Improved fertility diagnostics could boost bird conservation breeding programmes, say scientists

A new analytical method is revealing how conservationists can further boost breeding programmes dedicated to saving some of the world’s most threatened species, according to new research led by the University of Sheffield and ZSL. 

Conservation zoos, and the global breeding programmes they facilitate, are vital to maintaining or restoring populations and boosting genetic diversity of threatened birds. But not every egg produced in a breeding programe hatches into a chick. 

New research by an international team of scientists - from ZSL’s Institute of Zoology, the University of Sheffield, University College London, University of Canterbury and the Department of Conservation in Aotearoa New Zealand - has shown that new diagnostic methods could help amplify the success of conservation breeding programmes, by offering a better understanding of why eggs don’t hatch.

When an egg fails to hatch, its exterior and contents are subject to a visual examination with the naked eye. This is known as a macrscopic examination. As part of the research, published in the journal Global Ecology & Conservation, the results of 174 macroscopic examinations were compared with an advanced fertility diagnostic technique involving a microscopic examination of the egg’s contents using fluorescence microscopy, which can reveal early signs of fertilisation not visible to the naked eye. 

The diagnostic technique using fluorescence microscopy found that 65.5% of 174 unhatched eggs studied had been fertilised. In comparison, the macroscopic examination suggested that only 5.2% of the eggs had been fertilised. The result of the study challenges previous understanding that egg hatching failure was due to eggs not being fertilised.

While the large cost of advanced fertility diagnostics makes it currently unsuitable for all eggs to be be studied through this technique, the research team hope this more accurate picture will further the success of breeding programmes, as conservation practitioners can continue to refine their management decisions - and research can focus on why embryos are not reaching maturity. 

The mean hatching failure rate of bird eggs is approximately 17%, with eggs not hatching primarily due to either not being fertilised, or the embryo failing to mature and survive. Writing in the paper, the scientists said fertilsation failure was rare, and the reason the eggs were not hatching was linked to early embryo mortality. 

Professor John Ewen, from ZSL’s Institute of Zoology and Sihek Recovery Program Team Chair, said: “Sihek are one of the most threatened birds in the world. Today’s entire population is descended from a tiny handful of just 29 birds, brought under human care as part of an emergency rescue mission. We’ve already seen great success, last year releasing birds into the wild for the first time in almost 40 years, but as we would expect for such a genetically limited population, eggs failing to hatch remains a major hurdle. This study unlocks essential knowledge that will help us do everything we can to safeguard Sihek and other highly threatened birds. We’re hopeful it will lead to even more tiny Sihek chicks hatching in the near future.” 

How the new knowledge is being used

From colourful kingfishers who have disappeared from remote islands, to Britain’s endangered wading birds losing key breeding grounds to human developments, hundreds of species of birds are currently at threat of extinction. The issue is set to worsen as threats such as climate change intensify and put key breeding and wintering grounds at risk.

A stronger, more nuanced understanding of why embryos might not be developing will help inform decisions from conservationists and zookeepers working with some of the world’s most threatened species, highlighting the importance of re-pairing male birds previously considered infertile in case they are more compatible with another female. 

Keepers at ZSL’s two conservation zoos are working with partners to continue testing select eggs to help inform management decisions of birds under their care. 

The research team behind the paper are already working with the Sihek Recovery Partnership, an international project of conservationists - including from ZSL - working to recover one of the world’s most threatened birds, a turquoise and cinnamon coloured kingfisher from an island in the Pacific, known as the Sihek*. There are only 127 Sihek left in the world, and it is classified as Extinct in the Wild by the International Union for the Conservation of Nature. 

Last year, nine Sihek - hand-raised within participating North American zoos with support of keepers from ZSL’s London and Whipsnade Zoos - were released into the wild, making them the first Sihek in the wild in almost 40 years. The release marked a significant step in the mission to re-establish a viable wild population of the birds. The research team will be working with the Partnership to support conservation work, with the hope of further increasing the breeding success of the species.