57

 In Model III (Figure 13) both fish history and bird

history are important. Climatic events of previous years

influence biological events of the current year. This

model includes the age structure of the birds, and only

those birds 5 yrs old or older are capable of breeding. It

also includes large populations of predatory fish (gar,

bowfin and bass) that expand in wet years, decrease in dry

years, and compete with wading birds for food fish.

 Model III uses differential equations similar or

identical to those in Models I and II but makes increased

use of.discrete equations to handle logic functions such as

migration, fish kills, and the incrementation of age

cohorts. Modules on the right-hand side of the diagram

denote.age classes: 1, 2, 3, and 4 yr olds, which do not

breed; and the breeding stock, which consists of birds 5

yrs of age or older. Age cohorts are incremented once each

year, with mortalities subtracted. The number of breeding

birds increases or declines, depending on whether annual

recruitment (4-yr-olds becoming 5-yr-olds) exceeds average

mortality of the breeding stock.

 The bird biomass tank empties abruptly each June and

remains empty all summer, indicating that migration out of

the region has occurred. When the birds return to the area

in the fall the bird biomass becomes the product of

"minimum weight" and "number of sexually mature birds."

Bird feeding and bird respiration switch on at that time,