Natural and Experimental Infections of Eustrongylides ignotus: Effect on Growth and Survival of Nestling Wading Birds

Abstract

-One of three Tricolored Heron (Egretta tricolor) nestlings experimentally infected with Eustrongylides ignotus larvae died two days postinfection. During eight days postinfection, the other infected birds consumed less food per meal, consumed less food in relation to body mass, regurgitated more frequently, and exhibited lower bill and mass growth rates than did three uninfected control birds. Prevalence of eustrongylidosis among free-ranging Great Egret (Casmerodius albus) nestlings from Lake Okeechobee ranged from less than 5% in 1989 and 1991 to 28.4% in 1990. During 1990, infected nestlings suffered 10.6 to 17.9% higher mortality than uninfected birds, with younger nestlings suffering proportionately greater mortality. Summarized across colonies and nestling ranks (i.e. hatching order), infected nestlings averaged 2.9% shorter bills, 4.1% shorter tarsi, and 6.9% shorter wings than noninfected birds. Among rank 1 (oldest) and 3 (youngest) nestlings, infected birds were lighter (6.3-9.0%), but among rank 2 nestlings from both colonies infected birds averaged heavier than their noninfected counterparts. These results for mass may be due to the interactive effects of sibling rivalry. Eustrongylidosis generally had the greatest impact on those nestlings otherwise disposed to lower growth rates, whether due to colony location or nestling rank. The results emphasize the importance of monitoring diseases whenever prefledging growth and survival are of interest. Received 5 May 1993, accepted 17 November 1993. EUSTRONGYLIDES IGNOTUS INFECTIONS in nestling ciconiiforms, especially those of the family Ardeidae, appear to be associated with anthropogenic habitat alteration (Spalding et al. 1993), and can have a devastating effect upon reproductive success (Weise et al. 1977, Roffe 1988, Spalding et al. 1993, Spalding and Forrester 1993). In this study, we examined the effect of this nematode parasite, which burrows through the stomach wall, on the growth and survival of nestling ardeids in the laboratory and in the field. We experimentally infected Tricolored Heron (Egretta tricolor) nestlings to gather details about the effects of parasitism on young nestlings. We also monitored nestlings at Lake Okeechobee, Florida from 1989 to 1991 to gain information about prevalence and to measure the effects that eustrongylidosis might have on the growth and survival of nestlings in the wild. We provide evidence concerning both the mechanisms and magnitude of the effect of eustrongylidosis on ardeid nestlings and show that the impact on growth and survival results can be significant even during periods of only moderate prevalence. METHODS Experimental infections. -Six Tricolored Heron nestlings were collected from Frank Key, Florida Bay (25?06.3'N, 80?54.7'W) on 21 May 1990. Each was the intermediate-sized nestling of three siblings and was estimated to be between two to four days of age based on bill length. Nestlings were marked and placed in individual containers within an incubator. They were fed ad libitum four times each day with thawed fish or with fresh fish that had been examined and found to be free of eustrongylid larvae. Nestlings could take fish directly from a dish of shallow water or, if necessary, were offered fish held with forceps. They were never force fed. On the evening of the second day, three randomly selected birds (no. 2, 3, and 6) were each given four eustrongylid larvae per os prior to feeding. An average of 2.7 larvae per bird (range 1-10) have been found in very young naturally exposed birds (Spalding and Forrester 1993). Larvae were obtained from naturally infected mosquitofish (Gambusia holbrooki) collected in a canal in Gainesville, Alachua County, Florida, and maintained alive in the laboratory. Larvae were extracted from fish, placed in a gelatin capsule, and fed to nestlings immediately. Uninfected control birds (no. 1, 4, and 5) were given empty capsules. The nest container was examined