Introduction

Life depends on an essentially continuous exchange of mass and energy between living organisms and their environment.  Energy transfer between organisms and their environment is critically sensitive to changes in ambient temperature, wind, solar radiation and humidity.  The ability of animals to sustain behaviors such as foraging or reproduction is limited by their capacity to balance energy intake with energy expenditure.  In our investigations, we addressed the hypotheses highlighted above, with emphasis placed on reactions of House Sparrows to cold temperatures.  Thus, our primary research question was:

how does variability in thermal microclimates affect the rate of heat exchange and energy expenditure of free-living animals?

Methods

In order to maintain homeostasis, passerine birds must adapt to the variability of their environment for survival, growth and fitness.  The design of our experiment was to first measure surface (skin) temperature of a House Sparrow under no wind conditions, and secondly under wind conditions of 2.0 m/s.  These measurements were taken directly from Infrared Thermography images of the bird contained in a holding chamber.  Also measured was the amount of oxygen consumed by the bird while being subjected to no wind and wind conditions. (slides from E. Zerba, Princeton University)    

Biophysical equations describing energy expenditure of an endotherm (bird or mammal).  LCT represents the Lower Critical Temperature.  At LCT, the ambient temperature remains constant as heat loss by evaporative means beings to increase.  Ts and Ta values were measured in laboratory experiments;  surface area, A, was derived from Zerba et al, (unpublished) using ellipsoid models.

 Metabolism chamber for measurements of surface temperature and metabolic rate of House Sparrows.  Chamber designed by E. Zerba, Princeton University.  Construction by G. Ward with assistance by R. Ray, Colgate University.

IR Thermography image of a House Sparrow exposed to 15°C and no wind.  Red colors represent higher temperatures grading into cooler temperatures with blue colors.

IR Thermography image of a House Sparrow subjected to wind speeds of 2.0 m/s at 15°C.  Note the decrease in red (higher temperatures) and increase in blue (cooler temperatures) from the image at left.

Effects of wind on surface temperature of a House Sparrow.  Surface temperature decreases with exposure to wind speeds of 2.0 m/s.  Error bars indicate two standard errors above and below the mean of three birds sampled.  Note the standard error bars do not overlap, indicating a statistical significance to the data set.  The significant difference is p<0.05.

Metabolic heat production of a House Sparrow exposed to wind speeds of 2.0 m/s.  Metabolic heat production, in watts/gram increases with increasing wind speed.  Error bars indicate two standard errors above and below the mean of three birds sampled.  Note the standard error bars do not overlap, indicating a statistical significance to the data set.  The significant difference is p<0.05.

Conclusions

Our hypotheses stated that birds exposed to wind will have lower surface body temperatures and higher rates of metabolic heat production compared to birds in still air and that Infrared Thermography (IR), coupled with biophysical models, can accurately predict energy expenditures of House Sparrows under thermal conditions typical of those experienced in nature.  This implies that energy demanding activities of birds inhabiting exposed microclimates may be compromised.    These hypotheses were supported in our laboratory experiments, and may be used to integrate future field and laboratory experiments to predict energy expenditure of free-living animals.  However, more conclusive studies must be conducted in order to more accurately determine the extent of these relationships.

Questions for further study

1)  What adaptations must birds exhibit in order to survive in variable climates?

2)  Does energy expenditure dictate microhabitat selection of birds? If so, how can we quantify the extent to which energy expenditure is affected by thermal conditions typical of those experienced in nature? 

3)  How can field and laboratory analyses be more fully integrated in order to quantify energy expenditure of free-living animals?