Instantaneous Versus Average Speed

In his paper documenting the research on the migration of the Arctic Tern, Egevang states that the flight speed of the Arctic Tern was measured to be 11.3 m/s which corresponds to a daily travel distance of 976 km. However the maximum average daily speed did not exceed 520 km. How can this be explained?

1. The Arctic Tern is capable of flying thousands of miles without landing, but as the numbers show it does not fly at full speed the whole time.  Assuming that it travels at about 10 m/s for part of the day, and for the remainder of the day it glides without making significant progress, how many hours per day must it travel at 10 m/s?
2. If we were to assume that the Arctic Tern travels at a constant speed, at what speed (in m/s) must it travel to cover 520 km per day?

Note:  The method used to measure the distance traveled by the Arctic Tern, in which a small device measures latitude and longitude twice a day through the measurement of light (details), as a accuracy of plus or minus 150 km. This fact can also be used to explore the meaning of accuracy in measurement. The 150 km accuracy is not sufficient to determined the speed of travel of one animal over a single day, because the uncertainty of position can be greater then the distance traveled in a single day. As the days go by, more data is recorded and the distances increase, 150 km becomes a small fraction of the distance covered. This then provides enough accuracy to determine migration paths and rate of travel.