Avian Conservation and Management

INTRODUCTION

Concerns about biodiversity loss have been increasing in recent years (Ceballos et al. 2015; International Union for the Conservation of Nature: https://www.iucnredlist.org/). Reintroductions are attempts to introduce species to parts of their historical ranges where they were extirpated, to reestablish disappeared populations (IUCN/SSC 2013). The number of published examples in which reintroductions have been used as a conservation tool has increased rapidly since the late 1990s (Seddon et al. 2007). Reintroductions have been highlighted as a potential solution to species loss due to anthropogenic-induced climate change (Armstrong and Seddon 2008, Polak and Saltz 2011, Morandini and Ferrer 2017). However, the success rate of these attempts has not been high (Morris et al. 2021). Difficulties have often been encountered with long-distance dispersal by released animals, which causes reduced growth or unfavorable composition in the reintroduced population during the early establishment phase (Armstrong and Seddon 2008, Richardson et al. 2015b, Berger-Tal et al. 2020). High prevalence and incidence of long-distance dispersal across taxa are suggested to be a significant problem that urgently needs to be addressed (Bilby and Moseby 2024). Dispersal of the released individuals is chiefly dependent on three factors: stress or confusion from unsuitable release procedures and environmental changes, a lack of conspecifics at the release sites, and exploratory behavior to find more suitable habitats and breeding sites (Stamps and Swaisgood 2007, Richardson et al. 2015b, Berger-Tal et al. 2020). If conservation practitioners and researchers do not address these factors, prolonged and long-distance dispersal can occur with released individuals (Stamps and Swaisgood 2007).

Release techniques and individual characteristics are important factors in preventing long-distance dispersal (IUCN/SSC 2013). Hard and soft release techniques have primarily been used for reintroductions. The hard release technique involves animal release immediately after transportation to the release site, whereas soft release allows the animal to acclimatize to the release site’s environment before release. A soft release could also include the provision of resources such as food, shelter, or refugia to the target individuals. Soft releases offer ample opportunities for released individuals to adapt to unfamiliar environments and reduce stress caused by release procedures, environmental changes, and access to food and other resources in the new sites (Stamps and Swaisgood 2007). Furthermore, they are predicted to enhance social cohesion, especially in cases in which multiple birds were housed in the same enclosure (Horwich 2001). In contrast, hard releases can cause greater dispersal, notably in birds that are released into unfamiliar environments without acclimatization (Nagata and Yamagishi 2016). Although soft releases seem to be a better technique than hard releases, a phylogenic meta-analytical approach indicated that both techniques had similar effects with respect to birds (Resende et al. 2021).

Extensive research has focused on age as a significant factor influencing post-release movement in various species (van Heezik et al. 2009). In territorial birds, adult individuals tend to establish exclusive territories, whereas juveniles disperse over longer distances than adults, primarily driven by their exploratory behavior (Krüger et al. 2014, Miller et al. 2017). In Common Terns (Sterna hirundo) and House Sparrows (Passer domesticus), older birds exhibit higher tolerance to stress and confusion resulting from environmental changes than do their younger counterparts (Heidinger et al. 2008, Lendvai et al. 2015). Therefore, releasing older birds is expected to mitigate dispersal. The origin of individuals, whether they are wild or captive-bred animals, has also been discussed as a factor influencing the post-release movement of released individuals (Crates et al. 2023). The perception of stressors can vary considerably between wild and captive-bred individuals (Jones and Merton 2012). Richardson et al. (2015a) emphasized the advantages of soft release for birds, particularly when dealing with captive-bred individuals. Similarly, Jones and Merton (2012) recommended that captive-reared individuals undergo soft release with support, whereas wild-caught individuals should be released immediately.

Various studies have independently examined release techniques and age to prevent post-release dispersal, yielding diverse levels of effectiveness (Clarke et al. 2002, van Heezik et al. 2009, Mitchell et al. 2011, Bradley et al. 2012, Hardouin et al. 2014). However, the optimal release strategy varies among taxa. Therefore, conducting releases in an experimental way can help to identify the optimal strategy. We hypothesized that if release techniques and appropriate age selection are not implemented for captive-bred territorial birds, the released individuals disperse over greater distances. Specifically, the hypothesis suggests that younger birds subjected to hard release are susceptible to various dispersal factors. Both hard and soft releases of older birds could result in their vulnerability due to the lack of conspecifics. Furthermore, soft release of younger birds could induce exploratory behavior. Therefore, we assumed that long-distance dispersal is likely in younger birds subjected to hard release. Understanding post-release site fidelity in reintroduced individuals is crucial for effective reintroduction management strategies (Berger-Tal and Saltz 2014). Even if the dispersal distance is small, a large home range can still hinder reintroduction success. Variation in post-release behavior may stem from simplified captive environments compared to those in the wild (Crates et al. 2023). However, the specific rearing procedures that influence post-release movement remain unclear.

The Oriental Stork (Ciconia boyciana) is classified as an endangered bird species (BirdLife International 2018). In Japan, the wild breeding population of the Oriental Stork was declared extinct in 1971 due to a substantial decline caused by factors such as overhunting, habitat loss, pollution, and inbreeding within the remaining population (Ezaki et al. 2013). To address this issue, we imported > 60 birds as founders between the 1960s and 2000s from Chinese zoos and natural habitats in Russia (Matsumoto 2020). In 2005 and onwards, captive-bred birds (1–7 years old) were released into Hyogo Park of the Oriental White Stork (HPOWS) as part of efforts to restore the former species range. Both soft and hard release techniques were employed, and some birds were equipped with satellite transmitters before release. Our study aimed to assess the effects of various release techniques, ages, and rearing histories on the dispersal area and distance of released Oriental Storks in Japan. Minimizing post-release dispersal is crucial for the advancement of reintroduction biology (Armstrong and Seddon 2008). The results obtained by evaluating different release methods provide valuable insights for enhancing release protocols and contribute to the formulation of effective conservation strategies.

METHODS

Subject birds

The Oriental Stork source population originated from migratory birds that breed in the Russian Far East and migrate to southeastern China for overwintering (Shimazaki et al. 2004, Fan et al. 2020). Oriental Storks are long-lived species that attain maturity at a minimum age of two years (Ohsako et al. 2012). We released individuals based on pedigree diversity through captive breeding at HPOWS and zoos. Most of the released or fledged Oriental Stork individuals were marked with color bands to facilitate individual identification. We focused on 18 subject birds (10 males and 8 females) that were captive-reared at HPOWS (Appendix 1). The sex of birds was determined through DNA analysis of blood samples (HPOWS, unpublished data).

Release site and year

The release sites were located within the breeding area that was previously inhabited by the Oriental Stork population (Fig. 1; Hyogo Prefectural Board of Education and Hyogo Park of the Oriental White Stork 2014). Hunting of wild Oriental Storks is currently prohibited by law, “stork-friendly farming” has been implemented (farming that does not rely on chemical fertilizers and pesticides), and artificial nest poles have been installed around the release site. The site is predominantly characterized by paddy fields, with secondary forests in the surrounding landscape. To minimize the potential risk of accidents for released storks, e.g., collisions with artificial structures due to disorientation caused by rapid environmental changes or lack of familiarity with the wild, we deliberately selected an open area with few artificial structures and little pedestrian traffic. To mitigate the risk of storks being attacked by conspecific species, we selected areas located outside the territories of such species. To facilitate the acclimatization of storks during soft releases, we selected paddy biotopes > 1 ha, providing year-round feeding grounds. The timing of release during the day was selected when the weather was not too stormy.

The release sites were all within a 40 km radius. We soft-released 11 birds between 2006 and 2018 (one male and one female in 2006 and 2007, three males and one female in 2013, one female in 2017, and two males in 2018; Appendix 1). The birds left the enclosure within the first few days after opening the roof net. We hard-released seven birds (one male and two females in 2006, two males and one female in 2007, and one female in 2018).

Release techniques and ages

Over five years following the experimental release in 2005, the reintroduction plan was designed to test four release methods: (1) soft release of first-year birds, (2) soft release of one- to three-year-old birds, (3) hard release of one- to three-year-old birds, and (4) captive females held in an open-roofed cage to allow them to breed with males living in the wild to release the fledglings. Although the first three methods were effectively implemented, the fourth method was executed only once because of an unexpected attack on the released female by a wild male on the release day. After the release techniques and ages had been decided, individuals were selected for the reintroduction plan. Details of release techniques, ages, and bird selection are included in Appendix 2.

Soft release of first-year birds

We followed a specified release process (Fig. 2). The initial step involved foster parent-rearing to the fledgling stage. We transferred two eggs from the captive-bred parent birds in HPOWS to the foster parent birds in the enclosure for soft release approximately 25–29 days after laying. The second step consisted of the soft release phase. After determining that the fledglings were flight-ready, the roof nets were opened, allowing the fledglings to exit freely. Throughout the first month, the roof remained open, and food was provided to support their transition.

Hard release of first-year birds

We predominantly subjected first-year birds to soft release. However, we also included one bird (214; Appendix 1) that underwent a hard release during its first year to compare its dispersal patterns with those of the soft-released individuals. The initial step for this bird was the same as in the soft release method used for first-year birds. The second step involved a hard release procedure. We placed the stork in a wooden transport box measuring 930 × 430 × 1050 mm. The bird was then transported by car to the designated release site, where it was released individually into the open field.

Soft release of one- to three-year-old birds

The first step involved a similar process to the soft release of first-year birds, with the exception that some birds were reared by their birth parents. The second step consisted of artificial rearing and training. After completing the initial step, the birds were transferred to individual cages, shared cages with multiple birds, or a training cage. Throughout this second step, the birds underwent multiple transfers between individual or shared cages and the training cage. Over a period of approximately 300–650 days, the birds were trained together with other birds to develop their flight ability, conspecific response, and foraging skills. The specific protocols followed during the training phase are outlined in Appendix 2. The third step involved acclimation. The birds were relocated to an enclosure designed to acclimate to the external environment, where they resided for at least three months after training. The fourth step encompassed the soft release. During this stage, we opened the roof nets of the enclosure, allowing the birds to exit freely. Supplementary feeding was provided if birds returned to the enclosure within the first month.

Hard release of one- to three-year-old birds

The first and second steps of the hard release process for one- to three-year-old birds were identical to the corresponding steps in the soft release method. The training was conducted for approximately 100–500 days. The third step involved the hard release of individuals. After training, the birds were reared in individual cages for some days and then hard released. The protocol followed was the same as that used for the hard release of first-year birds.

Transmitter attachment

To attach transmitters, all focal birds were captured by two staff members. We captured the birds by hand or used a fishing net with a diameter of approximately 60 cm. Storks were selected for transmitter attachment to minimize any negative effects. Transmitters were attached at least seven days before the birds were released to determine whether there were any problems with the transmitter or the individual. We used a solar-powered GPS Argos transmitter (Solar Argos/GPS 70 g PTT, Microwave Telemetry, Columbia, Maryland, USA). The size of the device was 9.70 × 3.84 × 2.49 cm, the length of the antenna was 17.78 cm, and the mass was 70 g. The total weight was 90 g, including the attachment, which was approximately ≤ 3% of the body mass (3700–5550 g) of the subject birds. The transmitter was attached using a custom harness made of tubular Teflon ribbon (width 19 mm, mass 17.8 g). Details of the transmitter attachment protocol are included in Appendices 2 and 3.

Positioning interval and duration

The devices acquired six global positioning system locations per day at 1-h (N = 8 transmitters) or 3-h (N = 10 transmitters) intervals and transmitted the locations via Argos every three days. Positions were recorded between 06:00 and 18:00 local time with a 12-h off-duty cycle. We used only five positions per day, derived from both transmitters at different intervals. We calculated the straight-line distance between two locations using QGIS Desktop 3.10.1 (QGIS Development Team 2019). Erroneous locations were filtered based on the maximum flight speed of 112 km/h of the related Black Stork (Ciconia nigra; Chevallier et al. 2010).

Classification of subject birds

Our subject Oriental Storks fitted with transmitters were classified into two groups according to age (first-year-olds = younger; one- to three-year-olds = older), and two based on rearing methods. Therefore, four combinations of release techniques and ages were used: S0 = soft release of first-year-olds (five males, two females), H0 = hard release of first-year-olds (one female), S1-3 = soft release of one- to three-year-olds (two males, two females), and H1-3 = hard release of one- to three-year-old birds (three males, three females).

Dispersal area and distance

We used a 25% kernel density estimate (KDE) and a 50% minimum convex polygon (MCP) to calculate the core area after release. An MCP of 50% has often been used in previous research on the home range of storks (Zurell et al. 2018, Xu et al. 2021). However, not only core areas, but those connecting core areas, have often been included with the MCP. To identify accurately the core areas frequently used by released birds, we employed both the MCP and KDE. Although a KDE of 50% is commonly used to measure the core area (Laver and Kelly 2008), we selected a KDE of 25% because a KDE of 50% included too many areas uninhabitable by storks, such as the ocean. We defined the dispersal distance as the straight-line distance from the release site to the center of gravity for the core area. When an individual had several core areas, the distance between the release site and the center of the core area containing the largest number of positions was used as the dispersal distance, and the sum of each core area size was used as the core area size. We computed a KDE of 25% and an MCP of 50% for each individual distribution as the core area using the statistical analysis software R version 3.6.2 (R Core Team 2019) and the package adehabitatHR version 0.4.16 (Calenge 2006).

Statistical analysis

The kernel density estimate was only used in the statistical analysis for dispersal because the core areas of released stork occurrence were fitted more accurately by the KDE than the MCP (Appendices 4–8). To determine the factors affecting core area size and dispersal distance, we used a generalized linear mixed model with gamma distribution and a log-link function, in which the explanatory variable candidates were: release technique (hard or soft release), age at release (first-year or one- to three-year-old birds), and sex. Individual identity was included as a random effect. Model selection was performed using the Akaike information criterion (AIC). The model with the smallest AIC was selected as the simplest, best-fitting model. Because the captive rearing history and release age were correlated, we analyzed the effects of captive rearing history on core area size or dispersal distance separately from the preceding analyses. Captive rearing history focused on the number of transfers between cages and training days, the maximum number of individuals reared together, and the number of released birds raised by foster parents or birth parents. To compare the effect of the birds’ rearing parents (birth or foster parents), we conducted a Mann-Whitney U-test. Statistical analyses were performed using R version 3.6.2 (R Core Team 2019) and the packages lme4 version 1.1.23 (Bates et al. 2015), MuMIn version 1.43.17 (Bartoń 2020), and car version 3.0.7 (Fox and Weisberg 2018).

RESULTS

Factors affecting core area size and dispersal distance

In terms of the core area size, the AIC identified the full model as the best fitting, in which only the effect of release age was significant (Table 1). The model indicated that the core area size was smaller for birds released as one- to three-year-olds than for birds released in their first year (Fig. 3). For both KDE and MCP, S0 was divided into two groups, which were larger than those for H0, S1-3, and H1-3 (Fig. 3).

In terms of dispersal distance, the AIC also identified the full model as the best-fitting model, in which only the effect of release age was significant (Table 2). The model indicated that dispersal distance was shorter for birds released as one- to three-year-olds than for birds in their first year (Fig. 4). For both KDE and MCP, the dispersal distance of S0 varied substantially among individuals (Fig. 4). However, the dispersal distance of S1-3 and H1-3 were relatively short, and their individual variation was relatively low.

Three of the eighteen subject birds started breeding during the study period, including one individual from the S1-3 (ID:391) and two individuals from the H1-3 groups (ID:384 and ID:389). Their core areas were relatively small (Fig. 3), and their dispersal distances were relatively short (Fig. 4).

Correlations between rearing history and core area size or dispersal distance

We found marginally significant (P = 0.05–0.1) negative correlations between core area size and the number of transfers between cages (r = −0.44, P = 0.06), the maximum number of individuals living together (r = −0.45, P = 0.06), and the number of training days (r = −0.42, P = 0.09; Fig. 5). Core area size tended to be smaller in released birds raised by birth parents than by foster parents (Mann-Whitney U-test, Z = 1.68, P = 0.09).

Dispersal distance was negatively correlated with the maximum number of individuals living together (r = −0.47, P = 0.05; Fig. 6). We also found a marginally significant negative correlation between dispersal distance and the number of transfers between cages (r = −0.46, P = 0.06), but dispersal distance and the number of training days were uncorrelated (r = −0.32, P = 0.36). Dispersal distance was smaller for released birds raised by birth parents than by foster parents (Z = 2.04, P = 0.04).

DISCUSSION

We assumed that long-distance dispersal and large dispersal area of post-release Oriental Storks are more likely to occur in H0 than in the other three groups. However, we found that the core area sizes and dispersal distances were not clearly greater in H0 than in the other groups. Therefore, our hypothesis was only partially supported.

Effects of release age

The statistical models indicated that release age was the most influential factor. In another carnivorous, territorial bird species, the Barn Owl (Tyto alba javanica), wild juveniles were observed to disperse farther from the release site than were older individuals. This behavior was attributed to the juveniles’ inexperience in hunting live prey and searching for suitable areas with abundant prey (Saufi et al. 2020). In contrast, a study of dispersal rates in soft-released Kokako (Callaeas wilsoni) showed no significant differences between adults and subadults at 7 days post-release (Bradley et al. 2012). Reintroduced Kokako are attracted to the playback of conspecific songs (Molles 2008). In species in which conspecific attraction plays a crucial role, variation in dispersal between younger and older individuals may be smaller because younger birds prioritize proximity to conspecifics over finding better feeding sites or establishing their own territories through dispersal. Therefore, selecting older individuals, preferably older than fledglings, for release could be an effective strategy to prevent long-distance dispersal in species that are relatively unattracted to conspecifics or are territorial breeders, such as Barn Owls and Oriental Storks.

Effects of release technique

A dispersal study of wild Barn Owls using five combinations of release techniques and ages, i.e., hard-released adults, hard-released juveniles, soft-released juveniles, soft-released adults, and soft-released hand-reared fledglings, only identified soft-released adults in the study area for > 30 days, indicating that the other release methods were ineffective (Saufi et al. 2020). Another study of adult, captive-reared, territorial Burrowing Owls (Athene cunicularia hypugaea) released in pairs concluded that soft-released birds were more likely to stay at the release site, for up to two weeks after release, than were hard-released birds (Mitchell et al. 2011). In both Barn Owls and Burrowing Owls, individuals that remained closer to the release site had either bred or formed bonded pairs during the pre-release acclimation period. In some cases in our study, both male and female Oriental Storks were released simultaneously; however, the 3-month pre-release acclimation period may have been too short for pair bonding. A study of a similar species with comparable life history traits, captive-reared Crested Ibis (Nipponia nippon) adults, examined trip distances up to six months after release and found that soft-released birds remained closer to the release site compared to hard-released ones (Nagata and Yamagishi 2016). Notably, Crested Ibis is a colonial breeder (Wingfield et al. 2000), whereas Oriental Stork is a solitary nesting species. These findings suggest that soft release may be more effective when birds are released in pairs or are susceptible to conspecific attraction.

Effects of release sex

We did not detect any sex differences in dispersal area and distance, possibly due to the different ages of the study birds. However, in a study of reintroduced Crested Ibis, adult birds showed sex differences in dispersal (Nagata and Yamagishi 2016). First-year females had longer mean dispersal distances (189.16 km²) than males (112.73 km²), similar to the natal dispersal pattern of White Storks (Chernetsov et al. 2006). In contrast, among one- to three-year-old birds, males had longer mean dispersal distances (7.22 km²) than females (5.68 km²).

Effects of rearing history

Captive rearing methods appeared to influence the breeding behavior of Crested Ibis post-reintroduction (Okahisa et al. 2022). Therefore, examining rearing method characteristics that affect post-release dispersal behavior is crucial. We found that individuals that were neither transported nor trained had only cage mates as their family (i.e., foster parents and two juveniles), were reared by foster parents until fledging, and had larger core areas and longer dispersal distances. Their rearing histories corresponded to those of soft-released first-year birds. Therefore, rearing histories can be an important factor in determining dispersal area and distance. However, we could not distinguish between release age and rearing method to determine which factor contributed more to dispersal. Therefore, future research is needed to investigate the relative contributions of these factors to dispersal.

Comparison with the wild population

Wild Oriental Stork juveniles, 4–21 days after fledging, exhibit individual variation in their MCP 50% home range (0.29–6.7 km²; Xu et al. 2021). Similarly, the core area of storks released in their first year varied significantly among individuals (ranging from 34 to 11,000 km²). Although substantial individual differences in the core areas are characteristic of this species, the differences were more pronounced in released captive-reared birds compared to their wild counterparts. This result might be attributed to the extended tracking period of the study birds.

The founders of Japanese-reintroduced Oriental Storks migrate from the breeding areas of the Russian Far East to southwest China along the coast from late July to late October. They then stay in wintering areas until late March (Shimazaki et al. 2004, Fan et al. 2020). Interestingly, we found that some first-year birds had core areas far to the west of the release site during the wintering period. Although these birds may exhibit similar migration behavior to the founders, further studies are needed to understand the relationship due to our relatively small sample size.

Effects of artificial feeding

Many reintroduced individuals depended on food intended for captive birds in the open cages of HPOWS. Three S0, four S1-3, and two H1-3 birds established their core areas to include HPOWS despite the small area (Appendices 5–8). Thus, food stealing could influence core area size and dispersal distance. HPOWS identified and recorded the occurrence of reintroduced individuals in open cages during the feeding of captive individuals. The dispersal areas and distances of one- to three-year-old birds were small and less variable, but their occurrence in open cages varied greatly from 0 to 86%. The occurrence of first-year birds in the open cage varied considerably depending on dispersal distance: 0% for the three long-dispersed birds, and 0–65% for the seven short-dispersed birds, varying on an individual basis. Therefore, feeding dependency in the open cage did not influence the dispersal of released storks.

Effects of bird density

Density-dependent effects also determine dispersal behavior (Matthysen 2005). Variations in the dispersal area and distance of the released first-year birds have been increasing annually, possibly related to the population size around the release site (Deguchi et al. 2022). The daily average number of occurrences of reintroduced birds in the open cage may be an indicator of the population density around the release area (11.6 birds in 2013, 9.39 birds in 2017, and 9.33 birds in 2018). The dispersal area and distance of released first-year-old birds in those years varied substantially among individuals released in the same year (area and distance, respectively: 34.58–11,002.42 km² and 0.13–474.03 km in 2013, 6017.11 km² and 338.85 km in 2017, 56.30–110.05 km² and 28.70–228.51 km in 2018). Therefore, density dependence may not have affected the dispersal of first-year birds. This intraspecific variation needs to be studied further in the future.

CONCLUSION

Based on our findings, we conclude that one- to three-year-old Oriental Storks had smaller and less variable dispersal areas and distances than did first-year birds. Hard-release techniques may be less problematic for Oriental Stork reintroduction if one- to three-year-old birds are selected for release. To prevent long-distance dispersal in species with similar ecological characteristics, such as being carnivorous, territorial, and unattracted to conspecifics, hard release of one- to three-year-old birds is recommended. However, our study was limited by a small sample size, and we emphasize the need for further research on other species with similar or different ecological traits to develop effective strategies for preventing long-distance dispersal in reintroduction efforts.

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