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Takes account of the extra day in Leap years.
Jan 1 = YearDay1, Feb 29 adds an extra day in Leap years so that March 1 is YearDay 60 in non Leap years but is YearDay 61 in a Leap year.
Typically females arrive 14 days later than males. The males have already set up territory and the females sneak in. They are much more difficult to net than the males, hence the apparently lower numbers.