This interesting manuscript addresses the paradox of the great speciator by analyzing the case of the silvereye, a bird widespread on several islands (thus able to disperse over long distances overwater) with different subspecies. Specifically, the authors analyze six candidate genes involved in the migratory and exploratory behaviours in search of a genetic mechanism that can explain a fast shift from a high dispersion/migration propensity (necessary to colonize of the islands) to sedentarism (which could explain how the subspecies differentiated). Here below I point out my main concerns.

Major issues
Title: this is certainly a catchy title, however I would tone it down a little, since the work was carried out on one species only (though a very interesting one) and the results are not 100% clear and striking. Moreover, I suggest to add somewhere the word "migration", as you did not consider only dispersion. I suggest: Candidate gene length polymorphisms are linked to dispersive and migratory behaviour in the silvereye: searching for a mechanism behind the "paradox of the great speciators"

Keywords: some word are already in the title, and that makes them useless here

Introduction
L60-69 In my opinion, this part is too long and has a little to do with your work. In the discussion you don't take up the topic. So, I recommend shortening this part by cutting out some of the examples
L114-116 Is the Tasmanian the only partially migrant population? Otherwise, how did you choose to consider this particular one?

Methods
Please, add the sampling size for each candidate gene
L147-149 I don't find the corresponding results (H-W equilibrium, allelic richness, and heterozygosity)
L151 How did you select this region?
L169 I would change this title in " Gene flow estimate"
L171 How many individuals make up the subset? How did you select them? Are they representative of all the sampling sites? The same for the 336 individuals at L174
L180 How was the script made?
L190-191 Please, add some details? How did you adjust the delta values? How did you calculate m, a, and f?
L204 and subsequent. Did you try to use the longer allele instead of the mean allele length? Dominance of the longer allele has been suggested for some candidate genes
that show Poly-Q polymorphism, see e.g. Saino et al. 2015 Molecular Ecology
L207-210 Please, could you add some details con the CREB1 and ADCYAP1 models?
L209 How did you calculate the dispersal propensity?
L237-241 I'm not sure that this inhomogeneous is the best one. Why didn't you estimate the age of these populations by means of the time-calibrated gene tree? Or at least, did you validate the estimates?
L253 Did you assess the distribution?
L262 How did you choose 20 bp?

Results
L276-282 Were these alternative values of k considered somehow? You did not discuss them
L283-289 What does it mean? Please, discuss it briefly
L291-304 I would present the genes in tne same orderd as in the introduction and in the methods
L296 Did you sequenced the Clock gene to assess yhe number of Poly-Q repeats? It would be useful for comparison with other species/papers (of course, I don't mean you have to sequence it only to answer this question)
L300-301 "NPAS2 showed some variation across populations but similar mean values for populations in the ANZO cluster". Actually, Lord Howe Island seems quiet different
L308-310 And Australia?
L314 In which gene? Please, add a measure of variability
L314-317 I would like to read some synthetic results. I think that moving all the results to the supplementary materials hinders the understanding of the paper
L325-326 Do you mean that a unit decrease in DI corresponds to a 5 bp decrease in CREB1? I don't really understand the x-axis of the Figure 5B and I'm a little confused by this result
326-328 What values do they correspond to?

Discussion
L339 I would add a brief description of the direction of the association
L339-342 This sentence ("third, more limited, etc.") is not very clear to me, I suggest you to rephrase it, adding also some details (e.g. how do the population differ with regard to DRD4?)
L360-361 What is the limit that separates the two groups (long vs short)? How were they compared? I don't find this part in the methods and results
L373 Please, add "in a partial migrant population" after "individuals"
L389 Dispersal or migration?
L389-392 I don't understand this sentence. Of course photoperiod changes a little close to the Equator. However, it is known that birds (and organisms in general) are able to identify even minor variations in photoperiod and adjust their phenology accordingly. Otherwise, birds that breed at high latitudes and winter around the Equator would never migrate at the right time
L392-395 I don't understand this sentence. It is plenty of evidence that the photoperiod is the trigger of migration and that the basis of this behaviour are genetic. And is not surprising that temperate populations migrate more than tropical ones. Do you mean that individuals bearing the same alleles react at different photoperiodic regimes by deciding to migrate or not? Moreover, what happened to these large flocks that migrated between the South and the North Islands? Are they disappeared?
L401 Are these alleles rare enough to justify not finding them in recently established populations? If they determine the propensity to migrate, they should be found in these populations
L408-410 How were these populations chosen? Why did you decide to limit the sample to these populations? Which are young and which are old?
L443-451 This part is not fully clear to me. Did the longer CREB1 alleles originated in Australia? Why does the connection with Australia maintain the migratory status of the Tasmanian population? Isn't it because Tasmania is more temperate? Or do you mean that Tasmanian individuals could migrate only because they are very close to a big island plenty of food and space?
L453-472 I suggest to shorten this speculative part. It is interesting but very long and adds a few to your work
L481-484 I would move this sentence at the beginning of the discussion
L485-486 Does this part agree with any previous studies?
L487 Pleas, add "in a population" at the end of the sentence

Figures and captions of figures
Fig. 1 Please, add a measure unit for time (e.g. years) in the figure or in the figure caption
Fig. 1 Was French Polynesia considered in the analyses?
Fig. 1 It would be interesting to size the dots according to the sample size
Fig 2 B is included also in fig. 3, please, remove the redundancy
Fig. 3 I see the names of the sites on the right instead of under the figure, is it right?
L508 I think you mean Ambrym instead of Malekula
Fig. 4A I would appreciate more a boxplot instead of this representation
Fig. 5 Does this figure originate from the brms models?
Fig. 5 Why is (only) Tasmania shown? It would be more clear if you add the main labels. E.g. where are the islands mentioned at L521-524?
Fig. 5A The figure would be more clear if the x-axis was in a log scale
Fig. 5B Why is 1.00 in the left and 0.00 in the right in the x-axis?

Minor comments
Abstract
L4 I think that is more a "propensity" than an "ability"
L6 Please, add "yet" after "identified"
L6 I would change "explore" with "explored"
L10 I suggest to add "and migration" after "propensity"
L17 Replace "further" with "larger" and remove "size"

Introduction
L31 Replace "is" with "have been"
L43 Please, replace "relative" with "relatively"
L50 Replace "or" with "and"
L79-88 I suggest to add at least the main references (one or two per gene). The SM do not always accompany the paper and the citation in there do not count
L108 Please, change "examining" with "at three"
L113 Please, change "correlated" with "associated"

Methods
L166 Please, add "for" before ""SERT"
L166 Please, add the scientific name of dunnock
L168 Please, add a reference for this sentence
L203 (and subsequent similar cases) R package brms version?
L250 Please, modify the brackets, it took me some time to understand that the bracket was not closed after "a". I suggest "For all brms (i.e. a) Bayesian etc."

Results
L330 Please, add "propensity" after "dispersal"
L334 I suggest to change "genetic changes" with "allelic changes"

Discussion
L354 "and those evolutionarily recent colonisation events" I think there is a missing word
L359 Remove the comma before "male"
L380 Please, change "correlated" with "associated"
L399 Please, add "However" before "The Tasmanian"
L448 I think that the depth of the sea does not influence the crossing

Figure captions
L501 "Individual mean allele lengths"