  The criterion for the initiation of breaking demonstrated numerically by
Barthelemy et al. (2015) has been investigated in the laboratory for
unidirectional wave groups in deep-water and extended to include conditions of
moderate wind forcing. Thermal Image Velocimetry was used to compare
measurements of the crest surface water particle velocity (Us) with the wave
crest velocity (C), as determined by an array of closely-spaced wave gauges.
The energy flux ratio Bx = Us/C that distinguishes maximum recurrence from
marginal breaking was found to be 0.840 $\pm$ 0.016 in good agreement with the
numerically determined value of 0.855. Further, the threshold was found to be
robust for different classes of wave groups of distinct characteristic
steepness at the breaking threshold. Increasing wind forcing from zero to
U{\lambda}/4/C0=1.42 increased this threshold by 2%. Increasing the spectral
bandwidth (decreasing the Benjamin-Feir index from 0.39 to 0.31) systematically
reduced the threshold by 1.5%.
