Understanding Bird Scent -- Part 2: Diffusion

In last month’s column, Understanding Bird Scent-Part 1…The Bird, we learned about how bird scent is created.  We know that there are scent rafts that create four different scent clouds that carry the scent into the atmosphere.  Once the scent is airborne, there are numerous atmospheric weather conditions that affect the scent dispersal and the ability of our pointing dogs to locate a bird. In this part, of our three part series, let’s examine those weather conditions that affect bird scent diffusion. We’ll also look at the effect weather has on bird movement which also produces scent diffusion.

First, however, let’s get the scent off the bird.  Every warm living creature has natural body air currents.  Warm air rises so the body air current of a bird goes from the bottom to the top of the bird.  Those body air currents transport the scent rafts, and resulting cloud, up and into the atmosphere.  That’s why we want our pointing dogs to carry their heads high…that’s where pointing scent is most concentrated.  And, as we go through this three part series, it’s important to understand that the most common method used for quantifying scent is scent concentration.

Now that our scent rafts are airborne, they are impacted by weather.  And, this is where it can become rather confusing…for both the dog and the hunter.  When one considers the variations of temperature, atmospheric pressure, and humidity from hour to hour through a 24 hour period, it can be compared to commuting between the tropics and the arctic in one day.  (Scent and the Scenting Dog, William G. Syrotuck 1972).

As difficult as it may appear, let’s analyze  weather conditions that affect scent and see what we can learn. We’ll look at air currents, wind, temperature, humidity, barometric pressure and the moon phases.

Let’s begin with the simplest: air currents in hilly or mountainous areas and  not influenced by wind.  As the sun rises and heats the ridges, air currents travel up the mountain.  This process is often referred to as an up-draft.  As the sun gets higher, the lower elevation becomes warm and the air current continues upward. At about 3:00 pm in the afternoon, a reversal begins.  The sun is lower in the sky and the ridges of the hills begin to cool.  This cool air runs down-hill.  With other factors being somewhat neutral, that means we start our hunt in the morning at the top of the hill or mountain and work our way down.  In the afternoon, as the air current reverses, we start our hunt at the bottom of the hill and work-up.  The resultant turbulence (the changeover beginning at about 3:00 pm) causes a specific layer two to three feet above the ground.  Rafts which fall into this layer will be kept airborne at the height of two to three feet for long distances.  This effect is relatively short lived, usually one hour or less. (Syrotuck 1972).  This tells us that the rafts (see part 1) carrying the scent in the scent cloud may be most detectable between 3:00 and 4:00 in the afternoon; which coincides with feeding activity which means a moving bird and more scent dispersal. All are positive conditions for a pointing dog.

Now, we’ll discuss wind (airflow). How wind affects scent can be a complete article itself. And, in How Scent and Airflow Works, Jennifer Pennington did just that.  Pennington, along with many others, suggests that a great way to see how wind can affect scent is to watch smoke. We’ve all sat by a camp fire and experienced how smoke shifts from one direction to another.  One minute we have smoke in our face so we move to the other side of the fire…and then very quickly the wind changes and we have smoke in our face again.  Wind is a tough hombre to pin down.

After diffusion from the bird,  Pennington identifies fourteen different types of airflow (wind) that can affect how scent is carried through the atmosphere.  Her document is ten pages so for our research, we’ll cover the basics.  We’ll begin with the Latimer Flow which is a straight flow off the bird.  There are no obstacles to block the air flow or significant wind to change the direction.  On a calm day, this might be what we would experience in large open fields found in the prairies.  For those of us who have hunted the prairies, we’ve all experienced those 100’ points in perfect weather conditions.

Next, we have turbulent airflow.  If the prevailing breeze hits an object such as a building (of any size), a pile of brush, large granite rocks (like what we find in New England), the flow (and scent being carried) tumbles over the obstacle and becomes turbulent on the opposite side.  That turbulence creates a very unorganized jumble of scent which tumbles around and would be difficult for a pointing dog to pick-up at any distance. 

Pennington’s next airflow is a coning plume.  This is most likely where the term “scent cone” originated.  Scent travels from a very narrow and dense concentration to a broader cone configuration. With a good breeze, the scent can travel a great distance.  Pennington states that this is ideal scent dispersion for dogs. 

Those three airflow (wind) types cover most of the situations the pointing dog owners will encounter.  A simple rule for the pointing dog owner/hunter is that airflow (wind), and the scent that is being carried, always follows a path of least resistance.  And the old simple rule of hunting into the wind remains valid. We’ll finish wind with the following:  Strong wind may disperse rafts and scent to the point where they cannot be detected, whereas little or no wind may limit the area over which they are transported. (Geiger 1965)

Now let’s look at temperature and humidity.  Scent creating bacteria needs moisture and warmth to grow and stay alive.  Without those two factors, bacteria (and scent) will die. Bacteria are 80% water. Very hot and dry atmospheres dry up the nutrients of the bacteria to the point where activity will cease. (Syrotuck 1972) Extended warm temperatures with no moisture will eventually lead to scentless birds.  My wife and I were hunting Manitoba several years ago.  It was so dry that when you put your foot down the vegetation crunched like rice crispies (pop, snap & crackle).  Vegetation was dead and creeks were dried up.  Our dogs, with very good noses, ran over the few birds we found.  Your author has a very good friend who has a 27,000 acre quail ranch in Kansas.  In November of 2016, they went 60 days without rain.  His dogs had the same issue…the birds had no scent.  Remember that uropygeal gland we discussed in Part 1?  With no water, that gland most likely becomes much drier and disperses less, if any, scent.   

Because there are so many variables, it’s hard to find research that provides perfect temperature and humidity levels for a scenting dog.  One short paper I discovered suggests that both a humidity reading and a temperature (Fahrenheit) between 40 and 60 is ideal.  Those levels provide enough warmth and humidity (moisture) for bacteria to grow and both provide a level of comfort for the hunter and dog.  Also, if the temperature and relative humidity levels are approximately the same, there will be dew on the ground in the morning.  The dew revitalizes bacteria that has fallen to the ground…and that creates stronger scent. How about freezing temperatures? Freezing or near-freezing temperatures retard bacterial action, extending the decay period but reducing decay rate. (Syrotuck 1972) 

Barometric pressure and wind have been used to predict fish and game activity for decades.  We’ve all heard the old adage many times:  Wind in the East, fish bite least, Wind in the West, fish bite best, When the wind is in the North, The prudent angler goes not forth, When the wind is in the South, It blows the hook in the fish’s mouth.  Although this old predictor talks about fishing, it also applies to game movement…and that includes upland birds.  John Alden Knight, Moon up – Moon down  (1972) explains this old angler’s guideline as follows:  Wind in the East-harbinger of a falling glass (barometer) and bad weather along our Atlantic seaboard.  The fish (and game) take to cover and are not interested in food for the time being. Wind in the West – good weather with a fresh breeze and a high or rising glass, probably indicating good fishing (and game movement). Wind in the South – warm, gentle breezes and good weather, with a steady barometer.  No wonder the fish (and game) go on the feed. Wind in the North- as a rule this is a strong, cold wind, shifting to the east.  That means adverse temperatures and a falling glass.  Obviously bad weather for fishing (or hunting).  Knight goes onto say that the effects of the barometer on fish and game movement had been proven beyond doubt.

The average sea-level barometer reading is 29.92.  Knight points out that as little as one one-hundredth of an inch downward in the barometer can shut off fish and game movement.  So, a higher barometric pressure promotes more game movement and with the high barometric pressure pushing downward, there is more scent concentration for our dogs.

One more offering from Knight’s Moon up-Moon down.  Knight feels that game move more during a high moon.  This fall, check your moon calendar and see if you feel more birds were located during a high moon.

Your author hopes readers have learned something about scent diffusion that helps make your next hunting season more successful.  The bottom line on when to go hunting….whenever you can.  Part 3 of this series will discuss the receptor of all this scent …the dog’s nose.

Literature Cited

Geiger, R.  1965, The climate near the ground.  Harvard Univ. Press, Cambridge, MA

Knight, John Alden, 1972, Moon up-Moon down, Solunar Sales Co.

Pennington, Jennifer, 2016, How Scent and Airflow Works, Virginia Search and Rescue Assoc.

Syrotuck, William G., 1972, Scent and the Scenting Dog

Paul Fuller is a life-long sportsman.  He’s been an outdoor writer since 1971. He’s the host and producer of the award winning Bird Dogs Afield TV show (www.birddogsafield.com) and produced the epic video Grouse, Guns & Dogs. Paul shot over his first German shorthaired pointer in 1961. Paul may be reached at paul@birddogsafield.com

Copyright 2017, Paul Fuller.