Provisioning Behavior of Male and Female Grasshopper Sparrows

—We examined the provisioning behavior of male and female Grasshopper Sparrows (Ammodramus savannarum) during 2002 and 2003 by videotaping nests (n 5 15) and subsequently reviewing tapes to quantify provisioning rates and identify prey items. There was no difference in provisioning rates of male and female Grasshopper Sparrows (P 5 0.13) with mean rates of 2.16 visits/hr for females and 1.86 visits/hr for males. Provisioning rates for males and females combined varied with nestling age (P 5 0.01) with rates lower for 1–4-day-old nestlings, increasing through day 6 and then declining for 7–10-day-old nestlings. Provisioning rates varied with brood size (P 5 0.026) with rates higher for broods of five than broods of three or four. Most prey items delivered to nestlings were grasshoppers (Orthoptera, 68.1%). Received 8 October 2010. Accepted 22 January 2011. The relative contribution of males and females in provisioning nestlings varies among species of songbirds. One parent may provision nestlings at higher rates than the other parent in some species (Grundel 1987, Cockburn 2006); whereas males and females contribute equally in other species (Rytkonen et al. 1996, Schadd and Ritchison 1998, Nordlund and Barber 2005). Variation among species suggests factors influencing parental investment may produce different provisioning strategies in different species. Olson et al. (2008), in a comparative study involving data from 193 species of birds, reported parental care by males and females was negatively correlated; variance in male care was significantly higher than for female care. Thus, across species, variation in total care is primarily explained by differences in that provided by males. Several factors can influence male parental care. For example, males may reduce levels of parental care to pursue extrapair matings (Mitchell et al. 2007). Males in multi-brooded species may improve their reproductive success by provisioning nestlings in first broods, increasing the likelihood of their mates subsequently re-nesting (Stodola et al. 2009). Risk of nest predation may also influence male care as Bjornstad and Lifjeld (1996) suggested provisioning by male Willow Warblers (Phylloscopus trochilus) insured rapid growth of nestlings, early fledging, and reduced risk of nest predation. These and other factors can potentially influence male provisioning behavior, but their relative importance in explaining variation in male care among different species of birds remains unknown. Thus, studies are needed of species that vary in behavior (e.g., mating strategies and number of broods per breeding season) and nesting ecology (e.g., nest predation rates). Previous studies indicate several factors, including variation in brood size and nestling age, can affect provisioning behavior of both males and females. Parents, for example, may increase provisioning rates as brood sizes increase (e.g., Bédard and Meunier 1983, Conrad and Robertson 1993) or there may be an inverse relationship between provisioning rate and brood size (e.g., East 1981, Trémont and Ford 2000). Some parents provision nestlings at similar rates throughout the nestling period (East 1981, Bédard and Meunier 1983, Schadd and Ritchison 1998), while others either continue to increase provisioning rates as nestlings grow (Nordlund and Barber 2005) or increase provisioning rates through the mid-nestling period before reducing rates a few days before fledging (East 1981, Conrad and Robertson 1993). Grasshopper Sparrows (Ammodramus savannarum) are socially and genetically monogamous (Ammer 2003), and typically breed in grasslands and lightly grazed pastures (Vickery 1996). These sparrows nest on the ground with clutch and brood sizes ranging from three to five and typically have low nesting success (25–50%) (Vickery 1996). Grasshopper Sparrows are also multi-brooded (2–3 nests/breeding season) (Vickery 1996). Both males and females provision nestlings (Vickery 1996), but the relative contributions of each are unknown. The objectives of our study were to examine the possible effects of brood size, brood number, and nestling age on the provisioning behavior of male and female Grasshopper Sparrows.