Seasonal Trends in Movement Patterns of Birds and Insects Aloft Simultaneously Recorded by Radar

Round 1

Reviewer 1 Report

In this manuscript, authors use data from a single vertical-looking radar that operated continuously (year-round) to quantify birds and insects, and to categorize their movements as local or migratory based on their direction of flight. Unlike many other studies that focus on measuring and describing either only bird or only insect movements during specific periods of the year (e.g. during migration seasons), this work offers an estimate of the magnitude of insect and bird activity throughout the year, as well as insights of the onset and duration of their migrations. While the results presented are relatively simple, they are also interesting.

I believe this manuscript would benefit by establishing a clearer link between the concepts of migratory and “directed” flights, as well between local and “non-directed” flights. However, I also believe that the terms “directed” and “non-directed” are not the best choice. For example, in lines 48 – 49, authors say that “movements can be broadly categorized as migratory (with a certain seasonal directional preference) and non-migratory or local (randomly distributed).” However, local movements can be highly directional (non-random), e.g. daily flights from or towards roosts. I get that what authors mean here is that during non-migration periods the aggregate of flight directions do not concentrate around a well-defined mean. I’m sure they could provide a better definition than mine, but definitely better than just saying that local movements are randomly distributed. However, if authors have evidence that the latter is the case then please cite. Similarly, the title of subsection 2.2 (line 88) speaks of directed and undirected movements, and in lines 89 – 90 authors say “movements recorded by the radar include both migratory (directional) and local (non-directional)”. However, animal movement inherently involves directionality. My comment here goes in the same direction as above, please provide a more fitting definition for the concepts of migratory and local movements, and considering renaming this subsection. Furthermore, in Figure 4 we can see that the distribution of insect’s nocturnal activity throughout the year seems to be unimodal. In this case I wonder if one can speak of migration, since migration typically involves two-way movements and one would expect a bimodal distribution. Thus, it might be good to define early in the manuscript what the authors mean by migration, and/or the criteria they used to classify radar-detected targets as performing migratory or local flights.

The Conclusions section does not contain conclusions based on the results of this work, rather it is an extension of the Discussion with thoughts of what else could be done with similar data collected at larger temporal and geographical scales.

Line 48 – 49: If you have evidence that non-migratory or local flights are randomly distributed, please cite.

Line 51. “They” who?

Lines 59 – 61. While this is somewhat true, there’s been a lot of research in the last couple of years using weather radars; further, a good deal of efforts aimed to answer “how” aerial animals move even before the term aeroecology was coined. Please reword and/or acknowledge some of the existing research.

Lines 89 – 90. Animal movement typically involves directionality.

Line 214. Unclear. Is there a period missing after the references (to split sentences)?

Lines 217 – 218. “at the end of April”?

Lines 216 – 219. Please cite.

Lines 219 – 220- How do you know that some echoes originated from bats?

Lines 225 – 226. Please cite.

Figure 3. Please explain in the figure caption what each column represents - perhaps just add “(left column)” at the end of line 164, and “(right column)” at the end of the figure caption. Consider highlighting in each of your panels the DoY that make “typical” spring and autumn migration seasons.

Figure 4. Consider highlighting spring and autumn instead of even/odd numbered months.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

Review remotesensing-1191295-peer-review-v1,

Seasonal trends of movement patterns of birds and insects aloft simultaneously recorded by radar

General comments

Thank you for this interesting work and nicely written manuscript! I really enjoyed reading it. In my opinion is a valuable work that is of interest for both the scientific community and management. This study uses radar information to identify diurnal and seasonal patterns in directional (as an indicator for migratory movement) and non-directional (as an indicator for local movement) movement of birds and insects. The authors found distinct seasonal and diurnal differences in these two movement strategies for both birds and insects, providing important information for management and conservation. I have only minor comments, mostly on the terminology. For example, I think the authors use the term ‘dispersal’ in a wrong context as dispersal describes movement from one place to another without coming back. As the authors correctly emphasize, their data provide information on collective patterns and not individuals. Thus, their data cannot distinguish dispersal movement and therefore this term should be not used. Next, I recommend adding information on months to figure 3 next to the Julian day. In the discussion, you talked about what is happening a given month/season, and for readers less familiar which date a given Julian day represents, it might be difficult to follow your argumentation. Please find below my specific comments.

Specific comments

Line 17: your method does not differentiate single individuals and thus these estimates represent individual flights not individuals as some records may be repeated flights of the same individuals. Please rephrase ’ We estimated at least 3 million bird and 20 million insect passages over a 1 17 km transect annually’

Line 19: change to seasonal migration peaks

Line 19+21: consider using ‘directional’ instead of ‘directed’ movement, because you to not access the starting and endpoint of the movement of a given individual, but the collective pattern

Line 22: Difficult to understand what you mean with ’around 360°’ and how do you know that the movement you observed is ‘dispersal’ (i.e., animals never coming back) and not local/regional movement? your data cannot differentiate these movement types. Please rephrase to ´While in spring and summer, insect movement was non-directional, in autumn their movements concentrated towards southwest, similar to birds’

Line 35: ’natal dispersal’ as seasonal movement pattern is misleading in this context, because dispersed animals never come back and your data do not tell these details. Please use ’local-scale daily movements’ instead

Line 44: do you expect an anthropogenic impact as a form of competition for this space (e.g. by noise, pollution, airplanes)? If so, you might change to ’multi-use aerial habitat’ Line 49: local movements are not random, but reflect a utilization of the resources distributed locally. Please rephrase to ’…and non-migratory (locally distributed).’

Line 53: you might want to considering developing this sentence even further by adding ’…as collision with wind-turbines or vehicles’ (Grilo et al. 2020, Front Ecol Evol, https://esajournals.onlinelibrary.wiley.com/doi/10.1002/fee.2216; Muñoz et al. 2015, Biodivers Conserv https://link.springer.com/article/10.1007/s10531-014-0831-2)

Line 147: your method does not differentiate single individuals and thus these estimates represent individual flights not individuals as some records may be repeated flights of the same individuals. Please rephrase ’ ….to yearly estimates of 3,662,819 bird and 23,290,168 insect passages of the 1,147 km transect over the study area’

Figure 3: add a header ”spring” and ”fall”.

Figure 3 text: please change to ’ X-axis represents Julian day (DoY).’

Line 171-190: please add the date next to the Julian day to make it easier for the reader to identify the relevant periods, e.g., (DoY 31 to 131 (02 Feb-11May) and 30 to 134 (31 Jan-14May)). Alternatively, you add vertical lines to figure 4 as an identifier for the different seasons. Or you label the Julian day in the x-axis with the months. Do you have any stats to add that support your statements, next to the figure?

Line 195-197: Consider rephrasing to ’Migratory, dispersal, and local movements affect animals’ motivation and direction to fly, thereby influencing the seasonality and magnitude of mass animal activity in the airspace from local to the continental scale’

Line 197-198: Consider rephrasing to ’Using a continuous temporal approach, our findings emphasize the parallel occurrence of local and migratory movements in both taxa, and in particular in insects (e.g., fall). This suggest species-specific timing of different movement strategies’ (--> can you draw such a conclusion from your findings?)

Line 244: are the major wind currents/directions more variable than in England?

Line 263: could be furher extended to ’….loss of abundance as well as modification in regional occurrence/movement patterns over time could be traced and investigated’ I recommend to develop the discussion further --> Please move parts from the supplementary material (line 24-35) to the discussion as they provide important information for a critical assessment of the findings of this work, and thus should be included in the main document.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 3 Report

This study presents results on movement patterns of birds and insects monitored with a radar system over a period of 2 years. The authors compare birds and insects over different times of the the year, across the two migratory seasons and during day and night. The manuscript is well written and easy to follow. Figures are clear and self-explanatory.

I have two major criticisms to this study. The first is that the authors use directional and non-directional movements to tell apart migratory and local movements. This is, however, an oversimplification of a much complex problem. The same authors state (line 128): "Be aware that migratory and local movements are only defined by the scatter of the individual flight directions, and not by the direction itself." It does not seem they are in the position of validating the method since they have no information on which species they are actually tracking. I find very confusing across the text the use of directed movement and migration (or non-directed and local movement) as synonyms.

The second problem I see is that in the analysis on the movements of airborne species the authors have not included a single information about wind conditions. Wind patterns associated to measured movements would add a much powerful context to interpret results in Fig. 3 and 4. And maybe help to interpret statements like (line 189): "Generally, there was a stronger overlap between migratory and local movements in insect than in birds." The authors do not even discuss the lack of wind data in their analysis! The word "wind" is not even mentioned once in the text.

In addition, a more detailed methodological description is expected for this journal. The text in Supplementary Material could be shortened a bit and included in the manuscript leaving only the supplementary figures on a separate file.

Additional suggestions I have to improve the manuscript are reported by line order (and not relevance) below.

Line 18 - Why the fact that "peak local bird movement intensities during summer were of the same magnitude as migration peaks" was "surprising(ly)"? Is it normal to expect fewer movements in summer (during the breeding season) than during spring migration?

Line 51 - Consider changing "they" with "migration patterns".

Line 72 - It would be beneficial to briefly say why a rotating antenna is able to detect directions and when the antenna was rotating. Any specific time of the day? For how long was it rotating?

Line 74 - Consider replacing "Sempach" with "experimental site".

Line 76 - A brief note on how "credibility" is defined in the study and why values > 0.2 are considered "credible" is needed.

Line 82 - Once again "standard procedures" is a term too vague for the technical level of details expected in this journal. Some more details of the procedure are needed here.

Line 96 - Replace "clear" with "significant"

Figure 2 - From where the 40º SD assumption for each bin came from? (EDIT: I now see it the supplementary file. Please make it clear). Furthermore, is it a SD for all sample sizes a good assumption in the first place? I am wondering if the shapes of the curves in Fig. 2 and S1 are not an artefact of the underlying assumptions of the model used.

To add to the above comment. Why binning the data? Rayleigh test can be performed on raw non-binned data as well. I am sure the authors have their reasons but they should try to convince the reader that their approach is solid. No matter the method, there will always be problems with small sample sizes. Maybe a lower sample size limit could be used to avoid under/over estimations?

Line 144-149 - Does TTR have a unit of measurement? It is strange to read means and ranges in this paragraph without units.

Line 144-146 - Differences in means should be reported with an appropriate statistical test!

Line 147 - Here I read for the first time that the study area is 1 km transect. Such information should be detailed in the methods section.

Line 150-160 - What about TTR for spring and autumn? Any difference? If yes these are important for interpreting Figure 3.

Figure 3 - For completeness, it would be nice to indicate that the left column refers to "spring" and the right one to "autumn".

Lines 169-190 - "Deviance" from which model? Please explain the use of deviance in M&M.

Line 170-190 - Ranges in parenthesis have been mathematical/statistical defined or are done by eye-balling distributions in Fig. 4? Please be clear.

Line 199 - If so, cite Kemp et al. already in M&M.

Line 201 - Which two measures? Please state them explicitly.

Line 219 - It was not stated in M&M that also bats could be detected. It should be fixed.

Author Response

Please see the attachment.

Author Response File: Author Response.docx