Official websites use .gov
A
.gov website belongs to an official
government organization in the United States.
Secure .gov websites use HTTPS
A lock (
) or https:// means you've safely
connected to the .gov website. Share sensitive
information only on official, secure websites.
Schematic of the four photoperiod schedules birds were exposed to during the experiment. (a–d) The 24-h skeleton photoperiod, the control 24-h day, the 27- and the 21-h day. The descriptions on the right-hand side indicate the number of subjective days per calendar year, as well as the number of light hours. Note that double plots over 48 h are shown.
