| dc.description.abstract | Many species of birds migrate nocturnally. In nocturnally migrating birds, endogenous
circadian and circannual rhythms serve as biological pacemakers (clocks). They are responsible
for an individual’s migratory behavioral pattern (Zugunruhe). Circannual rhythms
provide the major basis for the initiation of migratory restlessness both in autumn and
spring. Circadian rhythms on the other hand, play an important role in regulating the increase
of nocturnal activity that is necessary for flying long distances. The significance
of these endogenous factors is evident in migrating birds in the sense that, zugunruhe
keeps happening even in the absence of external inputs. Plasma melatonin concentration
is crucial to this phenomenon. Melatonin concentrations in most migratory birds increase
at night regardless of whether individuals are day or night active. In the course of migratory
seasons, many birds show restlessness and excessive activity during night. In migratory
populations, night level of melatonin is lower during the migratory period, when birds show
nocturnal restlessness, than before and after this period, when birds do not show nocturnal
activity. The operation of this endocrine system of oscillators is far too complex to
understand by intuition so we unavoidably have to resort to abstractions. Mathematical
models are therefore essential in understanding the mechanism of the components of this
system and to further determine the dynamics of these periodic oscillators. Scheper model
is a set of delay differential equations which describes the circadian clock. This model is
based on the total duration of the chain reactions and the nonlinearity between input and
delayed output of the protein synthesis negative feedback loop. In this study, two systems
of scheper equations were coupled to produced circannual oscillation depicting the seasonal
change in the melatonin level of nocturnally migratory birds. The coupled system
displayed realistic behavior of circannual rhythm with respect to period and entrainment. | en |
