  Polar Crown Filaments (PCFs) form above the magnetic polarity inversion line,
which separates the unipolar polar fields and the nearest dispersed fields from
trailing part of active regions with opposite polarity. The statistical
properties of PCFs are correlated with the solar cycle. Therefore, study of
PCFs plays an important role in understanding the variation of solar cycle,
especially the prolonged cycle 23 and the current "abnormal" solar cycle 24. In
this study, we investigate PCFs using full disk H{\alpha} data from 1973 to
early 2018, recorded by Kanzelhohe Solar Observatory (KSO) and Big Bear Solar
Observatory (BBSO), in digital form from 1997 to 2018 and in 35 mm film
(digitized) from 1973 to 1996. PCFs are identified manually because their
segmented shape and close-to-limb location were not handled well by automatical
detections in several previous studies. Our results show that the PCFs start to
move poleward at the beginning of each solar cycle. When the PCFs approach to
the maximum latitude, the polar field strength reduces to zero followed by a
reversal. The migration rates are about 0.4 to 0.7 degree per Carrington
rotation, with clear N-S asymmetric pattern. In cycles 21 and 23, the PCFs in
the northern hemisphere migrate faster than those in the southern hemisphere.
However, in the "abnormal" cycle 24, the southern PCFs migrate faster, which is
consistent with other observations of magnetic fields and radio emission. In
addition, there are more days in cycle 23 and 24 without PCFs than in the
previous cycles.
