this four-category division is a media classification; the research itself expresses shape continuously using the two measures mentioned above. This raises a question: how does egg shape connect to flying?
3. Why Might Flying Birds Lay Asymmetric or Elongated Eggs?
The explanation provided by researchers is as follows. Flying birds need to keep their bodies streamlined to reduce air resistance. Because it is difficult to widen the body's width, it is thought that pointed and elongated eggs are more convenient for producing large eggs. This is because the volume of the egg can be increased without increasing the width of its widest part [1]. For example, if the maximum width that can pass through the body is fixed, you cannot widen the side-to-side dimension, so lengthening the egg is one way to increase the contents. An elongated, pointed egg fits this method. However, this is just one explanation offered by researchers [1]. It is not a result that confirmed how flying birds accommodate eggs inside their bodies. The study showed only that flight power and egg shape correlate broadly [2]. So, what reasons have been given for the pointed egg of the common murre, known for pointed eggs?
4. Did the Common Murre's Pointed Egg Evolve to Stop Rolling Off Cliffs?
Common murres breed on cliff ledges, and their eggs are pear-shaped and pointed. It has long been explained that if they roll, they only move in a circle and are less likely to fall off the cliff, or that they spin like a top and stop. In 2017, a group led by Birkhead actually investigated this. The eggs did not spin like tops. While it was confirmed that they roll in an arc, the impact on whether they fall off the ledge was found to be small [4].
In the same year, the same group investigated the relationship between egg volume and pointedness in two species of murre. The relationship was weak, and volume could explain only about 3% of the differences in shape. This result casts doubt on the long-held idea that "preventing rolling was the main reason for the shape" [5]. Some Japanese reports introduce the idea that they are "pointed to prevent rolling," but the conclusion of the papers is that this theory has been re-evaluated. This article follows the wording of the papers. If it is not for preventing rolling, why are murre eggs pointed?
5. What Idea Replaced the Rolling Theory for Murre Eggs?
As a candidate to replace the rolling theory, an idea focusing on the crowding of murre breeding sites was proposed. Common murres breed in very dense colonies. Therefore, evidence has been shown that fits the idea that pear-shaped eggs are strong against impacts from other individuals or are protected from dirt such as droppings and soil [6]. It is important to note that this is the stage of "evidence fitting the idea has been shown." It is not settled. The research showed that the most widely known explanation is unlikely to apply, and that another explanation is possible. Looking at it this way, egg shape may overlap with a broad tendency related to flight power and circumstances suited to each species' lifestyle. The study examining the broad tendency and the study examining one bird in detail answer different questions.
6. How Can You Measure Egg Elongation with Eggs from Your Kitchen?
Try lining up 3 eggs from your home on a desk. Compare the pointed end and the round end, and measure the length and the width at the widest part with a ruler. Dividing the length by the width gives you a rough guide to elongation. This is a simple guide for this article, different from the scale used in the research, where a larger number means a more elongated egg. See if eggs from the same pack differ slightly. If they are hard-boiled, you might try rolling them gently on a flat surface. See if they go straight or draw a circle. It will not necessarily result in the same outcome as a murre's egg, but you can learn how to look at rolling patterns. To prevent them from falling on the floor, do this with an adult, at a low height [4][5].