Abstract
White-throated sparrows increase fat deposits during pre-migratory periods and rely on these fat stores to fuel migration. Adipose tissue produces hormones and signaling factors in a rhythmic fashion and may be controlled by a clock in adipose tissue or driven by a master clock in the brain. The master clock may convey photoperiodic information from the environment to adipose tissue to facilitate pre-migratory fattening, and adipose tissue may, in turn, release adipokines to indicate the extent of fat energy stores. Here, we present evidence that a change in signal from the adipokines adiponectin and visfatin may act to indicate body condition, thereby influencing an individual's decision to commence migratory flight, or to delay until adequate fat stores are acquired. We quantified plasma adiponectin and visfatin levels across the day in captive birds held under constant photoperiod. The circadian profiles of plasma adiponectin in non-migrating birds were approximately inverse the profiles from migrating birds. Adiponectin levels were positively correlated to body fat, and body fat was inversely related to the appearance of nocturnal migratory restlessness. Visfatin levels were constant across the day and did not correlate with fat deposits; however, a reduction in plasma visfatin concentration occurred during the migratory period. The data suggest that a significant change in the biological control of adipokine expression exists between the two migratory conditions and we propose a role for adiponectin, visfatin and adipose clocks in the regulation of migratory behaviors.
Highlights
Small songbirds have evolved many strategies to cope with the costly rigors of long-distance migration
During the pre-migratory period, physiological and endocrine systems change in preparation for migration; birds become hyperphagic and alter their metabolism resulting in increased fat deposition [1,2]
Multiple isoforms of ADP were observed in the white-throated sparrow, not all of which have been observed in the domestic chicken; multimeric bands of similar size were observed in Japanese quail (Coturnix japonica)
Summary
Small songbirds have evolved many strategies to cope with the costly rigors of long-distance migration. During the pre-migratory period, physiological and endocrine systems change in preparation for migration; birds become hyperphagic and alter their metabolism resulting in increased fat deposition [1,2]. Many small passerines commence migratory flight after peak fat deposition has been reached [1], how each individual determines whether it has enough fat accumulated to initiate migratory flight is unclear [6,7]. Pre-migratory hyperphagia and fattening may lead to changes in metabolic signals, such as adipokines, that circulate in the blood of individual birds. Changes in metabolic signals could be utilized to initiate migration once fat stores reach a critical level. A relationship between body condition and migratory behavior has been highlighted in the literature, little is known of the physiological mechanisms linking endogenous information about fat stores and behavioral flexibility
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