The role of temperature and microclimate in the survival of wintering grassland birds

Over the past 40 years, grassland birds have declined steeply in North America, necessitating the study of limiting factors throughout their full annual cycle to mitigate this decline. Here we explored factors that may influence winter survival of two grassland specialist birds with steep declines; Baird's Sparrow (Centronyx bairdii) and Grasshopper Sparrow (Ammodramus savannarum). We studied the relationship of broad-scale ambient temperature and vegetation structure with winter survival of these species and explored the role of vegetation structure in providing microclimatic refuges for these overwintering sparrows. During three winters from 2016-2019, we monitored these species in the Marfa grasslands, Texas, using radio-telemetry. We placed radio-transmitters on >200 individuals of both species combined, and tracked them daily from mid-December to mid-March. We estimated ground cover in a 5 m radius circular plot for >= 20 locations per individual. We also placed temperature loggers at 40 bird points (of both species combined) and 40 random points to measure microclimate used by these birds. We then estimated winter survival probability using logistic-exposure and used general linear models to evaluate the relationship of ambient temperature and habitat covariates with survival. Winter survival over an 85 day period was lower for Grasshopper Sparrow than Baird's Sparrow (three-year mean = 65.92% and 85.54%, respectively), and ranged from 47% to 100% for both species. We found that minimum ambient temperature was the main factor limiting survival of sparrows. Microclimate temperatures were higher in bird locations compared to random locations and were warmer in shrub cover and tall grass than short grass or litter. These results indicate that microclimate may be important to sparrows on the wintering grounds and highlight the value of habitat structure for providing thermal refuges. Our results emphasize the need to maintain vegetative cover for grassland birds to protect against harsh weather conditions that may limit their survival.

Automated summary

Grassland birds in North America have experienced a steep decline over the past 40 years. In this study, researchers investigated the factors influencing the winter survival of two grassland specialist birds, Baird's Sparrow and Grasshopper Sparrow. They found that minimum ambient temperature was the main limiting factor for the survival of these sparrows, and vegetation structure played a role in providing microclimatic refuges. The results highlight the importance of maintaining vegetative cover for grassland birds to protect against harsh weather conditions.