Latitudinal Trends in the Metabolic Adjustments of the House Sparrow

Between latitudes 9 degrees and 59 degrees in North America, the House Sparrow, Passer domesticus, shows progressive increases northward in energy expenditures for temperature regulation, existence metabolism, reproduction, daily energy budget, permanent residency, and mobilizable energy. Energy requirements are greater in the winter than in the summer for temperature regulation, existence metabolism, daily energy budget, and mobilizable energy. Seasonal acclimatization to winter temperatures brings lower rates at equivalent temperatures in existence metabolism. However, latitudinal adaptation at colder northern latitudes brings higher rates at equivalent temperatures in existence metabolism. The number of broods produced per year is inversely proportional to the energy cost per brood, and the energy expended by the species for reproduction is approximately the same at all localities. The reduction in work capacity at high ambient temperatures increases southward, but is greater in desert climates with large daily and seasonal fluctuations in temperature than in the tropics with their uniform climates. The species may be limited in dispersal into desert regions by daily maximum temperatures that approach or surpass its upper limit of tolerance, but is capable of dispersing through the Central American tropics into South America. The species is limited northward by low temperatures at which it is unable to mobilize sufficient energy for existence and/or reproduction except where it makes use of artificial man—made shelters and food supplies. These variations and adjustments noted between geographic populations of the House Sparrow probably apply to bird species generally.