Managing Crane Pair Formation at the International Crane Foundation

Abstract

U nderstanding . com~a.tibili~y among cranes m a pamng situation is a valuable component to crane captive breeding programs (Derrickson and Carpenter, 1987). By studying cranes at this point in their life cycle, one can better prevent future management problems and enhance opportunities for successful breeding of pairs. Managing newly forming crane pairs can be instantaneous and trouble-free or it can drag on for weeks, with no guarantee of strong pair-bond formation. After all, the cranes have the final say in whether or not they find attractive the mate one has chosen for them. The person in charge can best serve the cranes by understanding the socialization process through proper interpretation of their behavior. At the International Crane Foundation (ICF), we choose an approach conservative enough to ensure safety, yet flexible enough to deal with each unique pairing situation.

A description of basic crane biology puts the captive pairing process in context. First, we know that a female crane must normally be strongly pairbonded with a mate before she is physiologically capable of egg production. This makes sense in terms of energy conservation since successfully passing on her genes requires intensive investment by both male and female in:

• defending a breeding territory

• behavioral synchronization leading to development of the female reproductive tract

• copulation

• selecting and maintaining a nest site

• incubating the eggs

• rearing and protecting the chick(s) for up to 11 months

Cranes become sexually mature at

between 2-4 years of age depending upon the species. Males produce sperm at 2-3 years of age and will continue to do so outside of a pair bond (Mirande, Gee, Burke, Whitlock 1996). At around one and a half years of age, the time of fall migration for temperate species, cranes will begin to seek out potential mates. The staging areas along migratory flyways thus serve as densely populated gatherings where young or solitary cranes assume the business of mate selection.

ln captivity, these dynamics differ radically. As such, pairs are matched based on the three following criteria:

• Genetics-ensuring that future offspring will be genetically fit.

• Behavior-seeking birds with a complimentary balance of dominance/submission and confidence levels.

• Age/Rearing History-i.e. pairing a younger female with an older confident male may decrease the time to first laying. Wild caught birds may calm down more in captivity if paired with hand-reared birds (Derrickson and Carpenter, 1987).

Cranes are, as a general rule,

monogamous. They may spend many years paired together if they share a strong pair bond. As such, it seems logical that in the wild they would choose mates exhibiting traits conducive to long-term relationships. This is played out in a period of assessment during the pair-formation process (Nesbitt and Archibald, 1981; Mirande and Archibald, 1990; Nelson, Small, Ellis, 1995). In one study, marked female Florida Sandhill and Greater Sandhill Cranes, on average, associated with between 3.6 and 6.0 potential mates before developing long-term relationships (Nesbitt and Wenner, 1987). In captivity, however, when caretakers place cranes X and Y together, mate selection has already been taken care of. Therefore, we need to understand the process by which cranes express whether or not they will develop a strong pair bond.

The preferred strategy for pairing at ICF is called "Forced Pairing," resulting in 71% of pairs laying eggs as contrasted to the "Flock Separation" method, averaging a 25% laying success rate (Mirande and Archibald, 1990). With "Forced Pairing," the aviculturist acts as matchmaker and chaperon to the birds (Burke, pers. corn., 1992). The birds are first placed in a subdivided pen (total pen size at ICF is 50' x 60' with a 14' x 14' adjoining house). The female occupies the back half and the male occupies the front half (house interior is halved). This simulates the position taken by the male in the wild when defending a mate.

The male is placed in the pen a few weeks early to allow for the development of territoriality. With the male already settled in and the new female settling in, observations are made of their interactions through the divider fence from a nearby blind. Data collection is usually performed by continuous sampling for one hour periods.

Cranes are strong visual communicators, so if it first appears that one bird sees the other as an intruder and not as a potential mate (shown by the display of agonistic behaviors), a visual barrier such as tennis wind break netting is placed between them. This greatly reduces stress levels and risk of injury.

At this stage the birds may express themselves via a large repertoire of behaviors in an attempt to "feel each other out." Some cranes will indicate that they want to pair up immediately (thus showing few, if any, agonistic behaviors), while others may display aggressively for several weeks (poorer candidates for socialization at such a time). Expect to see some of the following behaviors ranging from low to high intensity. The frequency of most of these behaviors is generally greater at the onset of the socialization process and lessens with time. Some are performed simultaneously or sequentially.