Why do eggs fail? Causes of hatching failure in threatened populations and consequences for conservation

Abstract

Reproductive failure is ubiquitous. However, research on the mechanisms underpinning reproductive failure is still lacking in most species. This gap in our understanding has particularly strong repercussions for threatened species and it hinders our ability to establish effective interventions to improve survival. In this review, we focus on why eggs fail to hatchone of the most critical and understudied aspects of bird reproduction. We identify the main drivers of hatching failure in threatened populations of birds and the key mechanisms that cause failure at different stages of development inside the egg. We then discuss the importance of management interventions aimed at reducing hatching failure in species of conservation concern. Our review highlights the need for a better understanding of the mechanistic basis of hatching failure in non-model bird species and identifies the methodological tools necessary to achieve this.

Physiological factors influencing female fertility in birds

“…Similarly, if an egg fails to hatch but was fertilized, then the cause of hatching failure must be referred to as embryo mortality, even if development arrested after only a few cell divisions. If fertility status cannot be unequivocally determined using the appropriate techniques [ 13 ], then the mechanisms of hatching failure cannot be conclusively known. Very early embryo mortality is likely to be mistaken for infertility when using traditional methods (e.g.…”

“…8: 202274 after only a few cell divisions. If fertility status cannot be unequivocally determined using the appropriate techniques [13], then the mechanisms of hatching failure cannot be conclusively known. Very early embryo mortality is likely to be mistaken for infertility when using traditional methods (e.g.…”

“…Birds are well suited to the study of reproductive failure, primarily because—unlike mammals—they produce large, well-protected eggs which make them easy to examine externally before they degrade [ 11 ]. It is also possible to precisely determine whether a bird's egg failed because it was unfertilized or because the embryo died very early, using microscopic methods, but many studies still fail to make the distinction [ 10 , 12 , 13 ]. Avian reproduction science also benefits from a wealth of knowledge from commercial poultry research.…”

Nature calls: intelligence and natural foraging style predict poor welfare in captive parrots

“…This seems likely because, while reproductive problems can reflect stress [ 20 , 107 ], they have poor specificity as stress indicators, being affected by multiple other factors (including genetic bottlenecks [ 17 , 61 ], a lack of appropriate imprinting opportunities and artificial incubation (e.g. [ 117 ]), all factors that would add noise to our analyses which we could not parse out (so increasing chances of Type II error). Furthermore, recent evidence suggests that parrot species which commonly breed in captivity tend not to be invasive if released [ 102 ], consistent with them being smaller brained, and that birds become less encephalized with domestication (e.g.…”

“…Derrickson & Snyder [ 62 ] suggested that endangered species are harder to breed in captivity than their non-endangered relatives (their examples including whooping cranes, Grus americana , being harder to breed than their common relatives, greater sandhill cranes, Antigone canadensis tabida ). Their hypothesis was plausible in part because small captive populations are so vulnerable to genetic drift and inbreeding [ 17 , 18 , 61 ], and to shortages of compatible mates and other management issues [ 117 ]. Consistent with this, our one robust finding related to population size: species represented by only a few pairs in captivity had the poorest reproductive rates relative to natural fecundity.…”