Birds cope with seasonal variation in environmental factors by adaptation of their physiology and life history. When seasonal variation is partly predictable, such adaptations can be based on endogenous circannual rhythms to anticipate reoccurring seasonal change (Helm 2009). The knowledge of local adaptation and adaptive potential of natural populations to seasonal variation is becoming increasingly relevant due to anthropogenic changes in the environment, such as climate change.
Although species have responded to climatic changes throughout their evolutionary history, a primary concern for natural populations and their ecosystems is the rapid rate of anthropogenic change. There is already compelling evidence that birds have been affected by recent climate changes (Walther et al.
2002; Parmesan 2006). Most studies report substantial evidence for changes in the phenology of birds, particularly of the timing of migration and of breeding onset; the implications of these responses, however, are not well understood. Many organisms advance the timing of their seasonal activities, thereby adapting to seasonal shifts in their environment (Coppack and Both 2002; Cotton 2003; Both et al. 2004, 2006; Visser et al. 2004). Study populations of African stonechats seem to follow a highly
conserved circannual reproductive schedule. I assembled in-depth knowledge of the life history, seasonal life cycle stages, and reproductive physiology focusing on a single population over the past three years. Birds from Kinangop, Kenya, did not attempt to breed outside the breeding season even when environmental conditions were optimal and other species within the same habitat initialized breeding. A long-term study on a geographically close African stonechat population at Lake Nakuru discovered similar findings (Dittami and Gwinner 1985). Neither heavy rains nor high levels of insect abundance were generally associated with any breeding activity outside the breeding season.
Interestingly, gonadal development during an extremely dry breeding season was uninhibited, and reproductive hormone levels did not rise with the subsequent belated onset of rain indicating the robust endogenous circannual rhythm of African stonechats (Dittami and Gwinner 1985). Our findings revealed substantial contributions of temporal and spatial isolation to population divergence in African stonechats. Considerable differences in the timing of breeding between populations on a local scale may promote population divergence and ultimately incipient ecological speciation processes.
The reliance on circannual rhythms is fundamental for timing essential annual activities, but may constrain the required fast adaptation to shifting seasonality in an era of climate change.
Understanding the endocrine mechanisms by which birds regulate the appropriate timing of breeding as well as the ecology and evolution of seasonal phenotypic traits will be essential for assessing whether populations can adapt their life histories and behaviors to cope with changes in the seasonality of their environment (Visser 2008).
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