The effects of Dipteran ectoparasitism on the growth and development of nestlings of the pearly-eyed thrasher (Margarops fuscatus) in the Luquillo Mountains, Puerto Rico
Authors:   , Arendt Wayne J. 
Published: December 2024

"In a rain forest population of the Pearly-eyed Thrasher instances of warble fly parasitism were found to be high. An average of 96.7 percent of all nests and 97.0 percent of all nestlings was parasitized during the 1981-82 breeding seasons. Almost 100 percent of the sampled nests were parasitized each month during the two year period. Although instances of parasitism were high throughout the study period, consequences of its temporal and spatial variability were noted among nests and nestlings, both within seasons and throughout seasons, and from year to year. Causes of this variability included fluctuations in the fly's population, which tended to increase over the breeding season in response to rainfall. Undoubtedly many more exogenous factors such as microhabitat variability, temperature, and humidity attributed to the differences in the observed instances of ectoparasitism, but they were not closely monitored. The reproductive success of the sampled thrasher population was greatly reduced as a direct consequence of warble fly parasitism, with nestling mortality averaging 50 percent for the 1981-82 breeding seasons. Although sample sizes of naturally parasitized thrasher nests were small for the 1979-80 breeding seasons, nestling mortality caused by warble fly parasitism at those nests averaged 60 percent (77 percent and 43 percent respectively). Though nestling mortality averaged 60 percent it was considerably higher for the wetter 1979 season and lower for the drier 1980 season. Similarly, though nestling mortality averaged 50 percent for the 1981-82 breeding seasons, it was higher (56.7 percent) for the wetter 1981 season and lower (42.8 percent) for the drier 1982 breeding season. Spatial variability in the instances and impact of warble fly parasitism was shown to be influenced by the ovipositing patterns of the adult female flies, which shifted with the host's ontogeny (dorsal and anterior preference in hatchlings, changing to ventral and more extensive parasitism as feather tracts developed) and seasonally in response to the rapidly increasing fly populations. It was shown that parasitism did not have to be heavy to cause debility or even death in thrasher nestlings. Thrasher nestlings showed a mixture of both rapid and gradual growth characteristics. Development paralelled that of small open-nesting passerine species during the first 1.5 weeks when growth was maximum, and conformed more closely to the patterns of typically slow-growing cavity nesters during the last half of the nestling period. Warble fly parasitism affected younger (smaller) nestlings during the first 1.5 weeks of development and warble loads did not have to be heavy to cause significant retardation in growth and even death in young nestlings. Moderate (31-60 larvae per nestling) and heavy ( [greater than] 60 larvae per nestling) parasite loads caused retardation and death in older (2-3 week old) nestlings. This study showed that body mass is not always an accurate determinant of body growth. Examples of extraneous factors (i.e., fly larvae, large seeds, prey skeletal fragments) were shown to contribute greatly to increased body weights. Heavy parasitism in the Luquillo Mountains populations of the Pearly-eyed Thrasher was thought to be the result of an apparent increased contact between parasite and host, which resulted from the host's recent increase in density. Even though warble fly ectoparasitism may continue to significantly reduce thrasher numbers, there are signs that the host is adjusting to heavy parasitism and may continue to be found in abundance in the rain forest."--Summary.