What kinds of studies are most needed to understand the effects of global change on nature? Two deficiencies stand out: lack of long-term studies  and lack of data on species interactions . The paper by Mennerat and colleagues  is particularly valuable because it addresses both of these shortcomings. The first one is obvious. Our understanding of the impact of climate on biota improves with longer times series of observations. Mennerat et al.  analysed an impressive 18-year series from multiple sites to search for trends in parasitism rates across a range of temperatures. The second deficiency (lack of species interaction data) is perhaps not yet fully appreciated, despite studies pointing this out ten years ago [2,4]. The focus is often on species range limits and how taking species interactions into account changes species range predictions based on climate alone (climate envelope models; ). But range limits are not everything, as the function of a species (or community, network, etc.) ultimately depends on the strengths of species interactions and not only on the presence or absence of a given species [2,4]. Mennerat et al.  show that in the case of birds and their nest parasites, it is the strength of the interaction that has changed, while the species involved stayed the same. Mennerat et al.  found nest parasitism to increase with temperature at the nestling stage. They have also searched for trends of parasitism dynamics dependence on the host, but did not find any, probably because the nest parasites are generalists and attack other bird species within the study sites. This study thus draws attention to wider networks of interacting species, and we urgently need more data to predict how interaction networks will rewire with progressing environmental change [6,7].
 Lindenmayer, D.B., Likens, G.E., Andersen, A., Bowman, D., Bull, C.M., Burns, E., et al. (2012). Value of long-term ecological studies. Austral Ecology, 37(7), 745–57. doi: 10.1111/j.1442-9993.2011.02351.x
 Tylianakis, J.M., Didham, R.K., Bascompte, J. & Wardle, D.A. (2008). Global change and species interactions in terrestrial ecosystems. Ecology Letters, 11(12), 1351–63. doi: 10.1111/j.1461-0248.2008.01250.x
 Mennerat, A., Charmantier, A., Hurtrez-Bousses, S., Perret, P. & Lambrechts, M.M. (2019). Parasite intensity is driven by temperature in a wild bird. bioRxiv, 323311. Ver. 4 peer-reviewed and recommended by PCI Ecology. doi: 10.1101/323311
 Gilman, S.E., Urban, M.C., Tewksbury, J., Gilchrist, G.W. & Holt, R.D. (2010). A framework for community interactions under climate change. Trends in Ecology & Evolution, 25(6), 325–31. doi: 10.1016/j.tree.2010.03.002
 Louthan, A.M., Doak, D.F. & Angert, A.L. (2015). Where and when do species interactions set range limits? Trends in Ecology & Evolution, 30(12), 780–92. doi: 10.1016/j.tree.2015.09.011
 Bartley, T.J., McCann, K.S., Bieg, C., Cazelles, K., Granados, M., Guzzo, M.M., et al. (2019). Food web rewiring in a changing world. Nature Ecology & Evolution, 3(3), 345–54. doi: 10.1038/s41559-018-0772-3
 Staniczenko, P.P.A., Lewis, O.T., Jones, N.S. & Reed-Tsochas, F. (2010). Structural dynamics and robustness of food webs. Ecology Letters, 13(7), 891–9. doi: 10.1111/j.1461-0248.2010.01485.x
The authors have revised the manuscript according to the comments of the reviewers and myself which has improved the manuscript and it is now almost ready for recommendation. Before I recommend the manuscript, I would like the authors to correct a typo which I found during my last reading (incorrect use of "however" on line 281) and to publicly deposit the data on which this manuscript is based according to PCI rules.
Dear recommender, Thanks for your reply. I have now done the last changes, uploaded the data and code online and formatted the preprint according to the PCI template.
First, I would like to apologize for how long it took me to make the decision – it was difficult to get reviewers in the summer. After considering two reviews and my own reading of the manuscript I ask you to revise the manuscript. Both the reviewers and I think that it would be a valuable contribution if you address the comments. I especially appreciate the temporal scale of the data and the strength of the evidence which comes with it. Currently the manuscript is narrowly focused on temperature effect on the parasites. In addition to reviewer comments, I would like you to explicitly consider aspects of host-parasite dynamics in the manuscript in addition to temperature. Could you test if there is evidence for dependence of the parasite population on performance or population size of the host in previous year, or in some other way relate parasitoid dynamics to host dynamics?
Summary: This study describes how local temperature affect blowfly abundance in blue tits nesting in a Mediterranean habitat based on a survey over 18 years. Authors convincingly show that environmental temperature is a potent mediator of parasite abundance, both within breeding seasons across years, and between different years.
General comments: I congratulate the authors on a fine contribution that will interest a range of ornithology and ecology scholars. The ms is well-written and clear for the most part, data were appropriately collected and analyzed, and conclusions follow. All in all, I found this neat paper, and have relatively few further recommendations offer. I have made note of some more itemized issues that you may wish to address. I hope you will find these useful.
Minor comments: 1. Line 20: Please italicize species names.
Lines 23-24: I think it would be easier to understand this effect should you express it as temperature differentials, e.g. what is a high “previous summer temperature”.
Lines 26-28: Sure, but blowflies are hardly range restricted as is?
Lines 29-32: This should be revised for clarity.
Line 54: What is the rate of activity? Movements per hour?
Line 87: You need to be more specific at this point – “physiological performance” is both vague and subjective.
Line 119: Which other breeding attempts would there be in the net boxes?
Line 120: It would suffice to say you visited boxes to determine start of breeding. References to Julian day are superfluous.
Lines 123-125: It would be prudent to state dimensions here.
Line 130: By “wing plumage color” you mean that you checked for a molt limit between the primary coverts + alula and the greater coverts?
Line 133: Because these females could not be caught?
Line 166: “Statistical analyses”
Lines 196-197: Were temperature indices for the different 3-month periods correlated?
Lines 210-215: How influential was the one very warm Fango summer for these results?
Lines 223-226: You could downplay this effect I think. It was prudent testing for it, but it is not essential for the Discussion to keep it in. Also, out of interest, if the loss argument holds where would the nest parasites otherwise have disappeared, and why?
Line 227: Thermal dependence is an awkward term, please swap for something less ambiguous.
Line 232: Why would you expect it to be?s
Line 239: I agree insofar that thermal limits to development are as likely in blowflies as in other insects. However, given the vast distributional range of the Calliphoridae, I would be careful with drawing broad conclusions about taxon-wide temperature tolerance.
Line 241: Summer heat, or warmer summer temperatures? This is an important distinction.
Line 247: “High” is rather subjective a term in this context.
Lines 241-250: This reasoning is in analogy with a recent study that manipulated nest temperature in blue tits (Andreasson et al. J Avian Biol 2018) found that nestlings in heated nests had higher body condition and suggested this could have been a result of increased parasite mortality at high environmental temperature.
Lines 271-277: This gets a bit repetitive.
Lines 281-285: You should have the data to test this?
Line 296: There are evidence for similar effects also in homeotherms, which you might consider acknowledging here.
Lines 291-307: I am not convinced by this reasoning, as there are already blowflies at latitudes considerably colder than at your study sites. I am not sure how much this adds to the ms.
Dear Editor, I found this MS very interesting and mostly well analysed and written. I have some, mostly minor queries intended as constructive with the aim to improve the paper. As detailed to authors, I think that one of my queries, that concerning making explicit the percent variance explained by two interesting factors, should be addressed ‘mandatorily’ as its dissection and eventual discussion may throw light on the role of two additional factors -genetic (bird host) and environmental (nest identity)- in this host-parasite system. I hope this review may be useful for the editorial team to reach a recommendation. Best regards.