Review of:
Fitness consequences of anthropogenic subsidies for a partially migratory wading bird
RSPB-2023-2695
Picardi et al.

Reviewer summary
In this manuscript, Picardi et al. investigate the relationships among urban habitat use/selection, seasonal migration, and nest survival in partially migratory wood storks. They hypothesize that anthropogenic food subsidies in urban areas will allow wood storks to forego seasonal migration, and that residents will have higher nest success than migrants (again linked to food availability and urban foraging). Their analyses confirm these hypotheses: they find that resident storks select more strongly for urban areas and that storks that forage near urban areas have higher daily nest survival. They conclude that anthropogenic food subsidies might have fitness consequences via migration and nest success and discuss other consequences of food supplementation in urban areas.

General comments
I agree with the authors that fitness consequences and mechanisms of population-level behavioral change are difficult and rare to study; this paper nicely addresses that gap by measuring behavior and one component of reproductive success simultaneously. The manuscript provides clear context for the study in the introduction and does an excellent job of contextualizing the results and providing ideas for further study in the discussion (though sometimes too extensively, see below). I have a few questions/suggestions about how the data were analyzed.

The manuscript contains two main analyses/results: that of the relationship between urban habitat selection and migration, and that of the relationship between urban foraging distance and daily nest survival. What is missing is the link between migration and nest survival/success. The introduction frames the study in the context of trade-offs of undertaking migration (L41-48, L61-63), but the explicit link between migration and reproduction is not made analytically. This pattern is finally shown in Figure 4, but the source of the figure wasn’t clear to me (were curves based on average urban foraging behavior of migrants/residents?).

I also wonder about the “distance to urban” variable. Is this really a measurement of urban habitat use or urban foraging? Given the high fragmentation and heterogeneity of this landscape, isn’t it possible that natural foraging areas are adjacent to urban areas? I would have expected a measurement of urban foraging intensity (e.g., proportion of foraging trips that are in urban areas) rather than this distance metric.

I am curious about the choice to report daily nest survival rather than probability of survival do 110 days. From a fitness perspective, what is the difference between a nest that fails at day 10 and one that fails at day 109, if neither produces offspring? Figure 4 shows that these differences in daily nest survival do scale up to nest success probability, but it seems the latter would be a more fitness-relevant metric, even if it contains less statistical information.

Finally, I found myself searching for more detail on the underlying data. Specifically, the study takes place over a relatively long time period; were there temporal trends in urban habitat use, migratory propensity, and/or nest survival over this period? Did urban areas expand (and how might that affect the results, given that the land cover data were static)? How many individuals switched tactics between years (and was switching associated with a change in nest location or foraging behavior)? The answers to these questions could give more context to the behavioral and reproductive responses of the storks to urban expansion.

Line-by-line comments
L42-44: I found this sentence difficult to parse and suggest rephrasing the “cost” as a “benefit” or the whole phrase as a cost-benefit tradeoff (rather than a benefit-benefit tradeoff). For example: “Individuals trade the benefits of inhabiting a familiar environment —albeit with limited resource availability for part of the year— and avoiding a risky and energy-demanding journey for the benefit of increased access to resources, reduced intraspecific competition, or pathogen escape”
Figure 1: I found the colors difficult to distinguish in the partially transparent points in panel C and suggest using shapes in addition to colors.
Line 152-154: Could available points be placed in the ocean?
Line 148-164: Clarify that one RSF was fit per bird-year. This becomes clear in the results but wasn’t obvious to me when reading this paragraph.
L 175, 177: I think you have phi written in two different fonts in the text vs. the equation – please clarify if these are the same variable.
Figure 3: The rug plot isn’t providing much information here because of the extremely high density of points at low values. I suggest trying partially transparent lines or another way of displaying the distribution.
L269-284: This is a great paragraph. I would be interested to know how often you saw interannual switching in your data.
L285-295: This paragraph is interesting but is fairly speculative and not all that closely tied to the results. I would trade more data description (see above) for this paragraph, if space is needed.