An in-depth exploration of the problems associated with hatching failure in birds, and the methods used to identify it, ahead of the Science and Conservation Event “Why do eggs fail?” on Tuesday 11 May.
Reproduction can be a costly process, representing a massive amount of investment on behalf of the parents, but producing offspring which go on to reproduce themselves is also often the only source of evolutionary fitness for many organisms. It follows that reproductive failure should be strongly selected against, and hence should be reasonably uncommon. However, in birds, on average 10% of all eggs laid fail to hatch, with much higher rates occurring in some populations of threatened species. For example, the California Condor (Gymnogyps californianus), Little Spotted Kiwi (Apteryx owenii), and Pink Pigeon (Nesoenas mayeri) have all experienced hatching failure rates of around 65%.
While many eggs may be lost to external factors such as predation, extreme weather, or abandonment, all of which can be understood and often even managed to some extent, questions remain around eggs that are incubated to term successfully but still fail to hatch. Suggested causes of these failures include environmental, genetic, disease-related, and behavioural drivers. Failed eggs have either experienced embryo mortality (where the embryo dies at some point between fertilisation and hatching), or they were never fertilised in the first place. However, eggs in this latter group can sometimes be hard to identify. If an embryo dies before visible signs of development become apparent, usually around 72 hours after fertilisation, the egg may be almost indistinguishable visually from an unfertilised egg and may be misidentified as having experienced fertilisation failure. Similarly, eggs which have failed to hatch can start to break down and become ‘addled’, which can disguise the earliest signs of development and cause such eggs to also be mislabelled as ‘infertile’. As different drivers, or combinations of drivers, are likely to result in an egg failing to be fertilised versus experiencing embryo mortality, it is important to establish the fate of an unhatched egg, in order to identify the potential drivers and apply the most appropriate mitigation strategies.
In a method using fluorescence microscopy, the germinal disc (a white spot on the surface of the yolk where the embryo will develop from if the egg is fertilised) and perivitelline layer (a transparent layer surrounding the yolk) are stained with Hoechst dye and examined under a microscope (Image 3). This method enables an investigator to locate sperm cells (to determine if sperm reached and penetrated the ovum – Image 4) and embryonic cells (to determine if fertilisation and embryogenesis – the development of the embryo – occurred). One study looking at eggs diagnosed as ‘infertile’ from five endangered species used such fluorescence microscopy to show that only 26% of these eggs had actually experienced fertilisation failure. Another study applying these methods to eggs of Blue Tits and Great Tits showed that with non-microscopic inspection alone, 50% and 32% of unhatched eggs respectively could have been misidentified as being unfertilised.
Very few studies accurately distinguish between fertilisation failure and embryo mortality using these, or similar, methods, but their results show that the assumption that undeveloped eggs are unfertilised is unreliable. This could have implications for our overall understanding of life history in wild populations, but has perhaps even more serious implications for both captive and free-living managed populations. For example, captive-breeding programmes of threatened birds often carefully monitor the breeding success of pairs and apply management interventions to mitigate issues and improve reproductive outcomes. A pair consistently laying ‘infertile’ eggs may end up being split up and paired with new mates based on the assumption that the male is not producing sperm or is otherwise having issues successfully fertilising the eggs. However, if these eggs are not actually unfertilised, but have instead experienced embryo mortality, a different issue may be at play which might not be resolved by remating the birds. It is similarly important to accurately establish a male’s fertility status through the presence or absence of sperm in eggs during selection of birds for a translocation event, as inclusion of an infertile male could threaten the translocation’s success.
Recent applications of these fluorescence microscopy methods include examining unhatched eggs of the Critically Endangered Kākāpō (Strigops habroptilus) in which 61% of eggs fail to hatch. 72% of visually undeveloped eggs were found to be fertilised, leading to reassessment of the previous attribution of undeveloped eggs to male infertility. The methods are also being applied to the Vulnerable Hihi (Notiomystis cincta), which has undergone a series of translocations to secure predator-free islands resulting in the successful establishment of multiple populations. A recent management question for this species regards whether gene flow can be ‘topped up’ or maintained between these separate populations by moving eggs, rather than adults or juveniles. As Hihi experience a hatching failure rate of 35% this work will help to capture the baseline information needed to model how plausible such egg translocations could be, as well as the impact on both donor and recipient populations.
Join the upcoming ZSL Science and Conservation Event ‘Why do eggs fail?’ on Tuesday 11 May to hear more on this topic from world experts, including some of the reasons that lead to female reproductive failure, and how the methods discussed might be beneficial to the captive-breeding programmes which play a key role in conservation of threatened species.
Select a blog
Our people are our greatest asset and we realise our vision for a world where wildlife thrives through their ideas, skills and passion. An inspired, informed and empowered community of people work, study and volunteer together at ZSL.
At ZSL, a key area of our work is the employment of Nature-based Solutions – an approach which both adapt to and mitigates the impacts of climate change. These Solutions, which include habitat protection and restoration, are low-cost yet high-impact, and provide multiple benefits to people and wildlife. We ensure that biodiversity recovery is at the heart of nature-based solutions.
A blog for lovers of ZSL London Zoo, bringing you extraordinary animal facts and exclusive access to the world's oldest scientific zoo.
Do you love wildlife? Discover more about our amazing animals at the UK's biggest zoo!
We're working around the world to conserve animals and their habitats, find out more about our latest achievements.
From the field to the lab, catch up with the scientists on the cutting edge of conservation biology at ZSL’s Institute of Zoology.
A day in Discovery and Learning at ZSL is never dull! The team tell us all about the exciting sessions for school children, as well as work further afield.
Every month, one of the pieces held in ZSL’s Library and at ZSL Whipsnade Zoo will feature here as Artefact of the Month.
Read testimonials from our Members and extracts from ZSL's award winning members' magazine, Wild About.
ZSL works across Asia, from the famous national parks of Nepal to marine protected areas in the Philippines. Read the latest updates on our conservation.
An Open Access journal for research at the interface of remote sensing, ecology and conservation.
