Long-duration GRBs stand out most when compared with the field population in terms of their specific star formation, or star formation per unit mass.  Because they often explode in dwarf (i.e., low mass) galaxies, the total star formation rate is not particularly elevated.  But per unit mass, their host galaxies are some of the most extreme star-forming galaxies in the Universe.  While they have comparable B-band luminosities, short-duration GRBs span a much larger range of star formation rates.  As a result, the median specific star-formation rate of short-duration GRB hosts is a factor of ~ 7 less, and is quite comparable to that in field galaxies.  Because the specific star formation rate is the inverse of the characteristic time scale to build up a galaxy's stellar mass, the smaller values in short-duration GRBs indicates that the progenitors track star-formation with a delay of hundreds of millions to several billion years..