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Wild animals typically organize activity around a 24-h day and daily timing across the year is optimized for both survival and reproductive success. Among-individual variation in chronotype, where individuals differ in when they begin or end their active day relative to a cue such as photoperiod, often exists within a population. Both intrinsic and extrinsic factors contribute to this variation and activity patterns may change across and within different life-history stages as energetic investment changes. Here we describe population level changes in free-living female great tit (Parus major) activity patterns of onset and offset of activity as well as assess variation and repeatability in daily activity both within- and across-breeding stages. We fitted individual females with accelerometers to track activity prior to nest building through chick rearing. Prior to clutch initiation females began their active day before sunrise, however, in the days prior to laying their first egg, activity was delayed until after sunrise. Females ended activity prior to sunset across the monitoring period and earliest during egg laying and incubation. In addition, females exhibited greater among- and within-individual variance in activity during parental care. Female daily activity was moderately repeatable within breeding stages and strongly covaried across several breeding stages. These findings expand our understanding of individual variation in activity patterns during reproduction and the potential fitness implications of chronotype in wild animals. [ABSTRACT FROM AUTHOR]

Although we have long understood that environmental variation affects both physiology and behavior, historically, most studies have limited or simplified environmental variation to focus more directly on traits of interest. Recently, a number of investigators have turned their focus toward attempting to incorporate such variation into studies of physiology and behavior, and not surprisingly, are finding that the results from studies that include more realistic variation, both from the environment as well as in physiological processes within individuals, can differ substantially from those of studies that attempt to hold the parameters constant. Understanding the role that this dynamic variation plays in shaping phenotypes is critical given that, under most predictions from future climate change models, increased variability in factors such as temperature and rainfall are predicted. [ABSTRACT FROM AUTHOR]

Research in captive birds and mammals has demonstrated that circadian (i.e. daily) behavioral rhythms are altered in response to increases in sex-steroid hormones. Recently, we and others have demonstrated a high degree of individual repeatability in peak (gonadotropin-releasing hormone [GnRH]-induced sex) steroid levels, and we have found that these GnRH-induced levels are highly correlated with their daily (night-time) endogenous peak. Whether or not individual variation in organization and activity of the reproductive endocrine axis is related to daily timing in wild animals is not well known. To begin to explore these possible links, we tested the hypothesis that maximal levels of the sex steroid hormone estradiol (E2) and onset of daily activity are related in a female songbird, the dark-eyed junco (Junco hyemalis). We found that females with higher levels of GnRH-induced E2 departed from their nest in the morning significantly earlier than females with lower stimulated levels. We did not observe a relationship between testosterone and this measure of onset of activity. Our findings suggest an interaction between an individual's reproductive endocrine axis and the circadian system and variation observed in an individuals' daily activity onset. We suggest future studies examine the relationship between maximal sex-steroid hormones and timing of daily activity onset. [ABSTRACT FROM AUTHOR]

Figure 1 in Bauer et al. (2016) incorrectly identified boxplots as representing means with standard deviations, while in actuality they represent medians within interquartile ranges. [ABSTRACT FROM AUTHOR]