Proximate mechanisms in behavior and evolution

highly tractable subjects for research in behavior and ecology. Studies of Eurasian tits and North American chickadees have made substantial contributions to our understanding of breeding phenology (Chapter 4; Chapter 5), food-storing (Chapter 2; Chapter 3), social behavior (Chapter 9), communication (Section III), foraging, and other topics. More recently, research on the Paridae has begun to address questions about the proximate neural and neuroendocrine causes of behavior. There are many reasons for the intense focus on these small, familiar birds. Most Parids are relatively tame, year-round residents that live in small home ranges and are easily attracted to feeders. Some, such as great and blue tits, readily breed in artificial nest boxes (though the most intensely studied North American Parid, the black-capped chickadee tends not to). These traits make it possible, at least in principle, to obtain complete observations on development, social organization, communication, and reproduction in individuallymarked birds over their entire lifetime. In both North America and Eurasia there are enough species to permit comparative analyses, and some species, such as the black-capped chickadee, have distributions large enough to permit comparisons between populations living in very different environmental conditions (Chapters 3 and 5). Most chickadees and tits thrive in captivity. Early in the 1700s Baron von Pernau kept marsh tits in his room to observe their food-storing behavior (Stresemann 1947). The ease of holding birds in captivity has made possible a great deal of research on behavior, cognition, and neurobiology in species for which there is extensive information on behavior in the wild (Chapters 2, 3, and 4). The integration of field and laboratory work has, for example, led to advances in research on control of the annual cycle. Although temperature, food supply, and other factors exert important effects on the timing of reproduction, photoperiod is the stimulus that initiates the sequence of events leading to gonadal recrudescence, mating, egg laying, and parental care. These behavioral and physiological changes are not, strictly, controlled by day-length but instead, as Phillmore and MacDougall-Shackleton describe, by an annual cycle in sensitivity to daylength. Successive stages of photorefractoriness and photosensitivity prepare birds to respond to increasing day-length in spring and initiate breeding. These responses include change in size of the song control nuclei HVC and RA, and the recruitment of new neurons into these structures. Some recent research also shows an annual pattern in the recruitment of new neurons into the hippocampus and change in the overall size of this structure, but data from different studies have not been consistent. There are different opinions about the function of such patterns (Chapter 3) and seasonal change in the hippocampus continues to be a topic of active investigation. Remarkably, for both the song control nuclei and the hippocampus, laboratory and field studies