The drive on luteinizing hormone secretion in castrated tree sparrows (Spizella arborea) exposed to short days is daylength independent.

For many, if not most, photoperiodic species of birds, short days are nongonadostimulatory. The tacit assumption that short days are also nonphotostimulatory was tested by determining whether plasma concentrations of luteinizing hormone (LH) in castrated tree sparrows exposed to short daily photoperiods (8 hr or less) are daylength dependent or independent. Castration of tree sparrows held on an 8-hr daily photoperiod evoked, within 2 weeks, a fivefold elevation in plasma LH concentration. Over the next 24 weeks, plasma LH concentrations of castrated birds were resistant, first to a stepwise reduction in daylength from 8 to 2 hr and later to a 2-, 4-, or 6-hr increase therein. At no time did LH concentrations differ among castrated birds held on the same or different short-day photoregimes. Conversely, at all times, LH concentrations of castrated birds, regardless of photoperiodic history, exceeded those of intact males held on an 8-hr daily photoperiod and sampled at the beginning of the experiment. These data, which argue that the drive on LH secretion in castrated tree sparrows exposed to short days is daylength independent and, therefore, likely intrinsic, verify the tacit assumption that short days are nonphotostimulatory.