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Problem with tracks estimated by SGAT

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I am working with geolocators data collected with Intigeo C65 devices on seabirds breeding in the Arctic (higher red dot on map1) and migrating in southern Atlantic in winter. Parts of the migration occurs during spring and fall equinoxes. I used a rooftop cablibration made in southern Quebec (lower red dot on map1) for about 14 days and I used the findHEZenith function to calculate a new zenith angle on the wintering area. Then I set a list of Zenith changing depending on the period of the year.

All my script is running, but for some birds, I have a strange movement up-north after a first staging period. When I create the initial path (thresholdPath function), everything seems to be okay, but when I run the estelleMetropolis function in 3 steps (burn-in, tuning, final runs), no matter how many iterations I do and no matter how many chains I set, the up-north movement appears. 

On the path on map2, the blue part is ± 10 days around equinoxes, gray is ± 15 days and black ± 20 days.

I tried changing all the parameters one by one, but the problem persists. 

Any idea what could cause this issue? 






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