Solving the Current Challenge:
Working for Successful Whooping Crane Nesting in Wisconsin
Prepared 2010; updated 2011
The number of young hatched and successfully raised by whooping crane pairs in the reintroduced population is below expectation with most pairs abandoning first nests after egg laying. Here is a review of nest productivity since the first reintroduced whooping pairs began nesting in Wisconsin, the studies that the Whooping Crane Eastern Partnership initiated, and the study results to date. Included is an explanation of the Bti treatment study that began during the spring of 2011.
History of Nesting Efforts
- 2005: First year that pairs in the eastern migratory population nested. Two nests were built and two eggs laid, but both eggs were lost.
- 2006: First successfully fledged chick in the eastern migratory population. Five nests built, all five nests were unsuccessful. One pair renested (#11-02 and #17-02) and hatched two chicks. One of the chicks (#W1-06) survived to fledging.
- 2007: Four unsuccessful nests. One pair renested, but this nest was also unsuccessful.
- 2008: 11 unsuccessful nests.
- 2009: 12 unsuccessful nests. Five or six pairs re-nested (it is unknown if there was a sixth renest). Two chicks hatched, but both chicks died within a month.
- 2010: 10 unsuccessful and 2 successful first nest attempts of the year, 2 unsuccessful and 2 successful second nests and 1 successful third nest. Seven chicks hatched to five sets of parents (two pairs had two chicks). One of the chicks that hatched was from a captive-produced egg. Two chicks, #W1-10 and #W3-10 (from the captive-produced egg), survived to fledging.
- 2011 – 20 first nests and two renests. Four nests hatched out four chicks, but none of the chicks survived to fledging.
Only one, naturally occurring, wild flock of whooping cranes exists. It breeds at Wood Buffalo National Park (Wood Buffalo NP) in Canada then migrates to Aransas National Wildlife Refuge (Aransas) on the Texas Gulf Coast, where it spends the winter. Our knowledge of whooping crane ecology is based on studies of this flock. Our expectations for breeding behavior and productivity in the eastern migratory whooping crane population also come from observations of this flock.
Wild Whooping Crane Breeding Ecology
Whooping cranes tend to form pairs when two to three years old. They mate for life but will take a new mate if the original mate dies. Paired cranes arrive at Wood Buffalo NP in late April and begin nest construction. They show considerable fidelity to their breeding territories, and normally nest in the same general vicinity each year. Several pairs have nested in the same areas for 22 consecutive years.
Wild whooping cranes may start laying eggs as early as three years of age although the average age of first egg production is usually five years old.
Whooping cranes generally nest annually, but may skip a year when nesting habitat conditions are unsuitable, or if they are nutritionally stressed, or for other reasons. For example, in 2005, 12 out of 70 known adult pairs failed to nest in Wood Buffalo NP.
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 percent 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 uncommon at Wood Buffalo NP, and renesting has only been documented a few times.
Whooping crane parents share incubation and brood-rearing duties. Except for brief intervals, one member of the pair remains on the nest at all times. Females tend to incubate at night and take the primary role in feeding and caring for the young.
Although they may lay two eggs, most pairs are not able to successfully rear both chicks. Only about 10 percent of whooping crane families arriving on their winter territories at Aransas have two chicks. The second egg in a clutch may provide insurance that at least one chick survives. In nests with two eggs, the first chick hatched has the greater chance of survival. Habitat conditions, including food availability and predator abundance, affect survival. In years when habitat conditions are good, crane pairs may raise two young.
Hypothesis Development and Testing:
Whooping cranes eggs covered with black flies.
Photo by USFWS; Richard Urbanek
We have proposed a number of hypotheses to explain the high rate of nest abandonment in the eastern migratory population. It is quite likely that more than one factor is causing the nest abandonment and that those factors may be interacting. Additionally, there are always random events that can disrupt nesting during any given year. The Whooping Crane Eastern Partnership has the complicated task of teasing out the interacting factors at the heart of the problem as distinct from random events that may also have disrupted nesting. The Partnership remains cautious and will not jump to conclusions about connections between observations of the birds and other factors like weather and black flies. Some relationships may appear to be cause and effect but may be unrelated or coincidental. The Partnership does not want to make important decisions based on relationships that turn out to be a coincidence.
Another constraint the Partnership faces is the small number of breeding pairs, which makes valid research conclusions difficult to reach.
In 2009 seven of 12 first nests failed within a 36 hour period. This suggests that an environmental factor contributed to the abandonments (e.g., limited food resources, biting insect harassment, disturbance). Some aspect of diet (i.e., quality, quantity, timing of availability), environmental temperatures, and the nesting inexperience of these relatively young birds may be contributing factors.
Biologists monitoring nesting whooping cranes observed large numbers of black flies on some of the incubating cranes during the time that the nests were abandoned, which lead to a cause and effect hypothesis. Black fly harassment as the cause of nest abandonment can be scientifically tested, by removing or reducing the number of black flies present and observing the outcome. As part of a two-year experiment, the Partnership will suppress black flies while monitoring nesting pairs. This experiment does not call for long-term suppression of black flies in the area.
The nest abandonment pattern in 2009 was similar to what has been observed in previous years. WCEP began research in 2009 that investigated the cause of the abandonments by collecting data throughout the nesting period on crane behavior, temperature, food availability, and black fly abundance and distribution. Results from the 2009 nesting studies did not provide sufficient data to indicate whether black flies were causing the cranes to abandon incubation before eggs hatched. Observers did note that, in some cases, the abundance of black flies near crane nests appeared to be much higher than would be expected compared to levels monitored elsewhere in the refuge during the same time period.
The results of the 2009 nesting studies are presented below, with the following cautions:
- Studies are unlikely to result in conclusive evidence of causes of nest failure after only one study season.
- Scientific experiments have greater power to reveal the causes of observed results when the size of the sample is large. Due to the small number of nesting pairs, it is difficult to draw valid conclusions about cause and effect relationships affecting nesting success of the whooping cranes.
- Incubating cranes are exposed to a large number of stresses, and it is likely that several factors may affect the cranes, quite possibly in combination.
Nest Monitoring Studies
Going into the 2009 nesting season, several theories were developed to explain the nest abandonment observed in previous years. Seven factors potentially contributing to limited whooping crane reproduction were tested. Those factors included: limited food resources, nest disturbance, inexperience among nesting pairs, captive breeding effects, incubation delays, nest habitat characteristics, and harassment by biting insects, including black flies. The following are explanations of some of the theories and the studies that were conducted during the 2009 nesting season to test those theories.
Limited food resources
This study examined the hypothesis that incubating birds were nutritionally stressed, so much so that the appearance of numerous prey items triggered by rising temperatures (e.g. the sudden appearance of large numbers of frogs) caused them to leave their nests to seek food.
Researchers observed what the birds were eating and the habitat they occupied while foraging. Additional sampling was conducted to determine what food items were available to the cranes at Necedah NWR, whether they ate them, or not. At least seven whooping crane nests were monitored with either cameras or observers in blinds, collecting information on nesting and non-breeding cranes at timed-intervals to determine the role of energy balance on nest abandonment. Supplemental food was provided to at least four of the seven nesting pairs, to see if providing additional food prevented nest abandonment.
Captive breeding effects
One hypothesis suggests that captive-reared birds will not survive and reproduce as well as wild-hatched individuals. Such effects have been noted in a number of bird reintroductions. The cause of captive-reared effects is not known. One belief is that wild-reared individuals, raised by parents who have learned to successfully survive and reproduce naturally have a benefit over individuals of captive-reared parents. Given the high level of parental investment exhibited by whooping cranes (as opposed to many bird species that spend little time with their young), this is a plausible hypothesis. While sample sizes are extremely limited, there is an indication that wild-hatched whooping cranes in the Florida non-migratory population survived better than their captive-reared parents.
Harassment by biting insects
Whooping crane decoy with real wings.
Photo by USFWS; Richard Urbanek
Another hypothesis for the cause of nest abandonment is that incubating birds were unable to tolerate the abundant blood-feeding black flies that emerged in large numbers with the first significant warm weather. Researchers observing nesting pairs in 2008 noted a correlation between the emerging black fly adults and the period of nest abandonment.
Another explanation for the nest abandonments is that they were not abandonments, but incubation delays. When both parents are absent from the nest, other than for nest exchanges, it is considered delayed incubation. Pairs on Necedah have left nests for as long as three days, then returned to again incubate. It is possible that pairs may be delaying incubation rather than abandoning nests. During the 2009 nest season, eggs were left in nests when both parents were missing to test whether they would return to incubate.
Results from the Nest Monitoring Studies
Results from 2009 Nest Monitoring indicate that limited food resources on breeding grounds and nest disturbance are unlikely causes of wide-scale nest failures. The study found that neither supplemental food nor allowing whooping cranes to return to temporarily abandoned eggs increased production. Food limitations on winter areas, causing birds to arrive on breeding areas lean, when they should normally be fat, could still be a problem and that hypothesis will be tested later.
Whooping crane nesting and comfort behaviors were affected by several species of biting insects, but no single species of biting insect caused all the nest abandonments.
By incorporating data that identified captive-reared versus wild-reared birds and age of pairs it became apparent that biting insect activity appeared to be working with both effects of captive breeding and inexperience among the whooping crane pairs to affect incubation length and nest failure date.
The study also showed shifts in whooping crane biting insect tolerance, with older pairs more tolerant that younger pairs. Although the maturing population will gain experience that hopefully results in increased production, our analysis indicates that this shift will not result in sustainable reproduction rates until approximately 2013. In the meantime, several management options to affect captive breeding effects and biting insect numbers can be explored. If any of these management options or combination of options work, whooping crane reproduction could be increased in the short-term.
2012 Black Fly Suppression Study - Bti Treatments
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