Host Migration and Size Do Not Influence the Prevalence of Most Chewing Lice (Phthiraptera: Amblycera and Ischnocera) on Shorebirds (Aves: Charadriiformes) across the World

Round 1

Reviewer 1 Report

Grossi et al. analyse large dataset with prevalence data for chewing lice on 110 species of shorebirds containing both previously published and their own data, collected across the world. The study covers nearly 30% of the species in the order Charadriiformes and includes all suborders, which make the dataset and the results obtained representative and valuable. The authors contribute to the knowledge about the chewing lice distribution with 15 new records of chewing lice for China and 12 for Australia.

The authors could not find that the host size correlated with the prevalence of the studied louse genera and they discuss the possible reasons behind this pattern. The also did not find a significant correlation with migration length and migration flyway, except for one of the louse genera in which the prevalence in short-distance migrants was higher than in the longer-distance migrants. However, I am interested to know if the authors tried to conduct separate analyses for gulls, auks, terns and waders? These groups are quite different ecologically from each other and analysing them together might hide some interesting intergroup patterns.

I have some additional comments listed below.

Lines 75-77 It does not become clear why the different lice genera are expected to be affected differently by the host migration. You discuss this later but it will be good to clarify it shortly here as well.

Table 7 is not needed since the same information is given in Table 9.

In Methods you write about “host length” while in tables you use “host weight”. Please correct/clarify this.

In my opinion the paper would benefit form a figure summarizing the supplementary table showing schematically a map with the different flyways and the locations where the data comes from (these might additionally show as pay charts the families of hosts and their share of the total birds examined in these locations). The size of the chart can correspond to the number of birds examined in every location.

Author Response

General comments: No such analyses of the individual groups were made, as the main focus is on migration and unfortunately most auks and gulls are here counted as short- (or at best intermediate-) length migrants, whereas most sandpipers and plovers for which data are available are long-distance migrants. Dividing e.g. waders from the rest would leave too few short-distance birds to compare with.  

 

Lines 75–77: We have clarified this with two examples.

 

Table 7: This have been deleted and the tables renumbered throughout the manuscript.

 

Host length/weight: This has been changed.

 

Figure suggestion: We feel the suggested graphical illustration, as outlined, would be more confusing than helpful, and have not added this. The reader would nevertheless have to refer to the supplementary table to find out what species were involved at each locality, how many birds of each species were examined, and how many of these were parasitized with each of the different louse species.

Reviewer 2 Report

Comments for author File: Comments.pdf

Author Response

Grammar mistakes indicated by the reviewer have all been corrected, and are not listed separately here.

 

Page 5, line 180: The reviewer asks why length was used as a proxy for size rather than weight. The simple answer to this is that for migratory birds, weight may differ substantially in the same bird between the beginning and the end of the migratory period, but the length of the bird does not differ in the same period. We clarified this in the Material and Methods section.

            However, we reran the same analyses using host mass instead of host length, and obtained identical results. We have added the relevant tables and figures to the manuscript.

 

Page 5, lines 185–187: We have rewritten this slightly; the 0–1 in the text is the same span as the 0–100% range given in the table. Percentages is what is normally reported, but this was recalculated to 0–1 range for simplicity in the analysis.

 

Page 6, lines 218–219: We have changed this to “updated checklist” for Canada, as there is no summary published of any records for Canada between 1989–2023.

 

Page 13, line 269: This has been changed to “prevalences”.

 

Table 5 – in this and the other tables, “length” was intended and this has been changed.

 

Figure 1: This has been changed to “prevalence”, and the text has been clarified. Note that we have also changed this figure, as the original manuscript figure had the letters mixed up. This did not change the conclusions of the manuscript.

 

Page 18, lines 372–375: There are a range of possible causes for this discrepancy, but here we are interested only in why smaller shorebirds more often have lice than smaller members of many other groups, including songbirds. As one explanation, we suggest that the bill structure and use in smaller shorebirds may be the reason their prevalence (and abundance) is more similar to that of larger shorebirds than that of smaller songbirds. As we write, larger-bodied shorebirds like auks and gulls are better analogues of the pigeons on which most of the research in preening has been conducted. Presumably with more data, hard-billed shorebirds that span many size categories (auks) would show the same correlation as in other birds, whereas those with soft bills would not. This is probably a better explanation than either reproductive capacities or life spans in lice. There is no published data suggesting that these factors are different in shorebird and songbird lice. We have therefore not changed anything here, and it is not clear that the reviewer was suggesting any changes.

 

Pages 18–19: We agree with the reviewer that a small section discussing the impact of dense flocks was missing, and have added this. However, we do believe the rest of the discussion here is valid for prevalence as well. For instance, even if opportunities for transmission are increased in shorebirds compared to other birds due to the dense flocking behaviour of the hosts, this does not mean that transmission would result in successful establishments of new populations on previously uninfected hosts. Long-term establishment success is in part connected to preening capabilities, which in general is based on bill morphology. This may of course affect different louse species differently. We are not aware of any group of songbirds or other hard-billed birds that form similarly dense flocks for which sufficient bird species have been examined for lice so that direct comparisons could be made without being caught up in the ecology of individual species.

 

Page 19, lines 432–434: The basis of the speculation is what we are comparing to: lice that drink blood or cause adventitious moult or reduce the insulation of the body may be expected to affect the bird more adversely than lice that can only cause damage to a smaller part of the host’s body. As the reviewer points out, populations of Saemundssonia head lice on a bird are usually much smaller than populations of other lice on the same host. However, we interpret this as mainly being a result of the limited amount of feather space for feeding and egg-laying on the head compared to the body/wings. There is no evidence that a smaller amount of Saemundssonia would consume a larger amount of feather tissue than a larger population of Quadraceps over the same time period, nor do we know of any published evidence that heat loss is so much higher on the head of the bird that this would skew the comparison.

            We have therefore not changed what we see as a reasonable (although currently untested) speculation for a speculation that seems to have a smaller basis in what is known about the system. We have, however, clarified parts of the paragraph.