The interplay of wind and uplift facilitates over-water flight in facultative soaring birds

Flying over the open sea is energetically costly for terrestrial birds. Despite this, over-water journeys of many birds, sometimes hundreds of kilometres long, are uncovered by bio-logging technology. To understand how these birds afford their flights over the open sea, we investigated the role of atmospheric conditions, specifically wind and uplift, in subsidizing over-water flight at a global scale. We first established that ΔT, the temperature difference between sea surface and air, is a meaningful proxy for uplift over water. Using this proxy, we showed that the spatio-temporal patterns of sea-crossing in terrestrial migratory birds are associated with favourable uplift conditions. We then analysed route selection over the open sea for five facultative soaring species, representative of all major migratory flyways. The birds maximized wind support when selecting their sea-crossing routes and selected greater uplift when suitable wind support was available. They also preferred routes with low long-term uncertainty in wind conditions. Our findings suggest that, in addition to wind, uplift may play a key role in the energy seascape for bird migration that in turn determines strategies and associated costs for birds crossing ecological barriers such as the open sea.

to better visualize and interpret this interaction. 251-259 -This is good discussion. I wonder if just the baseline uplift that the birds experience is enough, so they don't have to actively seek out stronger uplift to offset the energetic costs. Your point about turbulence is interesting as well. Perhaps there is some intermediate uplift strength (or deltaT) that is optimal, i.e., good uplift but not too turbulent. You could try testing this by including a quadratic deltaT term in the SSF. 268-269 -They could be more time-limited in the spring, however, so they may resort to flapping flight more in favor of a timely arrival on the breeding grounds rather than spending time seeking a more energetically optimal route.

Review form: Reviewer 2 (Adam Kane)
Recommendation Accept with minor revision (please list in comments)

Scientific importance: Is the manuscript an original and important contribution to its field? Good
General interest: Is the paper of sufficient general interest? Excellent

Quality of the paper: Is the overall quality of the paper suitable? Excellent
Is the length of the paper justified? Yes Should the paper be seen by a specialist statistical reviewer? Yes

Do you have any concerns about statistical analyses in this paper? If so, please specify them explicitly in your report. No
It is a condition of publication that authors make their supporting data, code and materials available -either as supplementary material or hosted in an external repository. Please rate, if applicable, the supporting data on the following criteria.

Do you have any ethical concerns with this paper? No
Comments to the Author This is a well written and interesting movement ecology paper that looks at over-sea migration in terrestrial bird species. The authors go to some length in their methods to address how species Introduction Bird morphology and body mass also have strong effects on flight costs too right. Line 78 -"as well as based on morphological characteristics" sounds a bit awkward here. It's just that the behaviour varies according to flyway, season and morphology. Line 87 -so this is when the sea surface is warmer than the air temperature. Line 94 -stated that there are 5 species tested but in the abstract it's 4. (This seems to be clarified on line 181 where the grey-faced buzzard is excluded but you can appreciate the confusion).

Methods
On the route selection analysis are birds less selective of the energetically cheapest routes at different points of the journey? E.g. near the end of the crossing when land is in sight. Line 190 -from not form. Line 192 -Was it possible for one of these generated steps to send the bird over land? Line 199 -It's worth defining 'wind support' here especially given it's importance in your findings. Lines 212 -213 -Presumably, these were 95% credible intervals? And I understand the analogy, but the terms aren't really significant in the same sense as a frequentist model.

Results and display items
All very clear and special mention to figure 2 which is a brilliant encapsulation of the work. Why rely on a simple correlation when relating delta T to w* when you have the GAM which shows a non-linear relationship?

Discussion
Good discussion on the value of the proxy delta T.

Supplementary Material
The supplementary figure legends have some question marks where they should refer to a specific table or figure. Figure S1 needs to specify the random effects are based on varying slopes. A minor point but the colour coding in S3 seems to be the opposite of what's natural to my mind -I'd think of orange as stronger than blue. Please stretch the x axis for Figure S4, it's quite hard to distinguish among the different birds. Great to see all of the code and data included.

07-Jun-2021
Dear Ms Nourani: I am writing to inform you that your manuscript RSPB-2021-0946 entitled "The interplay of wind and uplift facilitates over-water flight in facultative soaring birds" has, in its current form, been rejected for publication in Proceedings B. This action has been taken on the advice of referees, who have recommended that substantial revisions are necessary. With this in mind we would be happy to consider a resubmission, provided the comments of the referees are fully addressed. However please note that this is not a provisional acceptance. The reviewers and Associate Editor have some constructive critiques that will require some re-analysis of data.
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Sincerely, Dr John Hutchinson mailto: proceedingsb@royalsociety.org Associate Editor Comments to Author: Dear Dr. Nourani, Thank you for submitting your work to PRSB. Two reviewers and myself have looked at the manuscript and found it quite interesting. There are however some issues with the data analysis and transparency in the presentation. As mentioned by reviewer #1 it is not necessary to remove data because the hierarchical structure in the model will automatically "weight" your estimates. I found the model selection to be inadequate and unnecessary. Removing variables with nonsignifficant slopes is not a good practice. I really do not see the value of doing model selection here. You have a reasonable small model (few predictors) and a hierarchical structure that reflects the nature of your data. My recommendation would be to fit just one model (model 3) and report what you find.

Other comments:
Explain what data was used to fit the Gamma and von Mises. Also, did these distributions provided a good fit?
186. a tolerance of 30 minutes for hourly steps seems too high.
Priors and convergence diagnostics should be included.
It is a condition of publication that authors make the primary data, materials (such as statistical tools, protocols, software) and code publicly available.
Reviewer(s)' Comments to Author: Referee: 1 Comments to the Author(s) I reviewed a previous version of this paper for another journal. I generally liked it at the time and still like it now. The paper shows that uplift is widely available over the energy seascape, presents deltaT as a good proxy for uplift, and provides evidence that wind support seems to be the primary factor driving migrant route choice during water crossings. The latter is accomplished with a step selection function, which I think is a great application of the approach. I previously had major concerns about sample sizes for each species, especially in the context of drawing conclusions about soaring bird route selection on a "global" scale. Those have improved. I also had concerns about the number of steps used in the SSF for each individual; however, the authors have left that summary statistic out of table S1 in this version. Given that I know from the previous version that some of those sample sizes were incredibly small (e.g., 1), I need to see that those have improved to make a favorable recommendation for the paper.
Otherwise, I found the choice to subsample the species with larger sample sizes curios because the unevenness of the sample sizes among species shouldn't bias the global estimates (you're just throwing away information that could provide better estimates for those species).
Some line-by-line comments: 185-187 -you could use a continuous time model for the movement kernel and avoid doing this, sensu Eisaguirre et al. (2020, J. Animal Ecology). 189-191 -"over representation" shouldn't be a problem. Including more data for a species will only improve the estimates for that species and would not bias the global estimates, given the hierarchical structure in the model explained on lines 210-211. 192-194 -Seems to be some conflicting info here. Did you fit gamma and von mises distributions to the empirical step lengths and turn angles and draw from those, or did you draw from the empirical distributions? Either is fine, just sort of unclear as written. 200-202 -could you include some justification for 40 years? Seems like individuals might choose their route based on the predictability of conditions along routes that they've experienced, rather than variability over 40 years. So, would it be better to make this covariate specific for each individual by taking the variance over the approximate age each individual? Line 215-216 -did DIC and WAIC agree with CPO and MLik? 248-250 -I suggest consulting Avgar et al (2017, Ecology and Evolution) for some ideas on how to better visualize and interpret this interaction. 251-259 -This is good discussion. I wonder if just the baseline uplift that the birds experience is enough, so they don't have to actively seek out stronger uplift to offset the energetic costs. Your point about turbulence is interesting as well. Perhaps there is some intermediate uplift strength (or deltaT) that is optimal, i.e., good uplift but not too turbulent. You could try testing this by including a quadratic deltaT term in the SSF. 268-269 -They could be more time-limited in the spring, however, so they may resort to flapping flight more in favor of a timely arrival on the breeding grounds rather than spending time seeking a more energetically optimal route. Referee: 2 Comments to the Author(s) This is a well written and interesting movement ecology paper that looks at over-sea migration in terrestrial bird species. The authors go to some length in their methods to address how species Introduction Bird morphology and body mass also have strong effects on flight costs too right. Line 78 -"as well as based on morphological characteristics" sounds a bit awkward here. It's just that the behaviour varies according to flyway, season and morphology. Line 87 -so this is when the sea surface is warmer than the air temperature. Line 94 -stated that there are 5 species tested but in the abstract it's 4. (This seems to be clarified on line 181 where the grey-faced buzzard is excluded but you can appreciate the confusion).

Methods
On the route selection analysis are birds less selective of the energetically cheapest routes at different points of the journey? E.g. near the end of the crossing when land is in sight. Line 190 -from not form. Line 192 -Was it possible for one of these generated steps to send the bird over land? Line 199 -It's worth defining 'wind support' here especially given it's importance in your findings. Lines 212 -213 -Presumably, these were 95% credible intervals? And I understand the analogy, but the terms aren't really significant in the same sense as a frequentist model.

Results and display items
All very clear and special mention to figure 2 which is a brilliant encapsulation of the work. Why rely on a simple correlation when relating delta T to w* when you have the GAM which shows a non-linear relationship?

Discussion
Good discussion on the value of the proxy delta T.

Supplementary Material
The supplementary figure legends have some question marks where they should refer to a specific table or figure. Figure S1 needs to specify the random effects are based on varying slopes. A minor point but the colour coding in S3 seems to be the opposite of what's natural to my mind -I'd think of orange as stronger than blue. Please stretch the x axis for Figure S4, it's quite hard to distinguish among the different birds.
Great to see all of the code and data included.

Scientific importance: Is the manuscript an original and important contribution to its field? Excellent
General interest: Is the paper of sufficient general interest? Excellent Quality of the paper: Is the overall quality of the paper suitable? Excellent

Do you have any concerns about statistical analyses in this paper? If so, please specify them explicitly in your report. No
It is a condition of publication that authors make their supporting data, code and materials available -either as supplementary material or hosted in an external repository. Please rate, if applicable, the supporting data on the following criteria.

Comments to the Author
The authors did a great job of addressing my previous comments and concerns. I only have minor comments/suggestions below. line 47: consider "greater uplift" instead of "higher uplift" line 144: consider adding "The variable" at the start of the sentence (and maybe elsewhere too). Not a big fan of starting sentences with symbols. lines 202-204: consider rephrasing to reflect that you centered and scaled predictors to mean zero and unit variance. lines 232-234: I'm assuming Fig. S1 is showing the species-level means compared to the global (all centered and scaled?). If that's the case, I suggest rewording to something like: "The model results suggested a greater preference for wind support in Eleonora's falcon and a weaker preference in osprey (Fig. S1)." The term "significant" doesn't really hold the same meaning in Bayesian statistics as frequentist, so I think it's best to avoid it. lines 244-249: nice addition, considering the new result line 260: consider rewording to: "The map showing mostly positive deltaT indicates..." figure 3 legend: see comment above regarding rephrasing instead of using "z-scores" figure 4: nice way to show this interaction. sometimes these heat map representations of interactions are difficult to interpret, but I think it works well here. supplementary figures: not seeing supp. figure legends anywhere. I may have just missed them, but please make sure they are included.

13-Aug-2021
Dear Ms Nourani I am pleased to inform you that your manuscript RSPB-2021-1603 entitled "The interplay of wind and uplift facilitates over-water flight in facultative soaring birds" has been accepted for publication in Proceedings B.
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Sincerely, Dr Maurine Neiman mailto: proceedingsb@royalsociety.org Associate Editor Comments to Author: Dear Dr. Nourani, Thank you for the revised version of your manuscript. As pointed out by reviewer #1, we cannot find the legends for the supplementary figures. Also, the reviewer has some good suggestion to improve your manuscript. Beyon those, consider the following: line 132. Please explain the units used in this section. line 173. Consider joining this paragraph with the previous one. line 190. Where do these Gamma and von Mises distributions come from? Pressumably fitted to the data? line 210. I don't think is necessary to report how long it took the models to converge and on what machine. But, you should report how you assessed convergence.

Reviewer(s)' Comments to Author:
Referee: 1 Comments to the Author(s). The authors did a great job of addressing my previous comments and concerns. I only have minor comments/suggestions below. line 47: consider "greater uplift" instead of "higher uplift" line 144: consider adding "The variable" at the start of the sentence (and maybe elsewhere too). Not a big fan of starting sentences with symbols. lines 202-204: consider rephrasing to reflect that you centered and scaled predictors to mean zero and unit variance. lines 232-234: I'm assuming Fig. S1 is showing the species-level means compared to the global (all centered and scaled?). If that's the case, I suggest rewording to something like: "The model results suggested a greater preference for wind support in Eleonora's falcon and a weaker preference in osprey (Fig. S1)." The term "significant" doesn't really hold the same meaning in Bayesian statistics as frequentist, so I think it's best to avoid it. Decision letter (RSPB-2021-1603.R1)

16-Aug-2021
Dear Ms Nourani I am pleased to inform you that your manuscript entitled "The interplay of wind and uplift facilitates over-water flight in facultative soaring birds" has been accepted for publication in Proceedings B.
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Sincerely, Proceedings B mailto: proceedingsb@royalsociety.org Associate Editor Comments to Author: Dear Dr. Nourani, Thank you for submitting your work to PRSB. Two reviewers and myself have looked at the manuscript and found it quite interesting. There are however some issues with the data analysis and transparency in the presentation. As mentioned by reviewer #1 it is not necessary to remove data because the hierarchical structure in the model will automatically "weight" your estimates. I found the model selection to be inadequate and unnecessary. Removing variables with non-signifficant slopes is not a good practice. I really do not see the value of doing model selection here. You have a reasonable small model (few predictors) and a hierarchical structure that reflects the nature of your data. My recommendation would be to fit just one model (model 3) and report what you find.

RE:
We are grateful for all the constructive comments provided by the associate editor and the reviewers. We have now addressed all the concerns and comments, as detailed below. Regarding model selection, we agree with the associate editor and are now only reporting one model in the manuscript. We decided to go with model 2 instead of model 3, because we were interested to see whether and how the long-term variance of wind support can have an impact on route selection. With all the analyses done with this new version, we would like to point out that the results of the study remained largely unchanged.

Other comments:
Explain what data was used to fit the Gamma and von Mises. Also, did these distributions provided a good fit? RE: We have now clarified that the step lengths and turning angles of over-water steps along the tracks were used to fit the distributions (separately for each species-flyway combination) on Ls. 198-199 (tracked-changes file). We investigated the diagnostic plots to assess the fit of the distributions and overall found a good fit.
186. a tolerance of 30 minutes for hourly steps seems too high. RE: The tolerance of 30 minutes was chosen to maximize the amount of data that could be retained in the dataset (due to variation in sampling frequency of the original tracking data sets). We have now reduced the tolerance to 15 minutes. The number of data points is reduced, but not dramatically. Supplementary table S1 and modeling results are updated accordingly.
Priors and convergence diagnostics should be included. RE: This information is now added to Ls. 217-220 (tracked-changes file).
It is a condition of publication that authors make the primary data, materials (such as statistical tools, protocols, software) and code publicly available. RE: We have now uploaded all necessary data to reproduce the study on Dryad (https://doi.org/10.5061/dryad.r4xgxd2ct). Raw over-water tracking data, processed data used for step selection estimation, as well as any other data necessary to produce all the figures have been included. We have also updated all the R code necessary to reproduce the results and all the figures on Github (https://github.com/mahle68/global_seascape_public).

Reviewer(s)' Comments to Author:
Appendix A Referee: 1 Comments to the Author(s) I reviewed a previous version of this paper for another journal. I generally liked it at the time and still like it now. The paper shows that uplift is widely available over the energy seascape, presents deltaT as a good proxy for uplift, and provides evidence that wind support seems to be the primary factor driving migrant route choice during water crossings. The latter is accomplished with a step selection function, which I think is a great application of the approach. I previously had major concerns about sample sizes for each species, especially in the context of drawing conclusions about soaring bird route selection on a "global" scale. Those have improved. I also had concerns about the number of steps used in the SSF for each individual; however, the authors have left that summary statistic out of table S1 in this version. Given that I know from the previous version that some of those sample sizes were incredibly small (e.g., 1), I need to see that those have improved to make a favorable recommendation for the paper. RE: We are happy to know that the reviewer finds the improvements that we made satisfactory and thank the reviewer for the insightful comments. The table that the reviewer is referring to (shown below) did not include the number of steps (the units in our model), but sea-crossing segments. We defined segments as parts of a sea-crossing track separated by islands. Each segment contains multiple steps, depending on its length. In the table below (from a previous version of the manuscript), we had one migratory track for the Peregrine falcon from North America and this track was not broken up by islands, so the whole track was one segment. We have since decided to remove this information about the number of segments from the manuscript, because 1) it can be easily misleading or misinterpreted, 2) does not provide any useful information, and 3) depends entirely on the geography of the area that the birds are flying over. However, as the reviewer has pointed out, we have since collected additional data, which is reflected in the new version of Table S1.
The number of steps per species used in the current version is as follows (note that this is highly dependent on the distance of sea-crossing, as well as the number of individuals and years of data available):  Table S1 in the old version: