Nocturnal Predators Likely Cause Colony Failures

ArrayJanuary 16, 2015 at 11:45 am

[caption id="attachment_1225" align="alignright" width="300"]Typical time lapse camera and sound recorder setup. Typical time lapse camera and sound recorder setup.[/caption] The great blue heron was listed by MDIFW as a Species of Special Concern in Maine in 2007 because of a perceived population decline. Since I began working with herons in Maine, the first question I was to address was “What caused the apparent steep decline in the coastal nesting population?” Over the past six years I monitored both coastal and inland colonies to determine if that population decline is just a coastal phenomenon. In 2013, of 70 active great blue heron colonies we studied, ten failed mid-season, well in advance of expected fledging of young herons. The cause(s) of these colony failures were unknown, but disturbance by bald eagles, great horned owls, northern goshawks, or raccoons was suspected. In recent years, MDIFW has fielded numerous reports of bald eagles attacking great blue herons. This led me to believe that bald eagles may be the primary cause of disturbance and heron colony failures. If I could figure out what caused these colony failures, perhaps I would gain insight into the coastal population decline. This past spring and summer, my colleague and I attempted to learn what might cause a heron colony failure by “watching” and “listening to” the colonies around the clock. In early April, we deployed Day 6 Plotwatcher Pro HD time-lapse cameras at five great blue heron colonies that had failed in the recent past.  In addition, we deployed Wildlife Acoustic SongMeter2 Recording Units at three of these sites.  The time-lapse cameras recorded images every five seconds between dawn and dusk.  Although these cameras do not have infrared capability and do not operate at night, they were chosen because they can record a zoomed out view of most of the nests within a colony even when stationed 300 feet away, a predetermined distance chosen to avoid disturbance to the nesting birds.  To monitor heron activity at night, we tested the use of sound recorders to monitor potential nighttime disturbances.  The sound recorders were programmed to record sound for three minutes every half hour during the day (5am-8pm) and for three minutes every ten minutes during the night (8pm-5am). Three out of the five colonies failed again before the young fledged. Through the use of the cameras and sound recorders, we believe we have detected the culprits. [caption id="attachment_1218" align="alignleft" width="300"]Due to the density of snags and the distance from the upland shore, the nests are very hard to see on camera at the Carmel site. Due to the density of snags and the distance from the upland shore, the nests are very hard to see on camera at the Carmel site. (Click image to enlarge)[/caption] The Carmel colony, number WBC00791, was the first to fail. The camera and sound recorder were deployed on May 8th, at which time there 4 out of 6 nests were in the incubation stage. On May 19th, the batteries and memory cards were switched or replaced. On that date, 5 out of 6 nests remained active and an adult still incubated the eggs. On May 29th, the site was again visited but all nests were empty with no herons in sight. The video footage from May 19th to May 29th revealed a decrease in adult heron activity that began on May 26th. Unfortunately, there was “no video smoking gun”. However, the sound recordings captured intense heron distress calls during the 11:20pm and 11:30pm files for May 23rd, and during the 3:50am, 4:00am, 4:30am, and 4:40am files on May 25th. There were no other sounds recorded that provided clues of the cause of this distress. However, I suspect, given it occurred at night around multiple nests in trees within a wetland, the predator was likely a raccoon or fisher. The video camera was operating at 4:30am on May 25th, but heavy fog prevented us from “seeing” what may have transpired. The nests were all in the incubation stage during the first ground survey on May 2nd, and because incubation lasts 28 days, I suspect that the eggs had not yet hatched. We think the nest intruder was most likely a raccoon. We visited each nest tree on May 31st and found no signs of predators (no hair, claw marks, eggshells, feathers, or dead birds. The evidence may have been obscured by a significant storm that had occurred the previous week. [caption id="attachment_1219" align="alignleft" width="300"]We had a much better view of the nests at the Newcastle site. We had a much better view of the nests at the Newcastle site. (Click image to enlarge.)[/caption] The Newcastle colony, known as WBC00623, was the second colony to fail. The camera and sound recorder were deployed on April 28th, when at least 6 out of 15 nests were active: four were being incubated and two we believed were to be in the pre-incubation stage. On May 12th, May 27th, and June 9th, the batteries and memory cards were replaced. During each visit, all six nests, plus one additional nest built after April 28th, remained active. On June 9th, we observed young at all seven nests. A fuzzy head count of 22 nestlings between 2-4 weeks in age was recorded. Unfortunately, our June 18th visit field check revealed only one active nest with three young. A wing or carcass was visible in one of the failed nests. There had been a severe thunderstorm the previous night but we found no live or dead chicks under the nests in the water. Five adult herons were observed in the wetland. A site visit two days later revealed that all nests were now inactive and no adults were seen. Once again, the sound recordings were important for piecing together what transpired at WBC00623 between June 9th and June 20th. Heron distress calls were first heard during the night of June 11th. The distress calls were captured on the 11:30pm clip and lasted over two hours. Heron distress calls were also recorded heard on the nights of June 12th, 13th, 16th, 17th, and 18th. Again we suspect a raccoon or raccoons were the cause of the nest failure at this colony. [caption id="attachment_1220" align="alignleft" width="300"]Only the nest furthest to the right was active this year, despite 3 adults visible here. The correct date of this picture is 5/16/2014 (we hadn't set the date and time correctly on the camera!). Only the nest furthest to the right was active this year, despite 3 adults visible here. The correct date of this picture is 5/16/2014 (we hadn't set the date and time correctly on the camera!). (Click image to enlarge.)[/caption] The third colony that failed, referenced as WBC00688 in Waldo, was one that had a difficult time getting started. The site had three nests that remained from previous years but did not show signs of activity until May 14th. While four adults were observed on camera on several occasions, nesting was initiated at only one of the three nests. The adults nested and incubated for about a month before they abandoned the nest. A bald eagle was recorded on camera in the colony on May 9th, before the herons initiated nesting. Based on camera footage, one adult was at the nest in the final days, mostly standing in the nest and preening. There were no obvious signs of a second adult coming and going to feed young. Perhaps one of the pair was killed or had abandoned its mate, or the pair was inexperienced and did not know how to care for their young. We did not have a sound recorder at this site, thus no nocturnal observations are available to lend more clues as to why the site failed. Our results were not expected. Initially I believed bald eagles may be causing problems for some colonies. While bald eagles and owls were detected at the colonies, on video and sound recordings, there is no evidence they played a part in these three colony failures. Through this pilot study we learned the importance of nighttime surveillance to assist in the determination of colony failure. While the time lapse cameras were helpful, the distance from nests required at some sites reduced the image quality and their effectiveness in providing clues. It is important to also keep in mind that all colonies monitored with cameras and sound recorders in 2014 were inland sites in wetlands. Causes of colony failure may vary with habitat. Other predators may be a factor at inland colonies in upland pine stands and at coastal island colonies. Stay tuned! Check out this 2 minute sound clip during a disturbance event at the Newcastle colony.