Effects of weather on survival differ between dependent and independent juvenile Song Sparrows

Background/Question/Methods

The survival of juveniles to reproductive age is a critical component of population dynamics, and variation in juvenile survival can have significant impacts on population growth rates and contribute to population declines. A growing number of studies on the initial post-fledging period in passerines have shown that survival rates are low, variable, and sensitive to fluctuating environmental conditions. Despite widespread efforts to identify and project the effects of climate change on populations, few studies have examined the influence of weather on post-fledging survival. In addition, survival rates and the relative importance of weather factors may vary as birds develop from dependent fledglings to independent juveniles. I used 30 years of mark-recapture data (N=1,795) to examine the survival of juvenile Song Sparrows (Melospiza melodia) and the effects of weather factors during dependent (weeks 1-3), transitional (weeks 4-5), and independent (weeks 6-12) stages of development. 

Results/Conclusions

Juvenile survival within the first three months of fledging was lowest during the dependent stage, particularly the first week after fledging, and varied by year with a declining trend. Models including effects of total precipitation and average temperature in the previous winter on survival were strongly supported, but the effects varied by stage. Prior winter weather is believed to influence food availability, and had strong effects on the survival of independent juveniles, who must forage for themselves. In contrast, prior winter weather had little effect on the survival of dependent and transitional fledglings, who benefit from the care of their parents. The skills and abilities of parents may shield young fledglings from the effects of poor environmental conditions, while independent juveniles are fully exposed. Finally, the survival of independent juveniles was lower following warm, dry winters, so the projected increase in winter temperatures due to climate change may contribute to a long-term decline in juvenile survival. Examining juvenile survival by stage can provide new insights into factors driving variation in juvenile survival, which may in turn provide insights into variation in population growth rates.