Hi Ulises and Michael et al. It is really unusual when raptor floaters can actually be counted in the field and compared with occupancy data. That's an interesting graph. Another example of temporal variation in territory quality and its effects on floater numbers may be the effect of jackrabbit cycles and drought on territory occupancy in golden eagles in the deserts of western North America. The idea is that some territories may become temporarily unsuitable for occupancy (no food), forcing the territory holders into the floater pool. Jim Watson et al. has an interesting recent paper that reflects a bit on that issue (Watson et al. 2020. The Journal of Wildlife Management 1–10). I think what you are both seeing what might be relatively stable populations with adults moving in and out of the floater pool depending on temporal variation in territory suitability, i.e., availability of a nest site or enough food within the territory to sustain its occupancy. In a study of a resident golden eagle population (PLoS One 2017), I radio-monitored 51 floaters over a long period and only two acquired territories. Breeder numbers were meanwhile stable.

Hello Ulises; In the Western USA there is a great deal of energy invested in Western Bluebird breeding. People make nest boxes and set them out in likely habitat. Many of these projects have very long runs of monitoring data. I can't cite a particular reference but if you do a little searching on google scholar or make an enquiry of the Cornell University Ornithology Lab I bet that you will find material of interest. Best wishes, Jim Des Lauriers.

It has been examined in Aplomado Falcons:

Article Assessing the importance of artificial nest sites in the pop...

Hope it helps!

Thank you Christopher J W McClure !

Ulises; Here is the link to the bluebird project. They are serious amateurs with affiliations to the academic community.

https://bluebirdssc.wordpress.com/

Cheers, Jim Des Lauriers

James Des Lauriers , thank you very much for the information!

Hello Ulises,

Retired professor Bernd Heinrich spent a large portion of his academic career studying ravens and discusses this concept in some of his published books: floaters vs. the adult breeding population. If I remember right, he came to the conclusion that only a small portion of sexually-mature, adult ravens breed, even though individual ravens may form pair bonds early in life. Much of the population is composed of "floaters" or non-breeding adults. Many factors can influence this phenomenon from books/papers that I've read: food & resource availability, sex ratios in a population, suitable nest sites (eg, cavities or cliff ledges), available territories, etc. There was also an early paper (before 1960?) that experimented with Northern (Yellow-shafted) Flickers. The authors removed the male from an established breeding pair of flickers to see how many new males the female would accept; she ended up pairing with something like 20 males until the researchers stopped the test. Check the literature and academic databases for more references on this topic. Good luck!

For clarification on the flicker example above, all the new males accepted by the female (other than the original one) were "floaters": that is, wandering adult males that were just waiting for the opportunity to find a mate, nest cavity and/or territory. Flickers are dependent on tree cavities for nesting.

Thank you very much Stacia A. Novy !

Aside from direct counts, I think the only way to obtain a floater-to-breeder ratio is to project it from vital rate estimates. I'm sure you know this, but assuming that all floaters are adults (that being the proper definition) and that territory-holders and floaters have the same survival rates, you can calculate f:b by simple algebra (see Hunt, W.G. 1998. Raptor floaters at Moffat’s equilibrium. Oikos 82: 191–197). If the rates are different, then you can use the equations in Appendix 3 in Hunt, W.G., J. D. Wiens, P.R. Law, M.R. Fuller, T.L. Hunt, D.E. Driscoll, R.E. Jackman. 2018. Quantifying the demographic cost of human- related mortality to a raptor population. PLoS ONE 12(2): e0172232. doi:10.1371/journal. pone.0172232).

Hello; Mr. Hunt's suggestion incorporates a hazard. Do floaters and territory-holders actually have the same survival rate? Is there any literature supporting the assumption? In other species, unrooted individuals are at considerably greater risk than established ones. Acorn Woodpeckers immediately come to come to mind. It might be worth a bit of investigating. Best, Jim Des Lauriers

James is right, and the equations in the Plos One appendix 3 do allow for that difference. I uploaded both of those papers to my RG page.

Cheers, Grainger

Hello Ulises,

We conducted long-term monitoring of the Steller's Sea Eagle (Haliaeetus pelagicus) population for 12 years. Our data include both bird counts and inspection of nesting territories, so we have estimations of floater / breeder ratios and territory occupancy (which reflects nest site availability). So far we considered it in terms of average values, but if you're interested, we could see if there is any relation between these two variables. If so, please specify your request.

Hi Michael

That's an interesting idea and one worth pursuing.. I have your wonderful book but am traveling and do not have it available right now. Are the counts conducted in the nesting study area? I think you're saying that nest site availability varies enough over time to make the comparison. I am wondering what causes the variation. Are nests frequently destroyed in storms?

Hi Michael, I'm totally interested! On the top of my head and without knowing the system, it would be great to know if variability in occupancy is correlated with floater/breeder ratio, since floater numbers could rise up if there are less nest sites availability or if there are environmental conditions that could explain individual breeding choices. Glad to discuss more and help if I can. I let you my.email since I do not check here often: [email protected]

Thanks, really interesting!

Hi, I don't know if our data on peregrines in southern Scotland would help. We have a long-term dataset on breeding (50 yrs) and a mark-recapture study of the same population of about 20 yrs. We have information on nest sites, including those that become unoccupied over time, and some that appear to be suboptimal or at least appeard to be until they were occupied and then consistently successful . I have always thought we might somehow use those data along with other long term peregrine datasets (e.g. the midwest project that P. Redig was working on), and maybe other studies on other species to chip away at the issues around floaters. Amongst the raptors, I'd think useful datasets might exist for Common Kestrel, Osprey, spotted owl, Bald Eagle, also maybe white tailed eagle, red tailed hawk, Cooper's hawk.

Hi Michael J. Mcgrady , totally! That dataset seems really interesting! I think it's worth to check if there is a relationship between nest occupancy and floater/breeder ratio, especially for those spp associated with non-randomly nesting sites such as peregrines. Glad to discuss more and help if possible! I let you my e-mail: [email protected]

Thank you very much Grainger Hunt ! I'm just glad that you answered, your 1998 paper is one of my favorites on the topic!

Around 75 % of adult birds in our striated caracara population are floaters (based on counts are assuming that we encountered all nests in a certain area), and I would like to 1) see if this number is relatively high or just unrecorded in natural populations and 2) check to want extent it could attribute it to nest site restriction, which in our case would be posed by invasive ungulate's grazing

Dear colleagues, Ulises, Grainger, Michael and others,

Here is our data on territory occupancy and floater / territory holder ratio. There seems to be a negative relation (though not very strong) between territory occupancy and floater / territory holder ratio.

Although, I should warn that our determination of bird territorial status may be not very accurate since we didn't use individual marking. Here is short description how we made it. "The territoriality status of adults was initially identified in the field by expert judgment, based on the birds’ behaviour (e.g., territorial, defensive, courtship), location relative to nesting areas, and other circumstances of the observation (habitat, number of birds, etc.). Subsequently, these estimates were revised on mapped data, and the final judgment was made. In doubtful cases, adult birds registered farther than 1.5 km from the nearest sea eagle nest were classified as floaters, the threshold being based on our observations and energetic constraints of the species (Masterov, 1992)."

Understanding that this technique is far from perfect, I should argue that in many cases it works well since territory holders are often (by one or two birds) met within or not far from their nesting territories while floaters usually take part in big aggregations (together with immatures) on the sea shores.

From my point of view, this negative relation may mean that the population numbers remains more or less constant, and in some (unfavourable) years some territorial birds choose to skip breeding and become floaters.

Please comment whether this chart supports your ideas or not. Different species may behave differently. I can send the source Excel file via email on your request.

Hi Ulises and Michael et al. It is really unusual when raptor floaters can actually be counted in the field and compared with occupancy data. That's an interesting graph. Another example of temporal variation in territory quality and its effects on floater numbers may be the effect of jackrabbit cycles and drought on territory occupancy in golden eagles in the deserts of western North America. The idea is that some territories may become temporarily unsuitable for occupancy (no food), forcing the territory holders into the floater pool. Jim Watson et al. has an interesting recent paper that reflects a bit on that issue (Watson et al. 2020. The Journal of Wildlife Management 1–10). I think what you are both seeing what might be relatively stable populations with adults moving in and out of the floater pool depending on temporal variation in territory suitability, i.e., availability of a nest site or enough food within the territory to sustain its occupancy. In a study of a resident golden eagle population (PLoS One 2017), I radio-monitored 51 floaters over a long period and only two acquired territories. Breeder numbers were meanwhile stable.

Dear Grainger,

I'm really pleased that you have our Steller's book! I also have your classical papers on the Moffat's equilibrium and refer to them every time. About our counts: I understand that our approach was imperfect but we didn't have a possibility to track adult individuals. At the same time, we wanted to take floaters into account, so we decided that this is better than nothing. Thanks very much for the references to your paper and Watson et al. 2020! I will download them right now.

Dear Ulises,

We had published two article about floaters in a long-term woodpecker study in fragmented habitats that could be useful for you:

https://royalsocietypublishing.org/doi/10.1098/rspb.2017.0074

https://zslpublications.onlinelibrary.wiley.com/doi/abs/10.1111/acv.12566

Our results suggest that the occurrence of floaters, that may buffer the extinction risk of small population and reinforce the (re)colonization of empty patches, is related to the size and quality of habitat patches. We used the density of large oaks as a measure of habitat quality, since they provide forage for the middle spotted woodpecker. But, apparently in our study system, nest-site availability does not seem to be a critical factor inside the habitat patches.

Good luck with your interesting research!

Carlos

Hi Carlos Ciudad , thank you very much for your response and the works you've shared. It will be useful for my discussion to include data on variability on floaters in populations not limited by nest-sites, and I did not know about this case.

Gracias!

It is a pleasure to try to contribute in this fascinating and unknown topic, Ulises Balza.

¡Mucho ánimo con el estudio!

Hi all,

I'd say that floaters (nonbreeding adults) may be the most likely to arise from both: (i) habitat saturation, and (ii) in low-populated scenarios.

In the first case "i", floaters are likely to be attracted to high-quality and high-populated habitats (perhaps through conspecific attraction mechanism). This is what happens with our woodpecker population (Robles & Ciudad 2017 Proceedings B, see also Robles & Ciudad 2020 Animal Conservation). In that "i" case, resource availability (nest-sites...or food, shelter, etc) will play a critical role. In woodpeckers it seems that foraging availability is more important than nest-sites (as pointed out in a previous comment by Carlos Ciudad). But nest-site availability plays a critical role in the settlement of floaters in tit populations. This is what we can conclude from the high occupancy by tits after adding nest boxes in areas with lack or low natural cavities (see Robles, Ciudad & Matthysen 2011 and 2012 in Forest Ecol Manage).

In the second case "ii", the loss of mates led previously breeding adults to float in search of new mates during "long" periods. This is more likely to occur in low populated habitats, such as in severely fragmented environments. Under these circumstances, our floaters typically dispersed out of the previous breeding patch to search for potetial vacancies in other (populated) patches.

A high floater-to-breeder ratio can be an indicator of habitat quality and long-term population stability under the habitat saturation scenario, where floaters may buffer the extinction risk of populations by fast replacement of the lost breeders, but not under the low-populated scenario, where the occurrence of floaters is likely to be associated with Allee effects and/or detrimental stochasticity.

I hope this makes sense to you. Best wishes,

Hugo

Hello; Western Bluebirds in the western USA respond strongly to setting out nest boxes. In suburban and scrub habitats where old cavities are in short supply populations are sparse or absent. Various projects have shown that floaters quickly occupy nest boxes when they are available. So, nest availability seems to the the limiting resource for this species. This is likely to be true for most hole-nesting species and the distibution of these species is likely to be strongly correlated to population densities of woodpeckers. Best regards, Jim Des Lauriers

Thanks Hugo Robles and James Des Lauriers for your responses.

Do you know about there references on how a high floater-to-breeder ratio should look like? We have 2 to 5 floaters/breeder, and spatial distribution of nests suggest that a process of nest limitation has occurred since the introduction of grazers (red deer and goats) in the island. I argue that this floater/breeder ratio is another (indirect) evidence of nest restriction, but as floaters are usually less studied, I don't know if we could be in 'normal' rather than 'high' ratios. The study area is a dense penguin colony from which they obtain food, so it is certainly attractive to floaters, not necessarily from that particular population. We are also assuming that, as nest in this species is not elaborated and are mostly in the ground level (or at least human-with-no-equipment-accessible), that new nest-sites are less prone to be available with time.

In other words, we want to discuss if our f/b ratio could be an indicator of high quality of the site but only in food availability terms, being stability in (low) breeder numbers not necessarily an indicator of good population health.

Thank you!

Dear All

Am travelling so don't have much time to reflect on all this interesting stuff. Am so grateful to be part of this interesting discussion by all of you.

The response of bluebirds to additional nest boxes indeed makes floaters and equilibrium easy to understand as products of nest site saturation. And look at seabirds on pelagic rocks where every possible space has an incubating adult. Oystercatcher life-history is also revealing, particularly the work of Ens during the 1990s and also Kokko H, Sutherland WJ. 1998 Optimal floating and queuing strategies: consequences for density-dependence and habitat loss. Am Nat. 152: 354–366.

Ulises—Your observations of 2-5 floaters per breeder seems really high. Is there a high degree of floater interference of breeders trying to nest? If the grazers were introduced recently, then the high F:B might reflect the continued presence of former breeders, and the population will not be at equilibrium until a ~ stable age distribution finally develops. I don't know what is normal for F:B. I've only been able to model it from estimates of vital rates. Your direct counts are an unusual opportunity to look at F:B, competition for territories and other density effects.

During the early recovery of peregrines during the 1970s when so many historical sites were still vacant, there was clearly a recruitment deficit, and we often saw subadults as members of breeding pairs. Nowadays, they're virtually all adults, with floater incursions and fighting, sometimes fatally. Even so, one rarely sees a pair of peregrines on a substandard cliff, and that suggests that other factors besides a safe place to incubate are at play—perhaps defensibility or insufficient food within commuting distance.

Regarding Hugo's point about foraging habitat availability, I suspect that the cue is the female's inability to accumulate fat when attempting to hold a territory. Perhaps males prospect for territories while females prospect for territorial males with frequent offerings of food. Both sexes might go for a site where they have seen young raised in previous years and avoid those rarely successful.