Energy and Water Budgets of Larks in a Life History Perspective: Parental Effort Varies with Aridity

We compared physiological, demographic, and ecological variables of larks to gain insights into life history variation along an aridity gradient, incorporating phylogenetic relationships in analyses when appropriate. Quantifying field metabolic rate (FMR) and water influx rate (WIR) of parents feeding nestlings as measures of parental effort, we found that parental FMR and WIR of larks decreased with increasing aridity. Water and energy requirements of 6–8 d old nestlings were reduced in desert species. Nestling growth rate, clutch size, and number of clutches decreased with increasing aridity, and nest predation rates increased with increasing aridity. We combined FMR and WIR of parents and chicks, energy and water accumulated during growth, and brood size to establish energy and water budgets of parent–brood units. Parent–offspring energy budgets equaled 261 kJ/d for Hoopoe Lark, 164 kJ/d for Dunn’s Lark, 388 kJ/d for Skylark, and 347 kJ/d for Woodlark, a 27% reduction in the desert species when taking into account mass differences. Family unit WIRs were 46.6 g/d for Hoopoe Lark, 37.3 g/d for Dunn’s Lark, 101.6 g/d for Skylark, and 83.2 g/d for Woodlark. Parent–brood units of arid-zone species used 28–51% less water per gram mass than species from mesic areas. These results support the hypothesis that decreasing food and water availability favor lower energy and water requirements of parents and young, reduced growth rates, and smaller clutch sizes with increasing aridity. The decrease in parental effort with increasing aridity might reflect a lower fitness value of a single brood for arid-zone species than for larks from mesic habitats, suggesting that the probability of adult survival is higher in arid than in mesic areas.