Increasing Variability and Declining Breeding Success: A Major Challenge in the Conservation of the Critically Endangered Ardea Insignis (White-Bellied Heron)

Understanding the ecological and demographic factors influencing reproductive success is essential for effective conservation planning. This is particularly critical for species like Ardea insignis (White-bellied Heron), which is critically endangered and survives in fragmented populations. We monitored 3-4 active breeding pairs in Bhutan from 2016 to 2025, focusing on the first egg-laying date and reproductive outcomes. Although all nests were spatially dispersed (7-70 km apart), pairs showed high site fidelity for multiple years. Nests were primarily built on broad-leaved trees near major river channels, at elevations of 280-540 m, often located in proximity to the confluence of the main river and a major tributary. The nests were in secluded areas with minimal disturbance, although not exclusively away from human settlement. Most nests were located on the opposite bank from human settlements, in narrow valleys where fast-flowing rivers serve as physical barriers to human approach. We observed variation in the first egg-laying dates from year to year, with this variability increasing in recent years. We also observed that some pairs consistently initiated early breeding and achieved higher success rates, while others experienced unpredictable outcomes. Overall, clutch sizes varied from 2 to 4 eggs with a mean of 3.10 +/- 0.52 SD, hatchlings ranged from 0 to 4 with a mean of 1.55 +/- 1.23 SD, and fledglings also ranged from 0 to 4 per nest with a mean of 1.12 +/- 1.24 SD. We recorded renesting attempts following early clutch failures; however, all these attempts were unsuccessful. An extremely small population with increasing variability and unpredictable breeding output suggests that the population is at risk of extinction. We recommend management actions that prioritize the protection of all breeding pairs and critical habitats, as well as further research on breeding phenology and reproductive performance. Ardea insignis (White-bellied Heron) is one of the rarest and most critically endangered herons, and a species of significant global conservation concern. Fewer than 60 mature individuals are estimated to exist in the wilds of Bhutan, Northeast India, Myanmar, and China, with a few active breeding pairs recorded only in Bhutan and Northeast India. We observed variability in breeding timing, with first egg laying dates ranging from as early as February 9 to as late as June 13 within a similar breeding habitat. Successful pairs tend to maintain long-term bonds, exhibit high site fidelity, and consistently initiate early breeding, achieving higher success rates, while others experience unpredictable outcomes. We observed that at least 50% of the pairs that failed in the first attempt re-clutched, especially if the nest failed at an early stage. However, none of the replacement clutches (from renesting attempts) succeeded in fledging any chicks between 2016 and 2025. Nest failures were mainly caused by egg predation by monkeys and mesocarnivores, disturbances, late nesting, sexual conflicts, infanticide, chicks falling from the nest, adult mortality, and hazardous weather. Comprender los factores ecol & oacute;gicos y demogr & aacute;ficos que influyen en el & eacute;xito reproductivo es esencial para una planificaci & oacute;n eficaz de la conservaci & oacute;n. Esto es particularmente cr & iacute;tico para especies como Ardea insignis, que se encuentra en peligro cr & iacute;tico y sobrevive en poblaciones fragmentadas. Monitoreamos 3-4 parejas reproductivas activas en But & aacute;n entre 2016 y 2025, enfoc & aacute;ndonos en la fecha de puesta del primer huevo y en los resultados reproductivos. Aunque todos los nidos estaban espacialmente dispersos (separados entre 7 y 70 km), las parejas mostraron alta fidelidad al sitio durante varios a & ntilde;os. Los nidos se construyeron principalmente en & aacute;rboles de hoja ancha cerca de los principales canales fluviales, a elevaciones de 280-540 m, a menudo ubicados cerca de la confluencia del r & iacute;o principal con un tributario importante. Los nidos se encontraron en & aacute;reas aisladas con m & iacute;nima perturbaci & oacute;n, aunque no necesariamente alejadas de asentamientos humanos. La mayor & iacute;a de los nidos se ubicaron en la ribera opuesta a los asentamientos humanos, en valles estrechos donde r & iacute;os de corriente r & aacute;pida act & uacute;an como barreras f & iacute;sicas que dificultan el acceso humano. Observamos variaci & oacute;n entre a & ntilde;os en las fechas de puesta del primer huevo, con un aumento de esta variabilidad en los & uacute;ltimos a & ntilde;os. Tambi & eacute;n observamos que algunas parejas iniciaron consistentemente la reproducci & oacute;n de forma temprana y lograron mayores tasas de & eacute;xito, mientras que otras experimentaron resultados impredecibles. En general, el tama & ntilde;o de puesta vari & oacute; entre 2 y 4 huevos con una media de 3.10 +/- 0.52 DE, el n & uacute;mero de polluelos eclosionados vari & oacute; entre 0 y 4 con una media de 1.55 +/- 1.23 DE, y el n & uacute;mero de volantones tambi & eacute;n vari & oacute; entre 0 y 4 por nido con una media de 1.12 +/- 1.24 DE. Registramos intentos de re-nidificaci & oacute;n luego de fracasos tempranos de la puesta; sin embargo, todos estos intentos fracasaron. Una poblaci & oacute;n extremadamente peque & ntilde;a con variabilidad creciente y resultados reproductivos impredecibles sugieren que la poblaci & oacute;n est & aacute; en riesgo de extinci & oacute;n. Recomendamos acciones de manejo que prioricen la protecci & oacute;n de todas las parejas reproductivas y de los h & aacute;bitats cr & iacute;ticos, as & iacute; como investigaciones adicionales sobre la fenolog & iacute;a reproductiva y el desempe & ntilde;o reproductivo.