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    Post Meaing of a sentence

    The environmental factors that have resulted in the evolution of migratory behavior are not the environmetal factors that stimulate development of the migratory condition.

    Plz help me to understand the meaning of this sentence. How is the underlined part related to the underlined part?

    Thank you

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    Re: Meaing of a sentence

    Some enviornmental factors resulted in migratory behavior, and other environmental factors stimulate the development of the migratory condition.

    Sorry, but I don't know what 'migratory condition' means. If I did, I could help you further.

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    Re: Meaing of a sentence

    Hi Soup,

    There are two links concerning the matter in question:

    Many temperate zone species, including many migrants, are known to respond physiologically to changes in the day length with season (photoperiodism). For example, many north temperate organisms are triggered to come into breeding condition by the interaction between the lengthening days in spring and their biological clocks (circadian rhythms). Similar processes, acting through the endocrine system, bring animals into migratory condition.

    Migratory behavior: Information and Much More from

    NPWRC :: Migration of Birds

    Experiments have demonstrated that day length is the environmental stimulus that results in vernal premigratory weight gain. Light not only directly affects the hypothalamic feeding centers but stimulates adjacent centers in the brain to affect a shift in the bird's endocrine secretions, specifically increasing prolactin from the pituitary, corticosterone from the adrenal gland, and the sex steroids (e.g., testosterone) from the gonads. These hormonal changes facilitate the development of fat deposits resulting from the greater food intake caused by increased appetite.
    The premigratory state is also characterized by increased activity during the night, which is when most birds migrate. They become restless, perhaps in anticipation of the migratory flight. This behavior is seldom observed in the wild, but has been carefully evaluated in captive migrants. It has been shown, for example, that the intensity and duration of migratory restlessness in captives are correlated with the distance and period of migration in the wild population. Like premigratory weight gain, migratory restlessness is stimulated by long days through the effect of light on the hypothalamus, causing increased secretions of prolactin, corticosterone, and the sex steroids. Additionally, light stimulates the release of melatonin, a hormone produced in the pineal body on the top of the brain, which has also been shown to be necessary for the expression of this behavior.
    It is important to emphasize that the light stimulus is a function of length of the light period rather than because of the change in daylengths. It is also clear that the absolute length of the daylight period that is considered "long" varies with species, not only in terms of the daylength characteristics of their environments but in the daily period when a species' brain is receptive to the effects of light. Both the external and internal aspects of light stimulation reflect their geographic distributions. Thus, birds wintering in the tropics have evolved a response to that photoperiod which results in premigratory changes similar to that of birds wintering in the North Temperate zone under increasing daylength. Even birds wintering in South America initiate premigratory preparation in March and April under the decreasing daylengths of the austral fall.



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