Path coefficient analysis of correlation between breeding cycles of the common myna Acridotheres tristis (Passeriformes: Sturnidae) and its phthirapteran ectoparasites.

Louse population fluctuates seasonally. Several factors have been held responsible for the summer buildup of population of avian lice (Marshall A.G. 1981: The Ecology of Ectoparasitic Insects. Academic Press, London, 445 pp). It has been suspected that the breeding time of two haematophagous louse species (Ricinus picturatus and Menacanthus sp.) might be controlled by the reproductive hormones of the host bird, the orange crowned warbler (Vermivora cellata) (Foster M.S. 1969: Ecology 50: 315–323). Furthermore, testosterone implants gave rise to an increase in number of two haematophagous lice (Machaerilaemus malleus and Myrsidea rustica) on barn swallows (Hirudo rustica) (Saino N., Moller A.P., Bolzern A.M. 1995: Behav. Ecol. 6: 397–404). The present studies were undertaken to record the degree of synchronisation between the breeding cycles of a semidomestic passerine bird, the common myna Acridotheres tristis (L.) and its phthirapteran ectoparasites. Out of four phthirapteran species known to occur on Indian common myna, three species (Menacanthus eurysternus Burmeister, 1838, Brueelia sp. and Sturnidoecus bannoo Ansari, 1955) were taken into consideration in the present studies. Ten infested birds (both sexes) were weighed and deloused by lethal fumigation (Marshall 1981, op. cit.) every month during May 2000 to April 2001. The entire louse load was transferred to 70% ethanol and separated species-wise and sex-wise under stereozoom trinocular microscope. Total number of adult females of each species was specifically recorded. Feathers of the bird were plucked carefully from each region of body and placed in a glass jar. The bird was dissected in physiological saline to take out the gonads. The latter were cleaned, gently pressed between the folds of paper and weighed. Each feather was examined individually under the stereozoom trinocular microscope to record the number of fresh (unhatched) eggs of each louse species. Total number of live eggs of each louse species recorded from any bird was divided by total number of adult females of that species (subsequently averaged) to obtain the “egg index” in particular month. To derive the idea about the degree of synchronisation between breeding cycles of the host bird and its lice, an analysis of correlation between mean monthly live egg indices and mean monthly gonadal weights (in mg/100g of body weight) was taken into consideration (Fig. 1). The degree of correlation between live egg indices and mean monthly temperature as well as mean monthly photoperiod (averaged value of daily light hours) was also recorded. However, the values of Pearson’s coefficient of correlation (Table 1) give absolute measure of the extent of relationship between two variables (first and second), ignoring the effect of other variables (third, fourth ...) that may affect the first (dependent) variable. 200