Attwater’s prairie chickens at crucial nesting time

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It’s springtime on the prairie, and like most species, Attwater’s prairie chickens are working hard to produce the next generation.

Although we don’t know when hens select nest locations, nesting activities begin in earnest when the first egg is laid a few days after mating on booming grounds. This usually occurs in early- to mid-March. Lay intervals for domestic chickens range from 25–28.5 hours and is probably similar for prairie chickens. Time of lay appears to get progressively later each day until eventually a day is skipped in egg production. As a result, it takes approximately 14 days to complete the average clutch of 12 eggs, give or take.

So that all eggs will hatch at approximately the same time, incubation does not begin until the clutch is completed. Once incubation has started, the hen is fiercely dedicated to her clutch of eggs, usually leaving for only 45–60 minutes twice daily (morning and evening) to relieve themselves and find food.

The hen’s cryptic coloration and concealment of the nest site is her primary defense strategy against hungry predators like skunks, opossums, raccoons, coyotes, and snakes. She will typically remain motionless on her eggs in hopes of avoiding detection, even if potential predators are too close for comfort. However, if an intruder approaches within a foot or so of the nest at ground level, she puffs herself up, raises her short pinnae feathers on her neck, and hisses ominously at the threat, which may be many times her size.

If an attempt is made to take eggs from the nest, the hen will pummel the intruder with her wings, and inflict blows with her beak with sufficient force to draw blood. Of course, this fierce defense puts her own life in extreme jeopardy from predators, and we typically see a spike in hen mortality during the nesting season. If all goes well, the newly hatched chicks will leave the nest with the hen approximately 26 days after the onset of incubation.

However, research through the years has shown that on average, only approximately one-third of nests survive to hatch. If first nesting attempts are unsuccessful, hens usually will try again if there is enough time left in the season. June 17 is the latest we have observed a successful hatch, which means the hen has to start incubation by May 22. Males usually stop booming around this time as well.

Successful reproduction is critical to population growth. Good nesting and chick survival mean the population will grow; complete reproductive failure in a given year generally means that the population will drop by half the next year since adult mortality is fairly constant at around 50% a year. We have taken some fairly extraordinary measures to provide added protection to Attwater’s nests in an attempt to increase nesting success.

Of course, good grassland habitat is essential. We have also worked with Texas Wildlife Services since 1997 to manage potential nest predators prior to and during the nesting season. But we wanted to do more. We knew that fences of various sizes and configurations had been used with promising results to protect nests of ground-nesting species across several avian orders including ducks, sharptailed grouse, shorebirds, and seaside sparrows.

However, to our knowledge, such fences had not been used on prairie chickens, and certainly not for the endangered Attwater’s prairie chicken. The late Dr. John Toepfer first evaluated and refined the technique on non-endangered greater prairie-chickens. We began using nest fences on Attwater’s in 2000. While details have been fine-tuned through the years, the basic design has remained unchanged.

The process begins when telemetry indicates a hen has likely initiated incubation. By that time the hen is fully invested in her clutch and will usually hold tightly to her nest as described above. We approach the hen using radio telemetry in a tightening spiral pattern until we either see her on the nest, or we know exactly where she is setting; if she is not incubating, the hen will typically flush some distance from the observer.

If a nest is located, we then carry a roll of 3 x 100-foot hardware cloth and approximately 20 pieces of 4–5-foot long metal rebar to the site, and the predator-deterrent fence (3-feet tall, 25-foot on a side) is erected. Initially, the fence is constructed with the sides inclining in toward the nest to help the hen learn to go over the fence on her return trip to the nest after her twice daily recesses (typically she leaves the nest by taking immediate flight, but on the return trip she flies close and then walks the remaining short distance to the nest).

The bottom of the fence is “pinned” down with stakes made of rigid wire or cut up grating to discourage snakes and other animals from going under the fence. All of this is usually accomplished with the hen still on her nest approximately 12.5 feet away. Approximately two days after construction, the fence sides are placed in a vertical position, and then two days later the top six inches of the fence is bent outward at approximately 45 degrees to discourage snakes from climbing over the fence.

These fences are in no way predator proof. A raccoon or coyote could easily go over the top. However, they have no way of knowing that a prairie chicken nest is inside the fence, so usually, the fence deflects predators away from the nest.

Critical to successful implementation of this technique is promptly detecting hatch so that the fence can be dismantled to allow the young chicks to exit. In order to minimize disturbance at the nest site by repeated monitoring, we obtain an estimate of hatch by measuring egg length, width, and weight. Formulas previously published for other species and refined for the Attwater’s are then used to predict fresh egg weight.

The current egg weight is then compared to the predicted fresh egg weight. The predicted hatch date is estimated assuming 15% total average weight loss during the incubation period. These estimates are surprisingly quite good, and usually fall within 2–3 days of the actual hatch date.

This method is usually much more precise than other methods used to determine estimated hatch dates like telemetry, candling, or floating eggs. However, to make sure we are able to open the fence quickly after hatch, we start checking nests 5-7 days before the projected date in case hatch date estimates are off.

The predator-deterrent fences are a lot of work, but they are remarkably effective. A comparison of 224 fenced and 24 unfenced Attwater’s nests since 1997 at the refuge revealed a success rate of approximately 84% for fenced nests and only 13% for unfenced nests. Use of these fences is not without risk though. Eleven (4.9%) hens abandoned fenced nests. Six (2.7%) of these abandonments occurred immediately after the fence was built, while the remaining five (2.2%) occurred in mid to late incubation after the hen had been successfully traversing the fence and incubating her eggs for some time.

To place these abandonments in perspective, I conducted a review of five other studies from 1941–1986 on Attwater’s prairie chicken nesting which included a total of 127 unfenced nests. Abandonments reported for each study ranged from 0.0 to 15.8%, and averaged 6.2% (8 of 127) across studies. Therefore, even if all of the abandonments we observed were attributed to the fences, which I do not believe to be the case, the proportion we observed was still less than that reported for unfenced nests in the literature.

Two other losses occurred, bringing the total possible fence losses to 8-13 (3.6-5.8%), still below the abandonment rate for historical studies of unfenced nests. Given the substantial difference in success of fenced nests, we estimated that fenced nests produced 6.3 times more chicks than unfenced nests in our comparison. Using the higher average historical nest success of 32.2%, fenced nests would still be expected to produce 2.6 times more chicks. So from a risk-benefit perspective, the balance is substantially skewed toward benefits.

Of course, the hope is that wild populations will increase to the point that all this intensive effort will no longer be necessary in the near future. But in the interim, these predator-deterrent fences will remain an important management tool in Attwater’s prairie chicken recovery efforts.

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