Reintroduction of Captive-Reared Birds the Influence of Hand-Rearing and Release Techniques on Behavior and Survival in Three Species of Corvidae

Publication Date


Type of Culminating Activity


Degree Title

Master of Science in Raptor Biology



Major Advisor

John Marzluff


As recently as the early 1900s, the 'Alala or Hawaiian Crow (Corvus hawaiiensis) was abundant throughout forested areas on the leeward southern and western side of the island of Hawai'i (Baldwin 1969). Prior to the 1993 breeding season, the 'Alala was one of the rarest birds in the world with only 11 birds alive in the wild (Kuehler et al. 1995). The wild 'Alala population declined from multiple human-caused factors, including habitat reduction, exotic predators, and exotic diseases (Duckworth et al.1992).

In 1991, the U. S. Fish and Wildlife Service requested that the National Academy of Sciences, National Research Council's Board on Biology establish the Committee on the Scientific Bases for the Preservation of the Hawaiian Crow (here after referred to as the NAS committee; Duckworth et al. 1992). The NAS committee acknowledged that to expand the 'Alala population into historical, but now unoccupied ranges, and to substantially increase the number of crows in the wild, captive propagation and soft release reintroductions would need to be used. Soft-release reintroductions have many advantages over less invasive forms of management (i.e., protection and monitoring of the remaining population) they include: captive-reared birds can be released in areas where the present rare population does not exist; nestling and fledge-age mortality can be reduced by rearing birds in captivity; and pre-release quarantine periods allow birds to be screened for diseases and parasites (Duckworth et al. 1992). However, the NAS committee also addressed three disadvantages of captive propagation and soft-release reintroductions: 1) they are labor-intensive and expensive; 2) captive-reared birds might not learn the essential social and survival skills needed to survive after release; and 3) captive-reared birds held in captivity too long might lose the ability to adjust to life in the wild after release.

In 1993, a research project was initiated in Boise, Idaho to experimentally test these three concerns using three species of Corvidae as surrogates. Surrogate species, a common, taxonomically similar species with ecology similar to the rare species, were used to experimentally test the effectiveness of a soft-release method prior to the release of the rare species (Scott and Carpenter 1987; Louda 1994). Common Ravens (Corvus corax), American Crows (Corvus brachyrhynchos), and Black-billed Magpies (Pica pica) were chosen as surrogate species because they were abundant in southwestern Idaho and bracket the 'Alala in body size and social structure (Killham 1985; Bruggers 1988; Birkhead 1991). For example Black-billed Magpies and American Crows are smaller, while Common Ravens are larger than the Alala in body size. There is little information on the Alala's social structure prior to their decline. However, today's small population appears to have a social structure more similar to the Black-billed Magpies; i.e., they form smaller foraging and roosting flocks and remaining within a territory throughout the year.

The NAS committee's first concern regarding captive propagation and soft- release reintroductions was that they are very labor-intensive and expensive methods. Typically, birds are raised in captivity by parent-rearing (where the natural parents or a foster conspecific rears the young chick), cross-fostering (where the chick is reared by an adult ofa closely related species), or puppet-rearing (where the chick is reared in isolation by a human caretaker, using a parent-modeled puppet; Horwich 1989; Wallace 1994). In previous captive propagation attempts with the 'Alala, young chicks were raised by caretakers using the puppet-rearing method (Kuehler et al. 1995). To find a less labor- intensive and less expensive method of rearing young corvids in captivity, I used Common Ravens in southwestern Idaho as surrogates to experimentally test the differences in rearing social, altricial birds with a puppet (captive-reared birds were isolated from human contact) or without a puppet (captive-reared birds were allowed full visual and vocal contact with caretakers). This study forms the basis of Chapter 1.

The NAS committee's second concern with captive propagation and soft-release reintroduction was that captive-reared birds might not learn the essential social and survival skills after release. Because the 'Alala, like most other corvids, is a very social bird with an extended post-fledging dependency period (Munro 1960), this was a valid concern. Kleiman (1996) suggested that to prepare captive-reared animals for life after release, they could be paired with an experienced wild-caught individual, called a tutor, prior to release. To test this form of pre-release training, I used Common Ravens, American Crows, and Black-billed Magpies as surrogates to determine ifraising a corvid with a wild, older, conspecific tutor would increase social behaviors and survival after release compared with corvids reared without a tutor. This question forms the basis of Chapter 2.

The NAS committee's last objective with captive propagation and soft-release reintroductions was to determine the most appropriate age to release captive-reared birds. They were concerned that if captive-reared birds were held too long in captivity, the birds would lose the ability to adjust to natural conditions after release. Because both social and innate learning may occur during certain critical age-related phases or windows (Klinghammer and Hess 1964; Bateson 1979; Staddon 1983), this was a valid concern. Therefore, I tested the effect of age at time of release as a factor in both Chapter 1 and Chapter 2. I compared pre- and post-release behaviors and survival of captive-reared corvids released at fledge-age (in July) versus those released at post fledge-age (in November).

The NAS committee anticipated that this experimental work with surrogate species in Idaho would begin to address their concerns with captive propagation and soft- releases for the 'Alala. Little did the committee realize this information would benefit another tropical corvid.

The Aga or Mariana Crow (Corvus kubaryi) is another rare tropical corvid that has declined in recent years and may benefit from this surrogate research. The Aga population on the island of Guam has declined in recent years, yet for a very different reason than the 'Alala's decline. After World War II, the brown tree snake (Boiga irregularis) was accidentally introduced to the island of Guam via surface cargo movements of surplus U. S. military equipment (McCoid 1994). By the early 1970s, Guam's avifauna began to decrease significantly in distribution and density (Savidge 1987). However, these population decreases were blamed on disease and pesticides. It was not until Julie Savidge's work in the late 1980s when she determined that the cause of these declines was predation by the introduced brown tree snake. By that time, seven of the ten forest bird species native to the island of Guam had been extirpated, and the Aga population on Guam had dwindled to 100 birds (Michael 1987).

In 1994, the U.S. Fish and Wildlife Service and the Guam Division of Aquatic and Wildlife Resources (GDA WR) decided the current recovery efforts for the Aga were inadequate to save the Guam population. They determined aviculture support and reintroduction would be a necessary management tool in the recovery of the wild Guam population. Two of their concerns, similar to the NAS committee's, were that captive propagation and reintroduction typically 1) are labor-intensive and expensive, and 2) captive-reared birds might not express various wild behaviors, such as appropriate social and foraging skills (unpubl. report, GDA WR 1994).

Therefore, I anticipated that this experimental surrogate research would answer concerns of captive propagation and reintroduction methods for not one, but two endangered tropical corvids. Reintroduction efforts for rare species are often criticized for their single-species approach. Despite the many valid rebuttals to this argument (ethical, economic, and aesthetic; Carpenter 1983), this surrogate research goes beyond a single species approach. It is my hope that this research not only will directly benefit the , Alala and the Aga but also will encourage other researchers and managers to explore new methods of captive propagation and reintroduction. If restoration and conservation biologists are to justify the enormous resources needed to restore a rare species, we must be sure it will be a successful recovery.

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