A sparrow found in the northern reaches of the Rocky Mountains of North America shows a tradeoff between lifespan and reproductive period according to how high up it lives, and thus the weather conditions and food available to it.
Populations of dark-eyed juncos (Junco hyemalis; pictured) in Canada's Jasper National Park are regularly found living as much as 2,000 metres above sea level. Heather Bears at the University of British Columbia and her colleagues monitored the songbirds at four 2,000-metre sites and at four 1,000-metre-sites. They found that juncos at the higher elevation lived longer and produced 55–61% fewer offspring per year than those living lower down. When these high-altitude birds did reproduce, however, they invested more heavily in each offspring; their chicks were 15–20% more likely to survive into adulthood and weighed, on average, 11% more at 25 days old.