Foster and Kreitzman - Seasons of Life - Migration
Type of Spiritual Experience
I am now going to take the migration function and extract from Foster and Kreitzman's fascinating book a description where inter-system dependency occurs as well as inter species dependency.
The same pattern of triggers occurs for the migration, the use of the GPS system, the use of orbiting systems and angle of tilt. But there are a number of functions described for the birds themselves:
The functions which are all triggered by the inputs from the migratory identification system are:
- increase efficiency of fat production and storage.
- Increase pectoral muscles
- Increase enzyme production
- switch diet
- shrink liver, stomach or intestine
Once the bird has reached the right weight and achieved all these activities it will then fly. This is shown below diagrammatically but in much simplified form.
A description of the experience
Seasons of Life – Russell G Foster and Leon Kreitzman
Before they take off, migratory birds run through a repertoire of behavioural and physiological changes.
A marked increase in appetite and food consumption termed hyperphagia, begins about 2 to 3 weeks before migration and persists throughout the migratory period. This is accompanied by an increase in the efficiency of fat production and storage.
As a result, a migratory bird can increase its body weight through fat deposition by as much as 10% per day …. The ruby throated humming bird, which flies across the Gulf of Mexico and the sedge warbler which migrates from Britain to West Africa, lay down huge fat reserves relative to their size. This is necessary because both species complete their migration in a single sustained flight. Species that carry smaller loads of fat or fly longer distances typically fly in a series of stages, stopping to feed and put on weight along the route.
Additionally the pectoral muscles become larger and well supplied with enzymes necessary for oxidation (burning) of fat. Fat is the best fuel because not only is it lighter and less bulky than carbohydrates or protein, but it supplies twice as much energy per unit mass compared with protein and glycogen.........
One of the most remarkable all round performances is shown by the blackpoll warbler. Its over water flight to South America keeps it continuously in the air for 80-90 hours. … A typical blackpoll warbler at the end of the breeding season weighs about 11 grams. In preparing for its Atlantic trek it may.... increase its body weight to 21 grams.
Readying for migration often involves a switch in diet. Many autumn migrants switch from insects to a diet of berries and other fruits (high in carbohydrates and lipids). ….Migratory birds often shrink their livers, stomachs or intestines in anticipation of their journey.. shrinking their guts and other organs by half ior more reduces energy consumption. Their guts grow after they begin eating again... Most migratory birds are solitary for much of the year but start to flock together before or during migration. This social behaviour seems to provide improved predator avoidance, food finding and orientation