Nest-site selection, energetics of reproduction and comparative behaviour of mallard (anas platyrhynchos) and blue-winged teal (A. discors)

An organism's reproductive success may be affected by choice of breeding habitats and subsequent investment in offspring. Nest microclimate may profoundly influence energetic costs of incubation or embryonic survival, but few studies have evaluated whether birds select nest-sites based on microclimate. Likewise, increased investment of nutrients in eggs may have costs in terms of future survival and reproduction. These ideas have received only limited testing, particularly behavioural and physiological responses to clutch size variation of precocial birds. Therefore, I evaluated whether: (1) ducks select nest-sites for thermal features; and, (2) incubation behaviours, incubation periods, or energetic costs of incubation are related to clutch size. Blue-winged teal ( A. discors; hereafter teal) nested most often in grass or short shrub habitats, while mallard (Anas platyrhynchos) nested in all habitats. Within nesting habitats, microclimate followed a gradient from hot and sunny in grass-dominated sites to cool and shaded sites in aspen (Populus spp.) groves. I measured microclimatic conditions at mallard and teal nests and randomly located nonnest sites, to evaluate thermal consequences of nest-site selection. Duck nests received on average less solar insolation than nonnest sites, producing more moderate thermal conditions than at nonnest sites. When mallard and teal nests were concurrently monitored, teal selected warmer nest-sites than mallards. Thus, teal may choose warmer nests to offset costs of incubation, a hypothesis which has not been tested. Apart from egg formation costs, processes occurring during incubation may also limit clutch size of precocial birds. I tested if incubation periods were positively correlated with clutch size and found that incubation periods of captive mallards and wild teal, but not wild mallards, were longer for females incubating clutches enlarged by 50%. Incubating females may respond to increased energetic costs of incubation by: (1) losing more mass, (2) decreasing nest attendance, or (3) some combination of both. Mass loss and mass at hatch were unrelated to clutch enlargement, however, mass loss was positively correlated with mass at the start of incubation. While controlling for effects of weather, I found no change in incubation behaviours of mallards incubating enlarged clutches, but, teal increased overall recess frequency. Finally, I evaluated effects of clutch enlargement on incubation strategies, representing an integration of behaviour and body mass dynamics. When controlling for weather and body mass dynamics, clutch enlargement produced significant, yet subtle changes in incubation behaviours. Mallards took slightly shorter recesses, but maintained constancy, whereas teal increased recess frequency and decreased nest attendance. Energetic costs of incubation are controversial. I tested if incubation metabolic rates (IMR) were greater than resting metabolic rate (RMR). Incubation caused an increase in metabolic rate by mallard and red junglefowl (Gallus gallus spadiceus). I tested the assumption that IMR is positively correlated with clutch size and found that IMR of junglefowl within thermoneutrality was unrelated to clutch mass. Further work is needed to evaluate metabolic responses of precocial birds to clutch size variation. Overall, clutch size of precocial birds (particularly small species) may be limited by costs in terms of extended laying and incubation periods, as well as by subtle changes in incubation behaviours. Further integration of incubation costs with those associated with egg formation and brood-rearing, would provide a comprehensive and powerful analysis of factors limiting clutch size in precocial birds