LINKED PAPER
Life-stage and sex influence Philornis ectoparasitism in a Neotropical woodpecker Melanerpes striatus with essential male parental care. LaPergola, Joshua B. 2023. IBIS. DOI: 10.1111/ibi.13221. VIEW
Nestlings are vulnerable to a range of issues prior to fledging, with one common threat being infestation by ectoparasites. In the Neotropics, many species suffer from infestation by Philornis botflies (Diptera: Muscidae), which can impact nestling survival. Despite the ecological effects of such parasites and the potential implications for conservation, there are many gaps in our understanding of Philornis infestation, in particular the degree of host life stage specialisation. While nestlings are widely believed to be the primary targets of parasites with adult infestations being opportunistic or misdirected, recent work has suggested that the latter occurs more often than previously thought, with potential consequences for adult survival and reproductive success.
In a recent study in Ibis, Joshua LaPergola studied a population of Hispaniolan Woodpeckers (Melanerpes striatus) in the Dominican Republic to document Philornis infestation presence and intensity in both nestlings and adults, and determine the factors influencing ectoparasitism in adults.
Nesting, breeding behaviour, and ectoparasitism
Hispaniolan Woodpeckers are socially and genetically monogamous (LaPergola & Riehl 2022). Males and females share approximately equivalent diurnal incubation and brooding while males perform all overnight incubation of eggs and brooding of nestlings (LaPergola unpubl. data). Three non-mutually exclusive hypotheses were tested across six field seasons: (1) nestlings are more vulnerable to Philornis parasitism than adults; (2) nesting is associated with Philornis parasitism in adults; (3) Philornis parasitism is associated with incubation and brooding investment.
Figure 1. Probability and intensity of Philornis parasitism on adult and nestling Hispaniolan Woodpeckers. (a) Probability of Philornis parasitism plotted as raw data (adults represented by blue circles, n = 218 observations; nestlings represented by grey triangles, n = 554 observations) and model predictions from a generalized linear mixed model testing for an association with age, day of year captured and their interaction. The blue solid line and black dashed line represent model predictions for adults and nestlings, respectively. Raw data were artificially vertically separated to improve visibility of points. (b) Raincloud plot comparing adults (n = 40 observations) and nestlings (n = 123 observations) for the total number of Philornis wounds observed on infested individuals (i.e. only non-zero values for the total number of Philornis wounds). Sample sizes indicate the number of observations.
Infestation prevalence and intensity
In total, 218 adult records and 554 nestling records were collected from the Hispaniolan Woodpecker population. The results showed partial support for the first hypothesis, as although adults were as likely as nestlings to exhibit evidence of infestation, nestlings experienced greater intensity of infestation. This could be due to adults having lower accessibility to Philornis due to their well-developed plumage (Oniki 1983) vs. nestlings’ initial lack of feathers, or their mobility (Teixeira 1999) vs. nestlings’ confinement to the nest. Adults might also be more able to resist infestation than nestlings due to their immune memory.
Regarding the second hypothesis, nesting status of adults was not significantly associated with prevalence or intensity of Philornis parasitism, but parents with infested nestlings were more likely to also be infested than the parents of non-infested offspring, again partially supporting the hypothesis. While the lack of an effect of nesting status is suggested to potentially be an artefact of the sampling period mainly being in the nesting season, a more precise definition of the window of infestation would make it possible to discern the overlap between infestation and nesting and accurately compare nesting and non-nesting adults in future.
There was some support for the third hypothesis as males were 3.4 times more likely to host Philornis than females, although they did not differ in intensity of infestation. This could be explained by both sexes performing diurnal incubation and brooding but only males performing nocturnal incubation and brooding, which could increase their exposure to Philornis at night. However, as little is known about the temporal activity patterns of Philornis, this sex difference could also potentially be explained by sexual dimorphism in behaviour, morphology, and/or immunology (Zuk & McKean 1996).
Overall, the findings suggest that parasitism of adult Hispaniolan Woodpeckers may be part of a mixed strategy by Philornis, allowing them to reproduce when nestlings are unavailable or in short supply. Year-round monitoring of adult and nestling infestation and data on nestling availability and abundance would help to fully test this. This study also demonstrates that the Hispaniolan Woodpecker would make an excellent model system to not only study the biology of Philornis, but also to test potential Philornis management programmes prior to their use on threatened species which may be vulnerable to infestation.
References
LaPergola, J.B. & Riehl, C. (2022). Opportunity is not everything: Genetic monogamy and limited brood parasitism in a colonial woodpecker. Behavioral Ecology and Sociobiology 76: 72. VIEW
Oniki, Y. (1983). Notes on fly (Muscidae) parasitism of nestlings of South American birds. Le Gerfaut 73: 281-286.
Teixeira, D.M. (1999). Myiasis caused by obligatory parasites. Ib. general observations on the biology of the species of the genus Philornis Meinert, 1890 (Diptera, Muscidae). In Guimarães, J.H. & Papavero, N. (eds) Myiasis in Man and Animals in the Neotropical Region. Bibliographic Database: 71–96. Sao Paulo: Editora Pleiade.
Zuk, M. & McKean, K.A. (1996). Sex differences in parasite infections: Patterns and processes. International Journal for Parasitology 26: 1009-1024. VIEW
Image credits
Top right: Hispaniolan Woodpeckers (Melanerpes striatus) | Martin Gloor | CC BY-SA 4.0 Wikimedia Commons
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