Post-fledging dispersal and habitat use of a reintroduced population of the Crested Ibis (Nipponia nippon)

Dispersal is one of the most important life-history traits for animals. The study of dispersal has received increasing attention from animal ecologists during the past two decades (Clobert et al. [2001], Barbraud et al. [2003]). Knowledge of dispersal movements is fundamental to our understanding of many ecological and evolutionary processes, as well as for the design of successful conservation strategies (Paradis et al. [1998]). It is often defined as the permanent movement an animal makes from its natal site to the place where it reproduces or would have reproduced if it had survived and found a mate (Howard [1960]). Greenwood and Harvey ([1982]) suggested distinguishing between “natal dispersal” and “breeding dispersal”, the latter referring to the movements undertaken by adults between breeding sites. Post-fledging dispersal refers to the movements of the young after they become independent of their parents and leave their natal territory to move to wintering areas (Bull et al. [1988]). In the avian life-cycle this stage is poorly understood, but limited studies have documented post-fledging periods lasting up to two months (Morton [1991]; Baker [1993]). For migratory birds, some investigators have assumed that once the young disappear from their natal site they begin a slow movement to the south (Bent [1953]; Pulich [1976]). The young of resident species have been shown to stay in the general vicinity of where they were hatched, but will move into different habitats (Nolan [1978]; Rappole and Ballard [1987]; Bocetti [1993]).

The Crested Ibis (Nipponia nippon) was once one of East Asia’s most widespread and well-known birds, but deforestation and destruction of habitat has led to successive extinctions of Russian, Korean and Japanese populations in the 20th century (Yamshina [1967]; Archibald et al. [1980]; Yusuda [1988]). The Chinese population was presumed extinct for two decades preceding the 1981 discovery of seven birds in Yangxian County, Shaanxi Province (Liu [1981]). During the past 30 years, a great deal of conservation effort, both in-situ and ex-situ, has been undertaken by researchers and the local government. The wild population in China has increased to approximately 800 and more than 700 captive-bred individuals now reside in eight protection centers located in China, Japan and Korea. This rare species is now being reintroduced to protected sites within its historical range, with the goal of establishing stable, secure populations that may eventually qualify the species for delisting (Yu et al. [2009]).

The first reintroduction was at Zhaigou village, Ningshan County in China’s Shaanxi Province. From May 2007 to October 2011, 56 captive-bred individuals (29 males and 27 females) were released into the wild (Li et al. [2013]). From these birds, 14 breeding pairs have, so far, produced 46 fledglings. With a 62.5% survival rate for released birds, this first reintroduction program in China has had a promising start (Li et al. [2011]). Post-fledging dispersal is the original stage for juveniles, laying the foundations for the entire dispersal process, which to a certain extent, also determines the following series of the dispersal process. Therefore, the study of post-fledging dispersal can contribute to the study of the entire dispersal process, necessary to mark out successfully the strategies to protect the reintroduced populations. Also, the study of habitat utilization of the Crested Ibis helps us to understand the use of spatial resources and the effect of their variety on the movements of the birds. Post-fledging dispersal patterns and habitat utilization during the breeding period by the Crested Ibis in the wild have been studied recently (Yu et al. [2010]; Liu et al. [2003]), but dispersal behavior and habitat relationships in the reintroduced population remain unclear. Therefore, we designed the current study in order to explore the population dynamics and the habitat utilization of a reintroduced population of the Crested Ibis, attempting to provide theoretical and practical bases for reintroduction and conservation.

The Crested Ibis establishes a small breeding territory of no more than 50 m in diameter (Shi and Cao [2001]). The species is not highly territorial. To establish a new population, it is essential for released birds not only to survive successfully during the establishment phase of pairing and breeding, but also to disperse successfully into wintering areas. The onset of juvenile dispersal is usually more or less arbitrarily defined on the basis of some estimation of the typical size of the territory of adults (Ferrer [1993]; Walls and Kenward [1995]). Juvenile dispersal is characterized by an initial wandering and exploratory phase that contrasts with the restricted pre-dispersal movements. Such contrast is likely to be identifiable as a period of increased variability in the distance to the nest and hence could be used to identify the onset of juvenile dispersal. Thus, Soutullo et al. ([2006]) used a criterion of distance to determine the onset of dispersal. We monitored movements of the Crested Ibis following release using banding and radio-telemetry. Specifically, we sought 1) to document distance and direction of post-fledging dispersal, 2) to determine changes in habitat use between natal and dispersal sites, 3) to compare dispersal patterns between released and wild population in Yangxian County studied by Yu et al. ([2010]) and 4) to propose management suggestions for conservation and future release.

The Crested Ibis was widespread and locally abundant in Northeast Asia in the late 19^th century, when it nested in the Russian Far East, Japan and mainland China. It was also presumed to be a winter visitor to the Korean Peninsula (BirdLife International [2001]). The surviving wild population in Yangxian County and the reintroduced population in Ningshan County are residents that make only relatively short, seasonal movements between nesting and wintering sites (Shi and Cao [2001]; Yu et al. [2010]). However, in the past, the former population of Siberia (eastern Russia), northeast China, and Hokkaido (northern Japan) are known to have migrated south in winter (Yamshina [1967]; Archibald et al. [1980]; Yusuda [1988]). The Crested Ibis perhaps exhibits different patterns of post-fledging movements in their former and present ranges. We have no way of measuring the exact distance they travel to their wintering grounds at the Korean Peninsula, Taiwan or Hainan Island from the former breeding sites in Russia and northeastern China, but estimates suggest that this distance exceeds hundreds of kilometers.

In our study, the dispersal distances of the 25 individuals that completed the entire process of post-fledging dispersal were less than 16 km, suggesting that the post-fledging dispersal distance in the first year was highly correlated with the distance between suitable breeding and wintering habitat areas. In comparison, the natal home ranges of fledglings of the reintroduced population were occupied for significantly shorter periods than that of the wild population (21.1 ± 10.2 days, n = 36; t = 3.047, p = 0.004). Most fledglings moving southwest were concentrated at the mean direction (μ = 254.6°, š = 70.5°, n = 25) while fledglings of wild birds dispersed in a southerly direction (189.2 ± 46.5°, n = 36) with a mean dispersal distance of 20.3 ± 7.0 km (n = 36) (Yu et al. [2010]). The overall pattern of post-fledging dispersal of juveniles is similar for the wild and reintroduced populations, but the fledglings of the wild population dispersed four times as far as the reintroduced individuals. These findings are very useful for us to predict the distance and direction of dispersal under various landscape conditions of other released sites.

The process of avian dispersal begins when an individual fledges from its nest and is then guided by a series of decisions: where to move, what to eat, with whom to pair and ultimately where to attempt breeding. Post-fledging dispersal is one of the most important stages in the life-history of birds. Only upon finding a mate and attempting to breed can the dispersal process be regarded as complete. In our study area, juvenile ibis birds moved an average of 5.1 km away from their natal territory during the first year of their life. It took a minimum of two years for fledglings to become sexually mature. Sexual monomorphism in Crested Ibis fledglings makes gender assignment difficult to distinguish during the post-fledging dispersal period. Our observations of the dispersal of ibis fledglings indicate that they are not moving randomly. Similar to wild fledglings (Yu et al. [2010]) and resident Golden Eagles (Aquila chrysaetos; Soutullo et al. [2006]), the juvenile dispersal of our population reared in captivity is typically characterized by an initial exploratory and nomadic phase followed by repeated returns to the vicinity of the natal area. Siblings of each brood increasingly became independent of their parents at the beginning of August and all fledglings routinely joined ibis flocks that included other non-breeders in post-dispersal areas. Breeding opportunities may be more likely in these dispersal flocks than they would be if these young birds were to remain with the family group. In dispersal flocks, individuals may be capable of competing for new territories, thus becoming active breeders by dominating these new areas (Zack and Stutchbury [1992]). The fall/winter flocking period provides an opportunity to interact with possible mates and appears to be very important for sustaining the surviving population. We observed that, within dispersal flocks, adults frequently forced juveniles to roost on peripheral branches through a ritualistic pecking behavior. New pair formation occurs often following the flocking period in late winter and early spring (Shi and Cao [2001]).

According to the optimal foraging theory, the distribution and temporal use of habitat varies due to habitat quality (Lester et al. [2007]). The period that territories were occupied by the reintroduced population were 14.3 ± 7.0 days (n = 25) and 21.1 ± 10.2 days (n = 36) for the wild population (Yu et al. [2010]), which appears to reflect differences in food availability at different nesting sites. The nestlings of the Crested Ibis fledged approximately from mid-June to mid-July and were prevented from foraging in rice fields due to the inaccessibility of the flourishing rice shoots. Therefore, fledglings and their parents had to move to feed in alternate habitats such as rivers and grasslands during this period. In the following months, they returned to their main foraging sites (paddy fields; Table 2). From our study, we speculate that the proximate cause for dispersal of the Crested Ibis is increasing food availability during the conversion process of habitat, but potential pairs and pairing formation eventually might be found in colonies during the dispersal process and thus achieve population continuity, which is the ultimate factor for the post-dispersal of the birds (the remote cause).

Since 1999, great success for both the wild and captive populations has led some ornithologists to a more positive view regarding long-term reintroduction goals, restoring the species to its former range (Zheng [2000]; Litvinenko [2000]). To date, reintroduction programs have been conducted in 2007 in China (Yu et al. [2009]) and in 2008 in Japan ([Nishimiya and Hayashi]). The eggs failed to hatch on Japan’s Sado Island, but the worldwide program still achieved one of its first goals as new pair formations continued to occur and the number of fledglings produced continued to grow in our study area, Ningshan County (Yu et al. [2009], Li et al. [2011]). Recent investigation of the historical range for this species suggests that regions south of the Qinling mountains should be prioritized for future release efforts. Conservation of migratory endangered species entails preserving adequate breeding, stopover and winter habitats. This may therefore pose greater challenges for managers than resident species do (Serra et al., [2011]). Prior to future release in Siberia and the demilitarized zone (DMZ) at the Imjingak Pavilion in Korea, an extensive investigation should be conducted, because the condition of their former breeding ranges, wintering grounds, and stopover sites remains unknown.

In our study, eight recorded mortalities were split evenly between dependent and independent fledglings. Based on the temporal distribution of dead Crested Ibis birds of the reintroduced population in Ningshan (Li et al. [2013]), the time of death of the juveniles was concentrated in the May-August period, including the period of natal home ranges during post-fledging dispersal. Out of nineteen mortalities incurred among released adults during the 6-year period (three in 2007, three in 2008, four in 2009, three in 2010, two in 2011 and four in 2012), eight of them died from starvation, two were lost due to electrocution by power lines, four died due to predation or injuries attributable to predation (likely due to martens Martes flavigula, a small-size Mustelidae mammal) and one died from unknown causes (Li et al. [2013]). These findings indicate that starvation, especially in winter, can be a significant cause of mortality. We propose therefore the following suggestions to management.

The local government should promote a strategy to harmonize people and nature with a particular focus on sustainable agricultural practice that encourages local farmers to cultivate more rice fields within Crested Ibis release areas. The ibis, its habitat and the local communities will all benefit. Food (mainly loach, Misgurnus anguillicaudatus) should be supplemented to the rice fields where ibis regularly forage, especially during the winter when the temperature is below 0°C. Public education should be implemented to inform local people on the purpose of ibis conservation and on how to get involved. The effects on ibis populations of illegal hunting for frogs, fishes and commercial activities nearby shallow rivers should be made clear and all relevant resource protection laws should be strictly enforced.

The project carried out in Ningshan County, Shaanxi Province is a good example for reintroducing endangered species to their historical ranges. Compared with the wild population in Yangxian County, the juveniles produced from the released population exhibited a similar pattern of post-fledging dispersal. Our results are very useful for us to predict the distance and direction of dispersal under various landscape conditions of other released sites. The food availability during the conversion process of different types of habitat may be the proximate cause for dispersal, and potential pair formations which achieve population continuity are probably the ultimate factor for the post-fledging dispersal. These findings will be valuable for the protection of reintroduced populations of this critically and other endangered species.