Ecological and phylogenetic comparative studies of avian lice

Avian Lice (Phthiraptera: Amblycera, Ischnocera) are common, low pathogenic ectoparasites of birds. Their long coevolutionary history with their hosts make them worthy subjects for studying the ecology and evolution of parasitism.

Ecology of Avian lice
The abundance and population dynamics of lice are mainly affected by their host’s defensive efforts, life cycle, individual and species-specific traits. The host’s body size and development stage has been known to influence their louse load. Seasonal changes have also been long observed and suspected to be related to the host’s breeding season or moulting. Small falcon species are good subjects to study these questions as they are frequently infected with lice. They are also in the focus of many scientific and conservation programmes that provide valuable data on the birds.

Our studies aimed to investigate how individual traits of falcons affect their louse load at different stages of their life. The first one investigated how the sex, maturity, and clutch size of Common Kestrel (Falco tinnunculus) nestlings affect the abundance of their lice. Our second study investigated how static and dynamic traits of colonial Red-footed Falcons (Falco vespertinus) interplay with the dynamics of their louse subpopulations during the breeding period and how they affect the colonisation of new hosts by lice. Lastly, we investigated the effect of Amur Falcons’ (Falco amurensis) sex, age and body size on the abundance of their lice at a vast autumn migratory stop-over site.We collected ectoparasite samples from Common Kestrels in Hungary. We analysed the effect of the host’s sex, wing length and clutch size on the abundance of their lice.

We sampled Red-footed Falcon (Falco vespertinus) nestlings in two and adults in one breeding season. On the nestlings, we modelled the mean abundance of their louse species using the clutch size and host sex in interaction with wing length. For the adults, we used wing length and the number of days after laying the first egg, both in interaction with sex, as explanatory variables.

We sampled Amur Falcons in Nagaland, India, at major roosting sites. We modelled the abundance of their lice using the host age category (juvenile or adult) and wing length, both in interaction with sex, as explanatory variables.

We collected all ectoparasite samples with dust ruffling. We analysed the data using generalised linear (mixed) models with negative binomial distribution and log-link.The ischnoceran Degeeriella rufa (Burmeister, 1838) and the amblyceran Colpocephalum subzerafae (Tendeiro, 1988b) occurred on all three studied bird species 5 frequently. We analysed the abundance of these species in detail.  Laemobothrion (Laemobothrion) tinnunculi Linnaeus, 1758 was only rarely found on Red-footed and Amur Falcons.

On Common Kestrel nestlings, none of the investigated variables had a significant effect on C. subzerafae abundance. Contrarily, clutch size had a significant effect on D. rufa abundance. In small clutches (3–4 nestlings), the mean abundance of D. rufa was 3.7 times higher than in large (5–6 nestlings) clutches.On Red-footed Falcons, D. rufa abundances increased with the nestlings’ wing length. In one year, this trend was steeper on females. Initially, both louse species exhibited higher abundances on adult, breeding females, but it decreased subsequently through the breeding season. Contrarily, abundances were constantly low on adult males.

On Amur Falcons, host age significantly affected the abundance of C. subzerafae, being nearly four times higher on juveniles than adults. Juveniles were also more infested with D. rufa than adults. Additionally, the abundance of the latter species was lower on adult male Falcons than on adult females.We can think of two non-exclusive explanations why Common Kestrel nestlings in larger broods are more parasitised with lice. First, parental quality is known to affect the clutch size. If low-quality parents are also more heavily infested with lice, this could explain the higher louse load of their nestlings. According to the dilution hypothesis, lice can be considered long life-cycle ectoparasites. These are incapable to significantly raise their subpopulation size till the fledging of the chicks. This results in lower per nestling louse counts in larger clutches. According to our results, both the breeding parameters and the parents’ quality may affect their offsprings’ ectoparasite load.

It seems D. rufa postpones transmission until Red-footed Falcon nestlings develop juvenile plumage and choose the more feathered individual among siblings. On adult birds, the sexual difference in abundance could either be caused by the different plumage or the females’ preference for less parasitised males. Moreover, females likely have more time to preen during the incubation period, lowering their louse burdens. Thus sex-biased infestation levels probably arise due to parasite preferences in the nestlings and host behavioural processes in the adult falcons.Both louse species showed juvenile bias on Amur Falcons. Juvenile bias in ectoparasite infestation is common in nature, probably due to juveniles being immunologically naïve, more resource-limited, and may be inexperienced in body maintenance behaviours. Again, we found evidence of female-biased infestations. Being 6 sister species, the reasons for this could be similar for both Amur and Red-footed Falcons.In conclusion, we have seen that the louse load of small falcons is a subject of various traits starting from the early days of their life. Longitudinal studies could reveal more on this phenomenon if they could overcome the associated methodological hurdles.

Evolution of sexual body size dimorphism in Avian lice

Rensch’s rule (RR) postulates that in comparisons across closely related species, male body size relative to female size increases with the average size of the species. This holds true in several vertebrate and also in certain free-living invertebrate taxa.

We studied the validity of RR in avian lice in three families (Philopteridae, Menoponidae, and Ricinidae). We used published data on the body length of 989 louse species, subspecies, or distinct intraspecific lineages. We applied phylogenetic reduced major axis regression to analyse the body size of females vs males while accounting for phylogenetic non-independence.

Our results indicate that philopterid and menoponid lice follow RR, while ricinids exhibit the opposite pattern. In the case of philopterids and menoponids, we argue that larger-bodied bird species tend to host lice that are both larger in size and more abundant. Thus, sexual selection acting on males makes them relatively larger, and this is stronger than fecundity selection acting on females. Ricinids exhibit converse RR, likely because fecundity selection is stronger in their case. Body size is under strong selective pressure to escape preening, while the sexes’ relative size should still facilitate mating. However, it seems on a grander scale there are other selection forces at play as well. These are most likely not related to ectoparasitism since RR and converse RR occur in many free-living animal taxa.