d. How do we know about migration?

All day, column after column,
a squadron of feathers,
a fluttering airborne
ship
crossed
the tiny infinity
of the window where I search,
question, work, observe, wait

from "Migration" by Pablo Neruda

Throughout history there have been many inventive theories concerning bird's winter destinations: Aristotle thought redstarts transmuted into robins during winter, others believed that birds migrated to the moon; it was also long-thought that swallows hibernated at the bottom of ponds. We now know that only one bird hibernates - Phalaenoptilus nuttallii, a North American nightjar3.

i. Observation

One of the reasons why the migration of birds is relatively well-studied is that birds migrating during daytime are highly visible. In contrast, marine migrations are only now being understood, due to the difficulties of observing, catching and successfully tagging marine creatures. Systematic observation has been an important tool in revealing the timing, direction, regularity and duration of certain bird migrations. Observation programmes still play a key role today. The MIGRES programme records the passage of migrants across the Strait of Gibraltar (http://www.tarifa.net/nature/migreeng.htm).

Today cameras are enabling people to observe migrant birds the world over. Watch half a million sandhill cranes descend on the Platte river in the spring: http://magma.nationalgeographic.com/ngm/cranecam/ Or watch swifts feed their chicks in the tower of Oxford University Museum from May to August: http://www.oum.ox.ac.uk/swifts.htm

ii. Ringing

Emperor Frederick II, in the 13th Century, was the first to record the concept that migrants come and go in pursuit of food supply and heat, but he needed a method to follow birds once they took off. Ironically, the predominant method used to study migration today, known as ringing, had been used by the Romans centuries before. During a siege in the 3rd century, they attached pieces of parchment to the legs of swallows, effectively ringing them, but to be the bearers of human, rather than avian, news. Around 1250 AD, a German attached a message to the leg of a departing swallow, asking where it was going: the swallow returned the following year with a message from Asia⁴!