The only surviving self-sustaining migratory population of wild whooping cranes nests in Wood Buffalo National Park (WBNP) in Northern Alberta. Their breeding grounds are an area of boreal spruce forest growing on narrow ridges. (Figure 1.) The adult whooper pairs that nest there protect their hatchling chicks in accordance with some natural traits that man does not fully understand. The adults, and especially the young, face many difficulties.
Every year WBNP’s staff and the Canadian Wildlife Service join together to do two aerial surveys on the whoopers nesting grounds. In June the first survey is made to count the number of whooping crane nest. The second survey is made in August to determine the number of chicks that fledged (capable of flying).
WBNP officials reported that during 2014 that the survey team counted a record of 82 nest in June and 32 fledgling whooping cranes during August. In 2013, the survey team spotted 74 nests in the spring and 28 fledglings later in the summer. “That gives a success rate of about 40% when compared to how many fledglings we have when compared with the number of nests they started out with in the spring”, according to Stuart McMillan, Manager, Resource Conservation, WBNP. The success rate is based on an assumption of the survival of one chick per nest (more on this later). The surveyors observed 28 pairs with one fledgling chick, and two families with two fledgling chicks during 2014. Friends of the Wild Whoopers regards the 32 fledglings as a very good survival level.
So, what happened to the nest or chicks that may have hatched between the June and August surveys? Sixty percent of the nest did not produce chicks or, if they did, those chicks did not survive until the August survey.
The Aransas/Wood Buffalo population of whooping cranes needs many things in order to survive and reproduce. They need suitable breeding grounds, a reliable food source, an unpolluted habitat, a safe migration path and a healthy wintering area. Based on biological evaluations, WBNP does provide suitable, unpolluted breeding grounds with a reliable food source. So what are the limiting factors that caused either a 60 % nest failure or chick deaths between the June and August surveys?
Eggs are normally laid in late April to mid-May, and hatching occurs about one month later. The incubation period is from 29 to 31 days. Around 90% of the clutches contain two eggs. Whooping cranes may re-nest if their first clutch is destroyed or lost before mid-incubation. Egg predation is believed to be uncommon at Wood Buffalo NP, and re-nesting has only been documented a few times.
There are several animals that prey on the eggs and young of whooping cranes. Some examples are the American black bear, wolverine, red fox, gray wolf, lynx, bald eagle, golden eagle and raven. Adult whooping cranes have very few predators because of their large size.While the listed critters are potential nest predators, they are rarely a threat to adult whoopers.
The cranes build their nests in shallow waters as one means of protection. While in water it is more difficult for predators to catch adult whoopers unaware. Yet, while on land, it is easier for stealthy predators to creep up on them. Overall impact of predation on recruitment remains uncertain.
Chicks are particularly vulnerable until they are about 3 months old when they fledge (begin to fly). They are even more vulnerable in dry summers when water levels are low and nesting areas are more accessible to predators.
Whooping crane parents remain alert for evidence of predators and alarm calls may be given at sight of large predators. And, the parents may approach and threaten or attack small predators such as red fox. They may also give a distraction display toward a large predator such as a bear or wolf. Yet, the best efforts of the adults are not always successful in saving their chicks from predators.
From the time whooping crane chicks are hatched to the time they are adults, chicks and juveniles face many hazards. They may face death from predators, injuries, inadequate food and sickness. Scientists working with the cranes have estimated that approximately one-third, or less, of wild whooping crane chicks survive from hatching to breeding age.
So, again, what has happened to the nest or chicks that may have hatched between the June and August surveys? The answer is that we do not know precisely. But a research study performed from 1997 to 1999 provides some of the best information available to help us understand. That part of WBNP where the whooping cranes nest and rear their young is a huge wetland area. It is difficult to move around in the wetlands and to cover sufficient territory to perform detailed biological studies to learn about whooping crane problems. So, what we do know at this time is based on a few, but very informative biological studies and general observations on a sample of the whooping crane population.
A study of the survival and death rates of whooping crane colts (chicks) in WBNP was accomplished by personnel of Parks Canada and Canadian Wildlife Service from 1997 to 1999. The purpose of the study was to provide baseline information on causes of chick mortality and to gain information on why few whooping crane twins survive. Researchers used intensive aerial monitoring, ground observations and radio-telemetry to monitor twenty-two whooping crane pairs that had twin chicks in WBNP.
Transmitters were attached to 18 chicks hatched by the 22 adult pairs. Five (28%) of the chicks fledged (capable of flying); five (28%) succumbed to cumulative effects (head trauma, stress, exposure and infection); four (22%) were lost to unknown causes (three of these went missing after they had lost their transmitters).
Research personnel believed two chicks (11%) had been taken by a fox (Vulpes vulpes), one (5.5%) was lost to raven (Corvus corax) predation, and one (5.5%) died of pneumonia. Out of the 22 sets of twins monitored, one set of twins survived the summer. On another occasion the older sibling was taken by a fox and the younger chick survived. This resulted in 9% of the smaller sibling chicks surviving. Chicks being monitored that went missing did so between 7 and 22 days following hatching.
Interestingly, the researchers observed that most often it is the younger (smaller) sibling chicks that go missing for several reasons. The first chick to hatch (from the 2 eggs in a nest) is at least 2 days older and is heavier. The older chick displays severe aggression by repeatedly pecking the younger one, sometimes causing injury (head trauma, stress and infection). Then, the family groups depart the nest pond soon after the 2nd egg hatches and is on the move for the first week following hatching. All of these factors work together in wearing down most of the smaller sibling chicks and within 2 weeks the younger chick has perished or is unable to keep up with the family group and gets abandoned. Such natural conduct is distressing for humans to comprehend.
Based on their study the researchers determined several factors work against twins whooping crane chicks surviving. These include direct causes like predation from foxes and ravens, and indirect causes such as trauma (which the researchers believe to be primarily the result of sibling aggression), exposure and infection.
The research team wrote: “We believe the following scenario may be common in WBNP. Once the second eggs hatches, the older sibling displays aggression towards the younger colt, this aggression can be severe at the nest site as the colts are confined to a small area. The adults depart the nest pond when the younger colt is approximately two days old and the physical demands involved in keeping pace with the family wears down the younger colt. Over time the younger colt becomes weaker, lags behind and becomes vulnerable to predators or eventually is abandoned. While this may be the case generally, we did have two occasions where the younger colt fledged. We believe that the second (younger) colt acts as insurance in case the older colt dies in the first critical weeks of life. We believe that every wild whooping crane recruited into the Aransas-WBNP wild population is critical to the survival of this population and the long-term viability of the species. We therefore recommend that it is necessary to leave nests in WBNP with two eggs and let natural selection work to increase the fitness of the only self-sustaining wild population of whooping cranes in the world.”
by Chester McConnell, Friends of the Wild Whoopers
***** FOTWW’s mission is to help preserve and protect the Aransas/Wood Buffalo population of
wild whooping cranes and their habitat. *****