These papers are American and Korean respectively I'm afraid but still potentially of use.

http://mds.marshall.edu/cgi/viewcontent.cgi?article=1227&context=etd

Long story short, of the species sampled, only one species showed an increase in detection rate when the playback calls were used when compared to visual encounter surveys (Pseudacris c. crucifer = Spring Peeper)

The following paper only looks at one species of anuran (Rana nigromaculata = Three-striped Pond Frogs) but did show a marked increase in the detection rate when they used the calls of a single male, rather than a chorus of 5. 

http://www.kangwon.ac.kr/~frogkorea/Park/Sungetal2005_1.pdf

Hi Steven,

Good question. I do have some experience with playback usage for amphibian monitoring and detection. In the East of the Netherlands I monitor European tree frogs (Hyla arborea) as you know. On good, warm nights it is not really a problem to count calling males. On less favourable nights, which I usually skip but sometimes because of time constraints I will go out in the field, the males need some encouragement. Usually the don't call on cold(-er) nights but they can be enticed to do so. On my smartphone I have an app which plays the call of this species. After playing it for a few minutes (and if males are present of course) they will start calling making the succesrate of nights with less favourable conditions more succesful. I use the same technique in areas with the same species where populations-/male densitiy is low. The are more ready to call when more males are present (chorus). When I am not sure if a ponds is used for breeding I also play the call and see what happens. For midwife toads I have done the same but there is one problem with that species. They do not form a chorus, every male advertises for himself (and the ladies of course). This species I also got to call by clapping my hands f.e. More a detection method than a good way to measure the *(male-) population. Natterjack toads I also got to call with playback.

In Central America some hylid species also respond to a playback of their recorded call or by clapping your hands or mimicking their call with your own voice (f.e. species like Smilisca baudinii and Agalychnis callidryas.

Hope this helps a bit! Cheers, Tariq

Hi Steven,

Thanks Abhishek Raj for tagging my paper.

In our lab in Australia we use call playback for a wide range of Litoria species. However, there are a lot of species here that don't respond to call playback.

For our auditory surveys we estimate the number of callers prior to playback for three minutes, play calls (or imitate calls) for one minute, then record the number of answering males. We have found this to be useful as an abundance measure for calling activity, but it is always coupled with visual encounters. Our females and juveniles dont make sound so VES is needed and our males are very hard to spot in VES as they startle easily and hide (but females and juveniles don't hide). This covers us for male presence as long as we statistically account for changes in weather and change in calling as e.g. dropping temperatures reduce the amount of calling (however this cant account for changes in male energetic constraints).

There are a couple of things to consider if using call playback, I'm not sure if your interested in this side of things.

1. The number of calling males you use in playback (as previously mentioned)

2. The type of call you use (advertisement call, aggressive call, release call, unenthusiastic call) If you don't know what the calls mean, maybe choose the most common calls in good weather/breeding periods? A pilot study could help determine this and even make a neat note for call types.

3. Not all males within a chorus area actually call (satellite male behaviour)

4. We are going to modify our methods to also estimate the number of males calling during call playback. Sometimes males call during call playback but not afterwards, and kind of 'mirror' our calls, so actual response can be lost if only recording afterwards.

5. When I take volunteers out, I never rely on their ears. I have found with my species that untrained people massively overestimate the number of calling males as their calls are very loud and you feel your surrounded by 20, but its really 8

Good luck with your surveys.

Houston toad work has included call playback for detection of occurrence for the last twenty five years or so.  However, generally this is not conducted with any structured framework (e.g. calls are not consistent among recorded playback version used, not all surveys include playback, effort is not clear among surveys, nor do they differentiate response to call differently than simple detection without the playback).  With all of that, it is successful, importantly eliciting chorus response on occasions when the survey post would otherwise have been negative for that visit. We have utilized chorus detection inclusive of playback responses for occupancy, but I am not sure that how we would apply those data to population size (abundance). The probability of non detection among individuals and conditions (as noted by Tariq above) and the recording and observer bias (as noted by Melanie) are difficult to quantify clearing in making abundance decisions.  For whatever it may be worth we utilize two "metrics" physical detections and chorus detections annually, tracking both.  In our work a large percentage (50-80%) of all chorus detections one night apart are the same individuals.  My point is that you would need to likely utilize a single night so as not to bias the count with iterative duplicates of the same males. Your direction is one of much merit, as quantifying quality of call, observer bias, and then iterative repetition of chorus individual composition are all possible, and chorusing is a very trainable, accessible, long distance (hence large area) survey method. Hence a solution would enable many researchers in the work, but may be taxon specific.

That is a very good point Melanie concerning sattelite males. European tree frogs population also have these reproductive strategies and these are indeed the males you would miss if you are to rely on auditory surveys only. If possible I always combine the auditory surveys with visuals. Also to see if females are present which do not vocalize and in this species area always missed because the survey method is focussed on vocalizing males entirely. Your paper is very interesting! I breed Litoria at home and they are easy to entice into calling (a vacuumcleaner or loud noise usually does the trick).

Steven, I would be very interested to see if you can quantify the usage of playback in midwife toads also giving the challenges in this species I noted in my first answer. The population you monitor makes for a good case study for it is small and in a small area. Best of luck and keep us posted!

we have some experience with playback usage for detection the European tree frog (Hyla arbor). Especially in cooler nights it work fine to animate the males for calling. It works also to attrack some females if the sound device is placed longer time (approx 1 hour) on a spot

Thanks for all of your help guys, sorry it's taken an age to get back to you. I've noted everything you've said and I shall be discussing this over a cup of tea or two with my colleague tomorrow.The technique that I've used here (and some of you have used before) is more akin to the world of ornithology and the population I'm monitoring is in a unique environment - a small number of urban gardens.

First of all thanks to Matthew and Abhishek for sharing some great relevant papers, I'll take a read of these in the morning. Tariq thanks for opening with some great comments, some of which I hadn't though of which will help to start the discussion off between myself and Mark. Melanie, your comments have made me realise that something else we will need to start doing this year is monitoring air temperature as this is will influence when toads call. Like your anurans, our native common frog and common toad (also present where we have found the midwife toads) don't respond to playback calls of conspecifics. The type of call we are using is a mating call, one which when played in close proximity to a male elicits him to reply in order to try to defeat the potential competition. The call is at a pitch where it bounces off of objects easily, as if the toads are throwing their voices across the room. We've used playback to increase call rate of the toads so that we can find them and identify them visually. With our counts, only males we have both heard and seen have been counted although there are more males calling from nearby. The reason for this is not to overestimate the size of the population but this is of course biased in the fact that it doesn't consider females, juveniles or satellite males (as you've mentioned).

Michael, it sounds like what I've just described is similar to the method you use. To ensure that we don't count the same individual twice when we have identified where they are calling from and found them physically we are going to photograph the individual and use software such as Wild-ID to be able to recognise individuals. Males don't move around much but the females do - so we hypothesize we are more likely to find the same individual if it's a female in more than one location (based on our findings so far). Ochsenbein, you're methods are interesting, we have only been using a playback that lasts a few seconds (repeatedly) as opposed to an hour. Would you suggest using a longer sound file for the playback?

Thanks again all. Steve

Hi Steven,

There are some other climate/weather variables which may be useful to record. Although the paper is not from Europe, I thought it might help provide some other variables to record. There are a lot of things that will influence calling and therefore detectability and I'm trying to explore that for my species now. Good luck.

http://link.springer.com/article/10.1007%2Fs002650000256