https://doi.org/10.1177/1758155920971823 Avian Biology Research 1–9 © The Author(s) 2020 Article reuse guidelines: sagepub.com/journals-permissions DOI: 10.1177/1758155920971823 journals.sagepub.com/home/avb Introduction The Saker Falcon Falco cherrug (hereafter Saker) has a Palearctic breeding distribution that extends across pre- dominantly grassland and desert biomes from central Europe to eastern China. 1 The species has undergone popu- lation declines over three generations (1993–2012), with a median estimate of 47% decline (mainly in the former Soviet states of Central Asia), although uncertainty regard- ing the scale of this decline has resulted in the species being categorized as ‘Endangered’ on the IUCN Red List as a precaution. 2 Nonetheless, large populations exist else- where in Central Asia, primarily on the Mongolian and Qinghai-Tibetan Plateaus, which harbour an estimated 62% of the global breeding population. 3 Monitoring to detect population distribution, size and trends in these extensive and remote regions is logistically complex, 4 lead- ing to efforts to identify potential breeding areas using cli- matic envelope modelling, 5,6 determination of effective population sizes using genomic approaches, 7 and assess- ment of population trends by monitoring constant study plots, 8 or artificial nests, 9 or by inferring mortality rates from breeding turnover. 10,11 Avian reproductive organs exhibit development changes with sexual maturation and temporal changes associated with the stages of the breeding cycle. 12,13 Birds that incubate eggs develop brood patches, defeathered areas of skin on the Seasonal variation in gonad physiology indicates juvenile breeding in the Saker Falcon (Falco cherrug) Andrew Dixon, 1,2,3 Janelle Ward, 4 Sarangerel Ichinkhorloo, 5,6 Tuvshinjargal Erdenechimeg, 5 Batbayar Galtbalt, 5,7 Batmunkh Davaasuren, 5 Batbayar Bold 5,8 and Nyambayar Batbayar 5 Abstract We describe seasonal changes in the physiology of reproductive organs of Saker Falcons electrocuted on an electricity power line in Mongolia. Macroscopic examination of the gonads revealed asymmetry in testes size, with bias to the left body side. This asymmetry declined with seasonal increase in testes size during the main egg-laying period of the breeding season. In females, 91% possessed both left and right ovaries (N = 161); there was no visible oviduct associated with the right ovary and it was smaller than the functional left ovary. Both ovaries showed seasonal development in size, becoming larger during the peak egg-laying period. Hierarchical preovulatory follicular development was recorded in two females, with ca. 5 mm difference in the diameter of sequential follicles. Both sexes developed brood patches during the main incubation period, with adults more likely to exhibit brood patches than juveniles. Among juveniles, at least 82% of females and 91% of males were non-breeders without brood patches. The high electrocution rate at our studied power line provided a rare opportunity to examine the non-breeding component of the Saker population. Juveniles predominated in the non-breeding population during the main egg-laying period (89%, N = 65), with the proportion of adults electrocuted being significantly lower among females. Only a small proportion of juvenile females exhibited gonadal evidence of breeding, consistent with the low observed frequency of juvenile breeders at nests. The demographic composition of the non-breeding population is consistent with female mortality rates exceeding that of males, and potentially indicates incipient population decline. Keywords Avian reproductive organs, avian testes, breeding age, ovarian follicles 1 Emirates Falconers’ Club, Abu Dhabi, UAE 2 International Wildlife Consultants Ltd., Carmarthen, UK 3 Reneco International Wildlife Consultants, Abu Dhabi, UAE 4 Wildlife Health Solutions, Papamoa, New Zealand 5 Wildlife Science and Conservation Center of Mongolia, Ulaanbaatar, Mongolia 6 Mongolian Bird Conservation Center, Ulaanbaatar, Mongolia 7 Centre for Integrative Ecology, School of Life and Environmental Science, Deakin University, Victoria, Australia 8 Key Laboratory of Animal Ecology and Conservation Biology, Institute of Zoology, Chinese Academy of Sciences, Beijing, China Corresponding author: Andrew Dixon, Reneco International Wildlife Consultants, PO Box 61741, Sky Tower, Al Reem, Abu Dhabi, UAE. Email: adixonwales@gmail.com 971823AVB 0 0 10.1177/1758155920971823Avian Biology ResearchDixon et al. research-article 2020 Article