Clutch and egg size of Tree Swallows along an elevational gradient

How avian reproductive behavior changes at high elevations, and whether changes are the result of adaptation or constraint remains unclear. We compared clutch and egg sizes in two populations of Tree Swallows (Tachycineta bicolor), one at an elevation of similar to 2500 m a.s.l. in Wyoming's Bighorn Mountains and the other at similar to 1350 m at the base of these mountains. Females at the high-elevation site began laying 10 d later, on average, than females at the lower site. Females at the high site laid an average of 0.4 fewer eggs than those at the low site, a significant difference. Eggs were also slightly (2.3%), but significantly, smaller at the high site. Smaller clutches and eggs at the high site may have resulted in part from greater energetic constraints on females. Females at this site faced colder, windier weather during egg formation which would have increased thermoregulatory costs while simultaneously reducing the abundance of prey, i.e., flying insects. Laying a relatively small clutch at the high-elevation site may also be adaptive, however. Having a smaller brood, could help ensure there is an adequate supply of food for each offspring during bouts of inclement weather that are more severe at higher elevations. Also, if the delay in breeding inherent at hi g h elevations reduces the survival prospects of the young, then producing fewer young could enhance a female s chances of breeding again, perhaps at a different location.