Parental investment by the Northern Mockingbird: male and female roles in feeding nestlings

--Analysis of 3,293 feeding trips to nestling Northern Mockingbirds (Mimus polyglottos) in the first 11 days of nestling life showed that males and females fed young at similar rates. Young were fed a mixed diet of animal prey and fruits, and males and females fed similar volumes of animal prey and of fruits. Females displayed a monotonic increase in feeding rate during nestling life but did not significantly increase load size. Males significantly increased both feeding rate and load size with age of nestlings, and their feeding rate peaked in the mid-nestling period, when young grow most rapidly. Males and females fed broods of two and three young at similar rates. There was a biased breeding-adult sex ratio in this population, with males outnumbering females. The unbalanced sex ratio may allow females to demand a high level of male parental care in feeding nestlings and in other behaviors included in parental investment. Received 19 April 1985, accepted 10 September 1985. PARENTAL investment (PI) is defined as "any investment by the parent in an individual offspring that increases the offspring's chances of surviving (and hence reproductive success) at the cost of the parent's ability to invest in other offspring" (Trivers 1972). When offspring are typically aggregated in clutches, some components of PI (e.g. nest defense) may be viewed as a compromise among clutches separated in time and space rather than among individual offspring (Wittenberger 1981: 362-363). Parental care (PC) is the nongametic subset of PI comprising those behaviors involved in raising young to independence. In many monogamous bird species, both males and females invest substantial amounts of time and energy in offspring, particularly in feeding nestlings and fledglings. In many monogamous passetines, males and females make roughly equal numbers of food deliveries to nestlings (Kendeigh 1952, Lack 1968, Emlen and Oring 1977, Oring 1982, Greenberg and Gradwohl 1983). However, the schedule of feeding nestlings and the amounts fed by the sexes seldom have been quantified rigorously. We studied parental feeding of altricial nestlings by Northern Mockingbirds (Mirnus polyglottos). Parental feeding of nestlings is a major time and energy investment in offspring (Ricklefs 1974, Skutch 1976, Breitwisch et al. 1984). Therefore, selection should favor male and female distribution of investment in feeding 152 nestlings in proportions that maximize genetic contributions to succeeding generations (Fisher 1958). We attempted to assess the relative contributions by males and females to feeding nestlings; to establish if the patterns of feeding by males and females were similar, both daily and throughout the nestling period; to find if the quality of food brought to nestlings by males and females was similar; and to see if there was a relationship between brood size and relative investment by males and females in feeding nestlings. In light of the limited theory related to these questions, we submit our empirical findings and interpretation as a partial basis for future development of theory regarding apportionment of parental investments by males and females. STUDY AREA AND METHODS This study was conducted on individually colorbanded Northern Mockingbirds inhabiting the main campus of the University of Miami, Dade Co., Florida (Merritt 1985). We observed parental provisioning of nestlings during 25 12-h periods (0600-1800) for a total of 300 h of observations. Data were gathered on 12 broods of 9 pairs of birds from 5 April to 9 July 1981 (23 of 25 periods between 1 May and 9 July). Four broods were observed for one day each, 4 broods for two days, 3 broods for three days, and 1 brood for four days. Broods sampled on two or more days were observed on nonconsecutive days, with one exception (see Breitwisch et al. 1984). We observed 1 brood when the young were six days old (= day 6), The Auk 103: 152-159. January 1986 January 1986] Mockingbird Parental Investment 153 3 broods at days 10 and 11, and 2 broods for each of the nine remaining days (days 1-5, 7-9, 12). Mockingbirds in southern Florida fledge at about 12 days of age. Brood sizes ranged from 1 to 4 (œ = 2.4, SD = 0.79, n = 12). Feeding trips to the nest were recorded to the nearest second with digital watches. Using binoculars, we identified food items to major type (e.g. fruit, insect, spider, lizard), with finer levels of resolution whenever possible (e.g. species of fruit or order of insect). We estimated lengths of animal prey ( 8 cm; size classes with lower limit closed and upper limit open). We estimated relative volumes of foods on an arithmetic scale of 1-5; volume per centimeter was estimated for animal prey. Volumes of all individual fruits were between 1 and 5, inclusive, although parents frequently brought more than one fruit in a single delivery. Volumes of all animal prey - 1 cm in length were calculated as (volume per centimeter) x (length in centimeters). Daily mean load sizes (• volume/trip) ranged from 2.51 to 6.85 for animal prey and 3.11 to 6.50 for fruit trips. We calculated total volume of animals and fruits fed per nest each day and divided these values by (12 x brood size) to yield mean food volume.nestling•. h-•. Unknown foods were distributed between fruit and animal categories based on the proportions of known items fed by each parent. We assigned to unknown foods the mean relative volumes of animals and fruits fed by each parental sex to nestlings at that age.