AUTHOR BLOG: Not Too Many Sperm, Not Too Few

MaleZB and LTF
Male Zebra Finch and Long-tailed Finch. Photo credit: L. Hurley


Laura Hurley

Linked paper: Variation in the number of sperm trapped on the perivitelline layer of the egg in three species of estrildid finch by L.L. Hurley, K.V. Fanson, and S.C. Griffith, The Auk: Ornithological Advances 119:4, October 2017.

When you crack open your morning egg, you see the familiar yolk with its little white circle staring at you. That little white circle, the germinal disk, is the target sperm are aiming for to fertilize the big yolky ovum, but in birds one sperm is not enough to turn the egg into a chick. Multiple sperm must fuse with the ovum for this to happen, so lots of sperm are present at fertilization, and those that don’t fuse can become trapped between the two delicate layers that surround the yolk.

Hope I didn’t ruin breakfast for you. However, too many sperm reaching the egg can cause the development of the chick to fail, so there’s a bit of a Goldilocks situation—just the just right number of sperm are needed. The size of bird eggs vary widely—from hummingbirds to emus—and so does the number of sperm that reach their ovum. In general, the number of sperm varies with body size, but there is a lot of unexplained variation between species of similar size, within species, and even within a clutch of eggs. In our current paper, we explore variation in three similarly sized birds from a family of Australian finches to help us better hypothesize about what could be influencing sperm numbers.

This is part of a larger body of avian ecology work looking at how genetic, social, and environmental factors influence and regulate reproduction, development, and population dynamics ( This work involves a number of Australian species in both wild and captive settings, including Gouldian Finch, Zebra Finch, Long-tailed Finch, and Chestnut-crowned Babbler, as well as the invasive House Sparrow. We also use historical records to build models to help us understand the life history of birds across the whole of Australia—for example, opportunistic breeding ( and nest structure (