Horizontal transmission and reproductive rates of hen fleas in great tit nests

1. The transmission mode and reproductive rates of a parasite are usually associated with its virulence. In this study, two experiments were carried out to quantify horizontal transmission rates and reproductive dynamics of hen fleas infesting a population of great tits. Our results provide experimental data on certain factors affecting the population dynamics of an ectoparasite in a host population. 2. Immigration of adult hen fleas occurred in 72% (29 out of 40) of previously deparasitized great tit nests. The mean infestation intensity was 5.8 adult fleas per nest and did not vary seasonally. Adult hen flea distribution within the host population was aggregated and did not differ from a negative binomial distribution. 3. In nests experimentally infested with 40 adult fleas, two discrete flea cohorts were found at the end of the hosts' breeding attempts. The first cohort consisted of adult fleas, the second cohort was much larger and consisted of second and third instar larvae. No first instar larvae were observed in the second cohort. A small proportion of larvae was found in cocoons. 4. The day in the season and the daily mean temperatures during the birds' 'brooding period' (i.e. from the start of incubation until the last young fledged or died) were not significantly correlated with the number of fleas in the nests. These results suggest that climatic and seasonal factors do not have significant effects on flea reproduction. 5. The number of adult fleas and larvae was not significantly different between nests where all chicks died and nests where at least one young P, Cged. Host reproductive success affected the number and proportion of larvae in cocoons which was lower in nests of birds that failed compared with nests of birds that fledged young. The proportion of larvae in cocoons increased with the number of days in the 'brooding period' of the birds. 6. Among infested nests, the number of adult fleas and the total number of larvae were positively correlated with the fresh mass of the nests, suggesting that density-dependent mechanisms within nests affect flea numbers. Great tits appeared to increase the mass of their nests following experimental infestations with hen fleas. 7. In nests of birds that fledged at least one young, the number of adult fleas and larvae was not significantly correlated with measures of the hosts' breeding performance (chick mass and number of fledglings). These results suggest that flea reproductive success does not depend on the hosts' reproductive performance. 8. These results emphasize the need to study the effects of host responses on survival and reproductive rates of parasites, A knowledge of these effects is essential for the understanding of population dynamics, dispersal and life-history traits of the parasites.