To stay aloft, the birds took advantage of circular updrafts, originating under cumulus clouds, which carried them to heights of up to 1,600 meters (5,249 feet) with barely a flap of the wing. Once the ride was over, they could glide for long distances – up to 60 kilometers (37 miles) – before needing to find another updraft to regain altitude...."
http://www.csmonitor.com/Science/2016/0701/How-can-frigatebirds-stay-aloft-for-months-at-a-time
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