The bird dog's nose...

From an abstract on work by Johan B Steen, University of Oslo, Norway.

The ability to catch scent continuously while running, which may be an essential skill for many animals of prey, requires that ambient air flows inward through the nose also during expiration. In this study on  bird dogs, the direction of the air flow was detected by measuring the temperature of air inside the nostril. 

While resting, nose ventilation was synchronous with lung ventilation. While searching for ground scent, the dog was sniffing at a frequency of up to 200 s-1, a strategy which may create turbulence in the nasal passages and thereby enhance transport of scent molecules to the receptors in the ethmoidal cavity. When the bird dog was searching for game while running with its head high against the wind, it maintained a continuous inward air stream through the nose for up to 40 seconds spanning at least 30 respiratory cycles. We suggest that expiratory gas flowing at high velocity from the trachea to the mouth cavity creates a lower pressure than in the nose thus causing an inward sir stream through the nose during expiration by a Bernoulli effect.

So, a dog can scent birds when running even while breathing out. This contradicts what has long been believed by bird dog people - that dogs cannot scent while panting. It may also explain why dogs can scent a second bird while carrying the first in their mouth while retrieving.