Egg shape mimicry in brood parasites

Credit: laurent.breillat

Raising kids is hard work, costing you a lot of time, money and effort. But, what if you could drop off your kids permanently at someone elses place and let them handle the responsibility? For some species in the animal kingdom, that is exactly what they do.

​In fact, one percent of all birds (around 80 species) save themselves the effort of raising their young and cheat instead. These birds are called obligate brood parasites, laying their eggs in the nests of other species and leaving the hosts or “foster parents” to rear the foreign chicks for them.​

Given the substantial benefits of this reproductive strategy, obligate brood parasitism has independantly evolved 7 times among avian clades. You may be able to easily guess which eggs is foreign in a parasitised clutch, but it may not necessarily be so obvious to the host.

The most remarkable thing about brood parasitism is the birds’ ability to match the eggs of the host species in size, spottedness, background color, and darkness. This is known as egg mimicry. Brood parasite eggs well matched to its host eggs have a greater chance of survival than those that are poorly matched.

Egg shape is the least-studied component of egg mimicry although it has received more attention (but no evidence) in recent years. Underwood and Sealy (2006) found that bird species can recognize and eject objects from their nests based on their shape. Although birds can distinguish between different shapes, it was unknown whether brood parasites attempt to mimic the shape of their host eggs. 

For my project, I tested the following questions: 

1. Does egg shape and size mimicry occur between brood parasites and their hosts?

2.Does the accuracy of egg shape and size mimicry vary with nest type?

​We provide the first evidence in support of egg shape mimicry among cuckoos and their hosts and support previous experiments showing that hosts can discriminate and reject foreign eggs based on shape confirm that egg size mimicry can occur in cuckoos that exploit hosts with both open and closed nests. In contrast, poor visibility may limit the host’s ability to recognize foreign eggs based on their shape. 

Small egg size has evolved in many cuckoo genera possibly as an adaptation to parasitize smaller hosts. We suggest that hosts may use visual and tactile senses to distinguish between eggs based on their size.

We found that mimicry in egg shape between parasite and host eggs occurred in open nests but not closed nests. Both open and closed nests showed egg size mimicry. Credit: Marie Attard

How did we measure egg shape?

I photographed all eggs from parasitised clutches of three generalist cuckoo species to compare differences in egg shape and size between cuckoo-host pairs. Generalist parasites, which exploit a diversity of host species, face a challenge as they must overcome the potentially unique defenses of multiple host species. 

The brood parasites studied were the fan-tailed cuckoo exploits hosts with closed nests, the brush cuckoo exploits hosts with both types of nests, and the pallid cuckoo prefers to exploit hosts with open nests. Photographs were taken of the entire clutch of eggs, providing us with an egg silhouette. From this silhouette, we could create an egg outline to measure its shape and size.

PUBLICATION

Attard MRG, Medina I, Langmore N, Sherratt E (2017) Egg shape mimicry in parasitic cuckoos. Journal of Evolutionary Biology. 7(2): e31704.

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