In this article, I examine the turkey vulture’s sense of smell. An overview of morphological and physiological traits is presented first, followed by an analysis of the results of field experiments. The article addresses some pressing questions about the turkey vulture’s olfactory skills.
Can turkey vultures smell their prey or food?
Turkey vultures (Cathartes aura) have an excellent sense of smell. By smell alone, they can locate a mouse-sized carcass under leaf litter in densely forested areas. A turkey vulture’s superior olfactory skills result from a combination of nostril morphology and brain adaptations to detect traces of scent released by decaying carcasses.
A measure of an animal’s sense of smell is the size of its olfactory bulbs. Larger olfactory bulbs indicate a more developed sense of smell.
Turkey vultures are known for their ability to detect carrion by smell. They appear in the most unexpected places when a dead animal is lying around. In many cases, a turkey vulture’s presence has led to the discovery of small mammals’ carcasses. Before the bird’s arrival, there was no indication that a carcass lay nearby.
Turkey vulture brain adaptations to find food by smell
The Turkey vulture’s anatomical and physiological adaptations are unparalleled by any other animal. Their large and well-developed olfactory bulbs and powerful cellular mechanisms allow them to detect diluted volatile gas molecules in the atmosphere hundreds of feet away while flying.
In the 1960s, Kenneth Stager, a senior curator of ornithology at the Natural History Museum of Los Angeles County, showed that turkey vultures had an extremely large olfactory bulb, a part of the brain responsible for processing odors. At this point, however, it was unclear whether having large olfactory bulbs necessarily resulted in a greater ability to detect odors.
Scientist Nathan Grigg and colleagues (2017) examined the fresh brains of turkey and black vultures (Coragyps atratus) to determine what were the implications of having a large olfactory bulb.
One striking finding was that turkey vultures had about double the number of mitral cells than the black vultures despite having smaller brains.
Mitral cells are part of the olfactory bulbs whose function is to transmit information to the brain about the smell. Mitral cells can be interpreted as smell-sensitive elements or smell sensors in animals. More mitral cells imply a greater ability to detect smells.
Another striking finding was that not only did the turkey vultures have average larger olfactory bulbs, but they were about four times larger than that of the black vulture. Compared with other 143 animal species of known olfactory bulb sizes, the turkey vulture had significantly larger olfactory bulbs relative to brain volume.
Turkey vultures have nostrils adapted to detect smells
The turkey vulture’s nostrils or nares are adapted to detect the slightest whiff of odor in thin air. Its nostrils are wide open without a septum or middle wall that separates the nostrils. This means that one can see through a turkey vulture’s nose.
Some have suggested that the shape of the turkey vulture’s nostrils is an adaptation to keep it clean from pieces of meat that attach to the nose when the bird sticks its head into a carcass. I argue that it makes little sense.
The turkey vulture’s wide open nostril would collect all kinds of pieces of rotten meat and even block the air passages. The primary purpose of the turkey vulture’s nostril morphology is geared to detect smells.
The lack of a dividing septum allows permanent free-flowing air to come in contact with mitral cells and olfactory bulbs. The bird does not need to inhale air to detect any odors.
Measures of the turkey vulture’s olfactory skills
The most telling and conclusive field study (Houston 1979) measured the turkey vulture’s efficiency in finding carrion by smell. This study also established that they could tell -by smell only- the age of the carcass and choose the quality of carrion they preferred.
Setting: The study was conducted in a natural setting in the Tropical Rainforest of Panama. A first with a thick canopy.
Experimental trials: To find out the turkey vulture’s efficiency in finding carcasses, the researcher laid out 72 carcasses of dead chickens in separate locations. The carcasses were laid out in two days. The fresh set of carcasses was laid out the first day. The second set of carcasses was laid out the second say. The three, and four-day-old carcasses were those remaining, not consumed by the vultures. This means there were fewer three and four-day-old carcasses than one and two-day-old ones.
Results:
- Only one turkey vulture found the fresh chicken carcasses. Up to 52 turkey vultures found and descended to the two-day-old carcasses. Sixteen birds found the three-day-old carcasses. Surprisingly only three birds found and descended to the four-day-old carcasses, which were the most decayed and smelly to a human nose.
- This field experiment revealed that turkey vultures were able to find carcasses they were not able to see. They use their sense of smell to locate the carcasses located below a thick rainforest canopy.
- The study also demonstrated that turkey vultures had difficulty finding fresh or undecayed carcasses and needed the smell stimulus to locate their food.
- Perhaps more interestingly, the study demonstrated that turkey vultures were able to distinguish the age/quality of the carcass based on their sense of smell alone. They preferred and descended to two-day-old carcasses in significantly larger numbers than older carcasses.
Can turkey vultures use their sense of smell to choose their favorite food?
As indicated above, turkey vultures preferred fresh carcasses over older ones. However, the number of three and four-day-old carcasses left were fewer than the fresh and two-day-old carcasses laid out on the first two days of the experiment.
To confirm if indeed turkey vultures preferred fresh carcass over too decayed ones and if they are truly able to differentiate and choose their food in the air, by smell alone, a new field experiment was established.
Multiple sets of Two-day-old and ten-day-old carcasses were buried near each other on the forest floor. Turkey vultures found and descended to the area where the two types of carcasses were hidden and had to decide which carcass they would actually descend to based only on smells.
Many birds rejected the more decayed carcass and descended and consumed the fresher carcass. The study concluded that turkey vultures could recognize, solely by smell, the type, and age of carcass they prefer and reject decayed carcasses over fresh ones.
It would require more experimentation to determine what cues turkey vultures use to determine the age and quality of carcasses for food. The odor of death is complex and highly variable along the stages of decomposition.
How far can a turkey vulture smell?
The distance a turkey vulture can smell food has not been determined with certainty. I have not been able to find a source to confirm the distance that vultures detect the smell of decaying carcasses. It is a sure thing that vultures pick up cues “several hundred feet” from a dead animal, although a maximum distance may be variable and depends on prevailing weather conditions.
Indirect measures indicate that turkey vultures should be able to pick the smell of a carcass from a long distance. Common green bottle flies (Lucilia sericata) and other flies that lay larvae in the carcass of dead animals have a highly developed sense of smell. If they are around, they will find a carcass within minutes after the carcass begins to release an odor. On days with hot temperatures and considerable wind speed, green bottle flies have been found to pick up the smell of a carcass a mile away from the carcass’s location.
Although not determined through experimentation, it is likely that given the conditions such as a high temperature, hot air, high wind speed, and type and size of the carcass (large carcass release more decomposition gases), a turkey can detect the scent of a dead animal even further than a mile away from the carcass’s location.
Field studies that laid out bait for the vultures determined that turkey vultures flying downwind from the carcass were the first to detect the bait.
How does a turkey vulture use its sense of smell to find food?
To find food, turkey vultures fly over the landscape covering large areas as they test the air for the slightest scent of a decomposing animal carcass. They need a scent clue to begin tracking the odor trail.
Once the scent is detected, turkey vultures follow the scent trail in an undulating manner toward the origin of the smell. Then, the origin of the smell is reduced to a small area where the bird begins to fly in increasingly smaller circles in what appears to be an attempt to pinpoint more accurately the source of the odor. When flying in circles, turkey vultures fly in an undulating manner taking shallow dives that may be intended to triangulate the location of the carcass or simply gain momentum for a subsequent lift and change in altitude.
The turkey vulture then lands on a perch near the carcass’ location. On the perch, they often spend over an hour on what appears to be assessing the risk of descending to the ground. At this point, the odor is so strong to them that they fly almost directly to the spot on the ground where the carcass is found, even though the carcass may still be hidden under the leaf litter.
Why do turkey vultures fly in circles?
Turkey vultures in the air may be foraging for food or flying from one place to another. The flight of the turkey vulture is different when foraging for food than when moving between places.
When foraging for food, they cruise the landscape at altitudes of about 100 to 150 feet from the ground. The flight is buoyant and undulating and appears to be influenced by the wind speed and direction.
When moving from one place to another, turkey vultures are likely to be seen soaring in circles gaining altitude while riding the updraft thermals. Once they reach high altitudes, they then glide in a descending trajectory in one direction. Gliding flight generally implies losing altitude. Hence the bird will find another updraft thermal to gain altitude and repeat the process.
How soon after an animal dies, a turkey vulture finds the carcass?
The time it takes a turkey vulture to find a carcass depends on whether the carcass is visible and the birds find it by sight or hidden and found by smell.
Hidden or not visible carcasses: A turkey vulture can only locate a hidden or not visible carcass by following the odor trail towards its location. Generally, animals must be dead for a few hours before decomposing.
Turkey vultures rarely find a carcass 12 hours after death. The chances for them to find a carcass increase by the hour and occur within 24 hours after death.
Houston (1979), through experimentation, found that out of fresh and two-day-old chicken carcasses laid out as bait, most turkey vultures found carcasses 24 hours after death. Only one bird found a fresh carcass the same day or at least 12 hours after death.
This field experiment demonstrates that a carcass must initiate the process of decomposition and release smell/gases for a turkey vulture to be able to pick up the odor trail and follow it to the carcass.
Visible carcasses: Turkey vultures have great eye vision, capable of detecting small carcasses in the open, even while high in the air. A turkey vulture can find a dying animal using visual cues to find food. They can recognize when an animal is about to die and have been spotted standing by animals that were close to death.
Temperature influences the time a turkey vulture finds a carcass by smell
Decomposition rates are affected by the temperature. When an animal dies, fats, proteins, and other body components begin to break down and release biochemicals into the environment at different rates depending on the temperature.
At warmer temperatures, the process of decomposition and decay occurs more rapidly. Turkey vultures show up at carcasses once they can smell the trail of gases of death. As a result, when temperatures are higher in the summer, turkey vultures find carcasses more quickly than when temperatures are cooler in the winter.
Conclusions:
Turkey vultures have developed the most sophisticated sense of smell of any animal. Their sense of smell allows them to find small carcasses that are hidden. Nevertheless, for a turkey vulture to find a carcass, it must be in a state of decomposition and releasing gases. Turkey vultures prefer fresh carcasses to old and decayed ones but will accept old carcasses as well.
References:
- Houston, D. C. (1986). Scavenging efficiency of Turkey Vultures in tropical forest. Condor 88:318-323.
- Stager, K. E. (1964). The role of olfaction in food location by the Turkey Vulture (Cathartes aura). Los Angeles County Museum Contributions in Science 81:1-63.
