  We experimentally investigate fluctuations in the spectrum of ultrashort
laser pulses propagating in air, close to the critical power for filamentation.
Increasing the laser peak power broadens the spectrum while the beam approaches
the filamentation regime. We identify two regimes for this transition: In the
center of the spectrum, the output spectral intensity increases continuously.
In contrast, on the edges of the spectrum the transition implies a bimodal
probability distribution function for intermediate incident pulse energies,
where a high-intensity mode appears and grows at the expense of the original
low-intensity mode. We argue that this dual behavior prevents the definition of
a univoquial threshold for filamentation, shedding a new light on the
long-standing lack of explicit definition of the boundary of the filamentation
regime.
