240

during these years is unknown, but the same conditions that

cause poor nesting success probably also result in low

fledgling survival.



Age Cohorts

 The exact age of first breeding in l'ood Storks is not

known. Kahl (1963) estimated that they first breed at 3 or

4 yrs. Age cohorts followed from one nesting season to the

next in the Corkscrew breeding record gave a better fit to

first breeding at age 4 or 5. Neither fit, however, was

very good. Two paradoxes were evident: In 7 yrs of

successful nesting not all the adult birds in the

population nested, and in at least 1 yr more adult birds

nested than could be accounted for in the population by the

Corkscrew breeding record. This possibly could mean that

the birds that nest at Corkscrew Swamp Sanctuary are not a

"closed" population but rather a group of birds that

interchanges with an other group or groups. An alternative

explanation is that the number of breeding pairs for a

given year is in some way adjusted to fit prevailing

conditions and that not all the birds that are capable of

breeding in a given year always actually do so. This

mechanism was suggested previously (page 33) as one way in

which available energy controls the size of a population.

 It was difficult to test the suitability of a selected

adult survival rate because of the movement of birds into

and out of the breeding group each year. It appeared that