ANIMAL BEHAVIOUR, 2003, 65, 435-443 do;:l 0.1 006/anbe.2003.2086 @ Postfledging parental care in Savannah sparrows: sex, size and survival NATHANIEL T. WHEELWRIGHT*, KIMBERLY A. TICE* & COREY R. FREEMAN-GALLANTt *Departmentof Biology, Bowdoin College,Brunswick, Maine tDepartment of Biology, SkidmoreCollege, Saratoga Springs, New York (Received 2 August 2001; initial acceptance 11 February 2002; final acceptance 21 May 2002; MS. number: A9128) We investigated postfledging parentalcare in a philopatric population of Savannah sparrows, Passerculus sandwichensis, breeding on Kent Island, New Brunswick,Canadain an effort to understand the factors influencing adult birds' decisionsabout parental investment in offspring. Brood division was not based on offspring sex: male and female parents were equally likely to care for sons or daughters. The total duration of parental care, from hatching to independence,was similar for sons and daughters (median=23 days),regardless ,of the sex of the care-givingparent. The duration of parental care also corresponded closely to the time required for juvenilesto acquirebasicforagingskills. Despitehigh levels of extrapair paternity, male Savannah sparrowsinvested as much in postfledging care and were as effective as females in caring for fledglings, based on recruitment of fledglings into the breeding population the following year. Male parents were more likely to care for smaller fledglings and for offspring from early broods (presumablyto enable females to dedicate their efforts towards second clutches). Caring for fledglingswas costly for parents:survivorship decreased asa function of the duration of postfledging parentalcareand the number of fledglingscared for. Parental survivorship,however, was not affectedby the sex of the fledglingscared for. This studysuggests that sex-biased provisioning may be unlikely except in species with strongly sexuallydimorphic offspring, biased offspring sex ratios and sex-biased natal dispersal. @ 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour. life skills and face the highest mortality risks of their lives (Marchetti & Price 1989; Sullivan 1989; Weathers & Sullivan 1991; Wheelwright & Templeton, 2003). In addition, how much parents invest in fledglings from the first brood may determine if and when females lay a second clutch (McGillivray 1983; Verhulst & Hut 1996; VegaRiveraet al. 2000). One aspect of postfledging parental care in passerines (brood division) has been well established. Typically, within a few days of leaving the nest, each fledgling becomes associated with and hasthe opportunity to learn from one parent; that parent then assumes sole or at least primary responsibility for feeding and protecting one or more fledglings (Nolan 1978; Smith 1978;Edwards 1985; Price& Gibbs 1987; Byle 1990; Kopachena & Falls 1991; Anthonisen et al. 1997; Ogden& Stutchbury 1997;how- ever, see Wilson & Kikkawa 1988; With & Balda 1990). For many bird species the approximate duration of post- fledging parental care is also known (1-3 weeks in most passerines: Davies 1976; Edwards1985). Lessis under- stood about how parentalcare is allocated to fledglings of different size or sex (Harper 1985; Price & Gibbs 1987; Determining how much parents invest in their offspring is essential for understandingoffspring sex ratios, repro- ductive strategies and life-history evolution (Fisher 1930; Charnov 1982;Slagsvold etat. 1986; Clutton-Brock1988; Frank 1996; Sheldon 1998). A critical component of avian par~ntal investment remains poorly known, namely parentalcareof young from fledging to indepen- dence (Smith 1978;Weatherhead & McRae 1990; Ogden & Stutchbury 1997). Once nestling birds fledge, it becomes increasingly difficult to follow them and observe interactions with their parents. Given the paucity of data on postfledging parental care, r~searchers have had to estimat~ total parental investment in sons versus daughters, using proxies such as the relative size of male and female offspring (Dijkstra et al. 1998;Torres& Drummond 1999). The postfIedging period is important to understand in its own right because it is when young birds learn crucial Co"espondence: N. T. Wheelwright, Department of Biology,Bowdoin College, Brunswick, ME 04011, U.S.A. (email: bss@bowdoin.edu). C. R. Freeman-Gallant is at the Department of Biology, Skidmore College, Saratoga Springs, NY 12866, U.S.A. 0003-3472/02/$30.00/0 435 @ 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.