Phenological shifts in response to global warming have been widely reported for a large number species. However, the potential effects of phenological trends on individual fitness have rarely been assessed. In this study, we analyzed 32 years (1976 to 2007) of phenological data on nest initiation date (first and second annual clutches) of Painted Turtles, Chrysemys picta marginata. Data were collected from 865 individuals with a total of more than 4,000 observations in Southeastern Michigan, USA, a region characterized by a humid cold temperate climate. We investigated the association between nesting date and other attributes of females that could indicate an effect on their individual fitness. We used our results to identify potential effects of climate change on Painted Turtle populations in our study area.
Nest timing of the first clutch was strongly associated with spring temperatures (April-May average). Date of nesting was an average of four days earlier for each degree C increase in average spring temperature. Date of initiation of nesting was negatively associated with mean egg width, in warmer years, turtles nested earlier and produced wider eggs. However, changes in egg width took place only among smaller individuals that frequently produce only one clutch per year. Phenology of egg laying was also associated with clutch size, early reproduction in the previous year was associated with larger clutch sizes in the current year among all body sizes. Within the same season, we also found a positive relationship between early production of first clutches and the size of the second clutch.
Our results show that global warming in this relatively cool region could influence reproductive traits of these oviparous females in ways that may be interpreted as beneficial, especially among small size classes. Data shows that ambient temperature and number of females captured each year has increased during the study period. Together with a longer season to require resources, our results suggest a potential beneficial effect of early phenology on this population. However, the increasing trend in the number of captures plateaued around the year 2000, while temperature continued increasing, and after that the variability in number of captures increased substantially, indicating that other environmental or intrinsic processes are also influencing the dynamics of this population.