Pointy Wings, Pointy Eggs: Birds’ Flight Abilities Influence Egg Shapes

Birds’ eggs come in a variety of shapes — but why?
(Credit: The Trustees of the
Natural History Museum, London.)

Birds’ eggs come in a variety of shapes

People have always been fascinated by birds’ eggs. These eggs, which are surrounded by a calcified shell, come in a variety of colors and sizes — and even shapes. The most familiar shapes range from spherical eggs that are typical of many owls, to symmetrically elliptical eggs seen in hummingbirds, and a plethora of asymmetrically pear-shaped (conical) eggs in shorebirds — and a diversity of variations in between.

Bird cliff at Orkney Islands, Scotland. (Public domain.)
Variations in avian egg shape.
Left to right: brown hawk-owl (Ninox scutulata) (Credit: derived from
a larger image by Klaus Rassinger und Gerhard Cammerer, Museum Wiesbaden / CC BY-SA 3.0); maleo (Macrocephalon maleo) (Credit: Arisdp / CC BY-SA 3.0); common murre (Uria aalge) (Credit: derived from a larger image by Didier Descouens / CC BY-SA 4.0). (Credit: GrrlScientist / CC BY-SA 4.0).

Avian egg shapes lie along a continuum from symmetrical spheres to asymmetrical cones

“From the beginning, this was an interdisciplinary collaboration of evolutionary biologists, computer scientists, mathematicians and physicists,” writes Professor Stoddard in email.

Fig. 1. Morphospace of avian egg shape.
Average egg shapes for each of 1400 species (black dots), illustrating variation in asymmetry and ellipticity. Images of representative eggs are shown alongside their associated points in morphospace (colored red) to highlight variation in shape parameters. (doi:
10.1126/science.aaj1945)
Eggs of the graceful prinia (Prinia gracilis palaestinae; MHNT 225 Jaffa) are sort of an “average representative avian egg”. (Collection of Jacques Perrin de Brichambaut)
(Credit: Didier Descouens / CC BY-SA 4.0)

Avian eggs shapes are not related to taxonomic groups

Are avian egg shapes related to avian evolutionary history? In a word, no.

Fig. 2. Partitioning of egg variation among avian orders.
Black dots (one per species) (n = 1400) show the morphospace of egg shape in two dimensions (asymmetry and ellipticity). Colored polygons show the minimum convex hull plotted for all species within a subset of avian orders (Strigiformes, Apodiformes, Suliformes, Passeriformes, and Charadriiformes). Overlap in egg shape is extensive, even among ecologically and phenotypically dissimilar orders. However, some orders (e.g., Charadriiformes) occupy a much larger region of egg morphospace than others. doi: 10.1126/science.aaj1945

Avian egg shape is related to flight ability

Professor Stoddard and her team compiled a database with biometric, life history and environmental parameters for all species in their sample, and they used this database to build a biomechanistic model. When they tested their model, they found that egg shape (asymmetry and ellipticity) is not related to clutch size, developmental mode, environmental factors or nest characteristics — but one biometric did consistently stand out: hand-wing index (HWI). HWI is a widely-accepted proxy for flight efficiency and dispersal ability in birds (ref & see below).

Linear measurements used to calculate the hand-wing index: (WL) ‘wing length’ from the carpal joint to the tip of the longest primary feather; (SL) ‘secondary length’ from the carpal joint to the tip of the first secondary feather. (Santiago Claramunt et al., 2011, doi:10.1098/rspb.2011.1922)

Egg shaping starts before the egg is laid

The researchers turned their attention to understanding the physical processes involved in building an egg within the avian oviduct. The oviduct is a tiny but greatly expandable tube through which an egg moves after it leaves the ovary. It makes sense that the maximum size or width of a stretched oviduct is constrained by a small, muscular, streamlined body plan. But egg volume can be maximized whilst egg girth is maintained or reduced by creating an elliptical or asymmetrical egg that can pass more easily through the narrow oviduct whilst still holding enough nutrients to support embryonic development. Consistent with this idea is that birds whose eggs are larger than expected for their body mass also produce more elliptically-shaped eggs.

The most aerodynamic birds tend to lay eggs that are elliptical or pointy

At this point, you may wonder why some birds’ eggs are elliptical whilst other birds lay asymmetrically conical eggs. Professor Stoddard and her team think that evolution may provide some insights. For example, elliptical eggs probably originated in reptiles, and may represent an evolutionary balancing act between large egg volumes required by precocial offspring, and a limited oviduct or pelvic opening that constrains egg girth.

Some eggs of the common murre (Uria aalge). (Credit: The Trustees of the Natural History Museum, London.)

Source:

Mary Caswell Stoddard, Ee Hou Yong, Derya Akkaynak, Catherine Sheard, Joseph A. Tobias, and L. Mahadevan (2017). Avian egg shape: Form, function, and evolution, Science 356(6344):1249–1254 | doi:10.1126/science.aaj1945

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