Multistate modeling of brood amalgamation in White-winged Scoters Melanitta fusca deglandi

Female waterfowl may lose or abandon offspring shortly after hatch often resulting in the phenomena of post–hatch brood amalgamation (PHBA; Eadie et al., 1988). Potential fitness implications of this behavior has generated considerable debate (Eadie et al., 1988; P ys , 1995; Savard et al., 1998) about physiological or ecological costs and benefits to ducklings in amalgamated broods. Several researchers have proposed that PHBA is a result of, but is not limited to, accidental mixing (i.e., accidental mixing hypothesis), initial brood size at hatch (i.e., brood size and success hypotheses), or maternal female condition at hatch (i.e., energetic stress hypothesis) (Eadie et al., 1988; Bustnes & Erikstad, 1991; P ys , 1995). We studied PHBA in July and August, 2000–2001, in a population of White–winged Scoters on Redberry Lake, Saskatchewan, (52° 00' N, 107° 10' W), a 4,500 ha federal bird sanctuary and World Biosphere Reserve. Ducklings (n = 265 in 2000 and n = 399 in 2001) were captured in nests at hatch, given a uniquely– colored nape marker for individual identification, and re–observed during daily observation sessions. We were interested primarily in movement probabilities during the first two weeks after hatch, when most travel by ducklings occurs, and after which duckling survival was constant (Traylor, 2003). We used multistate modeling (Brownie et al., 1993) in Program MARK (White & Burnham, 1999) to test hypotheses concerning PHBA. We estimated probabilities of (1) staying with putative mothers and natal siblings, , or (2) movement to a foster brood followed by adoption by a foster mother and conspecific non–siblings, . We tested hypotheses about relationships between and hatch date, brood size at hatch, female condition at hatch and size, duckling condition and size at hatch, and weather within one week of hatching. An average of 37.7% and 9.6% of ducklings moved to foster broods in 2000 and 2001, respectively. PHBA was highest the first four days of duckling age in 2000 and the first ten days in 2001. Duckling movement to foster broods in 2000 was a function of hatch date ( = –1.24, 95% CL: –2.33, –0.15), female condition ( = –0.83, 95% CL: –1.52, –0.14), and female size ( = –1.26, 95% CL: –2.21, –0.32). In 2001, duckling movement probability was related to hatch date ( = 0.33, 95% CL: –0.07, 0.72), initial brood size ( = –0.69, 95% CL: –1.12, –0.25), female condition at hatch ( = 0.35, 95% CL: –0.08, 0.78), female size ( = –0.32, 95% CL: –0.74, 0.10), duckling condition ( = –0.54, 95% CL: –0.99, –0.10), and weather ( = 1.14, 95% CL: 0.63, 1.64