Do the remains remain? Small animal decay in the Alaka'i 

Kreuser, Abigail M., Winter, Tyler A., Crampton, Lisa "Cali" H.

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Introduction & Methods

Importance:

  • Trapping as management of invasive species is widespread through Hawai‘i.
  • Carcasses remain for an unknown amount of time after being killed.
  • Counters on traps account for more carcasses than what bodies are physically counted durring checks.
  • About 2/3 of traps have counters, and this will better help our estimate of kills.
  • If non-target animals are killed, would they be found?
  • Unique environment, stable temperatures, frequent rain, animal scavengers.
  • Fenced and unfenced trap sites may affect scavenging of carcasses.

 

Winter study with 32 bird carcasses, followed by summer study with 60 birds and 60 rat carcasses.

 

Hypotheses:

  1. If not scavenged, clear evidence of a bird would persist for four months in the Alaka‘i such as feathers and bones.
  2. Removal rates will be faster in summer due to increased rat population size and increased temperatures.
  3. Removal rates of rats will be greater than birds.
  4. Removal in unfenced area will be greater than the fenced area.

 

Methods: 

Winter study

  • Received 32 bird carcasses, 16 Zebra Dove and 16 Red-crested Cardinal, from USDA Wildlife Services in Lihue
  • Weighed and assessed bodies for wounds, and “banded” legs with unique zip tie combination.
  • In December, deployed carcasses on the Alaka‘i Plateau at our fenced Halepa‘aki field site.
  • Placed 30 carcasses on the rat trapping grid 50 meters away from rat traps.
  • Control carcasses, 1 Zebra Dove and 1 Red-crested Cardinal, placed in a cage near camp.

Summer Study

  • Received 40 Zebra Dove and 20 Red-crested Cardinal carcasses from USDA Wildlife services.
  • Received 60 Rat carcasses from Dr. Aaron Sheils, USDA APHIS
  • Deployed 60 carcasses at Halepa‘aki protected from ungulates by a fence, and 60 carcasses at Mohihi that is not fenced.
  • Three carcasses, one of each species, were placed in a cage for control.
  • 28 cameras were placed on carcasses 14 at each site, 7 on birds and 7 on rats.

 

 

 

Deployment Data:

  • Station ID
  • Species
  • Carcass ID (winter only)
  • Substrate on which it was placed
  • Position of the carcass’s body
  • Cardinal direction of the head and shoulders (Winter)
  • Bearing taken in summer study
  • Environment
  • Body Condition (0-6) (summer only)

Carcass Checks Winter 4 Month Schedule

  • December 17, 2018 day of deployment
  • December 27-28, 2018, 10 days after deployment
  • January 4-6, 2019, 20 days after deployment
  • January 28-30, 2019, 45 days after deployment
  • March 12-13, 2019, 90 days after deployment
  • April 20-23 and May 8, 2019, about 130 days after deployment

Summer study is following the same pattern of checks at 10, 20, 45, 90, and 130 days after deployment.

Check Data:

  • Station ID
  • Cardinal direction (winter) of the head and shoulders
    • Anterior bearing (summer)
  • Carcass present (Y/N)
  • Distance moved from original location
  • Arthropods present
  • Amount of bugs present on a scale of 0-none, 1-some or few, 2-tons/a lot
  • Evidence of rodent scavenging (Y/N),
  • Skin visible (Y/N)
  • Bones visible (Y/N)
  • Body condition(0-6)
  • Distinct detection (Y/N), carcass would be found during a quick 10 second scan of an area around a trap, and the evidence would suggest it is a bird.

 

Body Condition

  • 6 -fresh and just placed.
  • 5 -looks as if not touched or minimal decay or scavenging.
  • 4 -some skin, tissue, or bone visible, and at least 75% of body remains intact.
  • 3 -major damage of scavenging about 50% of body intact and majority of soft tissue remains.
  • 2 -soft tissue or muscles present with prominent bones and feathers/fur still bound to skin or cartilage.
  • 1 -a puddle of bones and feathers with no soft tissue remaining.
  • 0 -bird/rat is gone with little evidence the animal was there such as a few feathers or tuft of fur.

 

 

 

 

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Winter Bird Decay

Decay of a Red-crested Cardinal that represents the carcasses that stayed slightly more preserved. 

 

Image 1. Red-crested Cardinal E9-1 decaying. Photos from the left,: deployment, 10 day check (BC=5), 20 day check (BC=3), 45 day check (BC=2),  90 day check (BC=2), 130 day check (BC=1). Thought to have been scavenged inbetween 10 and 20 days after deployment because of body position change and exposed skin. 

 

Average body condition decreased rapidly throughout the 20 days after deployment. Then remained around a body condition of 1 on average. 

Figure 1. Body condition rapidly decreases during the first 20 days after deployment as soft tissue decays and  carcasses are scavenged. Then body conditions remain around 1, which is a puddle of bones and feathers, 45 days to 130 days after deployment.  Body conditions of 0 were not included in this box plot, and the days after deployment are not to scale. 

 

Initial body condition, weight, species, and relative environment had no significant effect on body condition after 4 months. 

 

Zebra Dove J13-2 photos from the top: deployment, 10 day check (BC=4), 20 day check (BC=2), 45 day check (BC=1), 90 day check (BC=1), 130 day check (BC=0.5).

 

 

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Winter Scavenging

Timeline of Distinct Detections 

10 Days- 80% (24/30) bird carcasses remained distinct detections.

  • One carcass placed in the flood plain and disappearance was noted after heavy rain.
  • The other five carcasses are presumed to have been moved and scavenged by rats, although it is possible that they were scavenged by cats.
  • E4-1, E5-1, E6-1, are in consecutive order, therefore, similar habitat. Rats may still reside in this area.
    • E4-1 feather clumps found after 10-day check. No evidence found at 19 days after deployment.
    • E6-1 few body feathers and primary feathers were left and were detected at 19 days, and 2 retrices remained after 45 days.
Image 3. Remains of E4-1, 10 days after deployment (BC=0). 

20 days- the same 24 of 30 carcasses were considered a distinct detection.

45 days- 73% of carcasses (22 of 30) remained distinct detections

90 days- 67% of carcasses (20 of 30) remained distinct detections

130 days- 63% of carcasses (19 of 30) remained distinct detections

 

 

 

Not all carcasses were removed by scavengers, some became not detectable because of environmental factors. 

Figure 3. Five bird carcasses were taken by scavengers after the first 10 days of deployment, and 1 was washed away  by stream flooding. Carcasses not found at later dates is possibly due to leaf coverage, and scavengers slightly moving the carcass.  The carcass not found for the first time after 130 days was never noted having been scavenged.  

 

 

Partial scavenging of bird carcasses by rodents adds difficulty to finding carcass, but does not ensure carcass will be missing. 

Figure 3. Eleven carcasses were not distinct detections after 4 months, and 19 were observed as distinct detections. Evidence of scavenging observed on 14 of 30 carcasses, 5 of which were still considered distinct detections.  

 

 

 

 

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Initial Summer Results

The trapping sites not protected from ungulates are more likely to have carcasses scavenged.

Figure 4. Number of carcasses scavenged and not scavenged compared at the two field sites. On the left, only 2 of 60 (both birds) were scavenged at the site protected from ungulates by fence. On the right, 15 of 60 birds were scavenged at the site not protected by ungulates. 

 

 

  • Animals that interacted with carcasses in order of frequency is pigs, birds, cats, rats, deer.
  • None of missing carcasses on camera were completely taken by rats, birds, cats, or deer, only pigs.
  • The two carcasses that were not present in the fenced site were stationed near the LZ near camp, which may be a better habitat for rats.
  • At the unfenced site, 9 of 15 remained a distinct detection although the main body was not present fur or feathers left behind made it evident an animal was there then scavenged.
  • From the unfenced site 9 carcasses scavenged were birds and 6 were rats.
  • Dry summer has caused desiccation which may affect the overall rate of decay.

 

 

Image 4. Pig munching down on a bird carcass. 
Image 5. Bird checking out and possibly scavenging on a rat carcass. 

 

Image 6. Cat caught sniffing carcass, and leaves the carcass untouched. 

 

 

 

 

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A Month of Rat Decay

 

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Conclusions and Looking Forward
  • According to the winter study, it seems likely that bird carcasses would be found during regular rat trap checks.
  • Prominent feathers and bones remain, and allow a person to differentiate between bird and rat such as jaw and leg bones.
  • Scavenging of the carcasses can make finding a carcass more difficult if it is no longer in specific location.
  • Scavenging does not seem likely after 20 days of a carcass being deployed.
  • Bird interest could be from abundance of grubs inside carcass.
  • At fenced site rats are likely primary scavengers, found carcasses in tree roots, or feather trails leading into holes.
  • Cats show interest in carcasses, but not likely to scavenge.
  • Trouble finding carcasses as a result of leaf litter; hoever, unlikely whole body could get covered by leaves when it is directly under a trap.

Future Direction

  • Compare rat and bird decay, and if likely to find both if killed after 4 months.
  • Compare summer versus winter decay and scavenging.
  • Improve trap checking methods.
  • Better associate the likelihood of carcass removal via the environment or scavengers with the counters we have on traps.
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Acknowledgements

Thank you to Bill Bukoski with USDA APHIS for supplying the bird carcasses for the winter and summer study. 

Thank you to Dr. Aaron Sheils with USDA APHIS for supplying the rat carcasses and the cameras and camera supplies used during the summer study.  

Thank you to Dr. Lisa "Cali" Crampton and Tyler Winter for help in designing the study and continuing logistics. 

Thank you to the awesome field crew that checked these carcasses repeatedly, viewed photos, and entered data, and always offered valuable insight. Justin, Erica, Lauren, Noah, Katie, Laura, Autumn, I appreciate you all. 

Thanks to DOFAW and PCSU. 

 

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