I typically would measure EWL simultaneous with MR. That allows me to assess evaporative heat loss in a more meaningful way. I gather you are also measuring for BMR while the bird is in its resting phase. But evaporative water loss often means more ecologically (although I actually do not know what it is you are looking at) during the daytime. I would recommend looking at both MR and EWL during the daytime, when it is warmer, in addition to what you are now doing. Of course, it might be useful to know why you are measuring oxygen consumption and water loss in the first place.

Ask Joe Williams at Ohio State.

In that case I would report both values. But, I agree with Hugh. I would measure EWL and MR during day. If you are measuring MR and EWL at night in a diurnal species, you are measuring sleep values, which will be always different to that what is characterizing a normally functioning animal. And you have a great example for that. Take into account that body temperature fluctuates and has its rhythm. How a sleeping bird at a TNZ can lower its Tb to the sleeping level? Evaposrative cooling would be a solution. Then, if Tb drops below a set value, it incereases its MR to keep Tb regulated. This is probably why your time of minimum MR and minimum EWL differ.

But if you are interested in the minimum EWL of a bird measure it at temperature below thermoneutrality. Should solve the problem (together with measuring a diurnal bird during its alpha phase).

BTW, this paper may be helpful for you: Aschoff, J. (1981). THERMAL CONDUCTANCE IN MAMMALS AND BIRDS - ITS DEPENDENCE ON BODY SIZE AND CIRCADIAN PHASE. Comparative Biochemistry and Physiology a-Physiology 69, 611-619.

Hugh, yes, I'm interested in BMR, which is why I'm measuring MR and EWL in my diurnal birds at night, during their rest phase. I expected minimum VO2 and minimum EWL to occur at roughly the same time of the night, but they don't. I agree that EWL would be more meaningful if measured in the daytime, and in freshly wild caught birds, as opposed to at night in my long-term captive birds, but BMR is the main focus of my study, not EWL. It would be interesting to measure my bird during the daytime too, to see how EWL increases; in that case what would MR be called, RMR? Thanks for the suggestion.

Michal, that's interesting what you said about Tb and evaporative cooling, there does seem to be a link. Thanks for the reference.

George, thanks for the suggestion, I will email Joe Williams.

Michal has made a couple of interesting points. He provides a possible reason for nighttime minimal MR and EWL to differ. I am not sure of the mechanism though, because most EWL is achieved through panting or some other active form (e.g., gular flutter), not common at night. Could enough be lost passively through breathing? That itself is an interesting question.

He also gives you a reference for Aschoff. It was Aschoff and Pohl who first recommended that BMR be measured at night in diurnally active birds. This gradually became standard practice to the point that some authors have defined BMR conditions to include measurement during the rho (inactive) phase of the bird's day. However, that is an extension of A & P; I don't think they ever went that far. Today, we recognize that BMR can shift as a function of season, body composition, even diet. That is, as Gabrielsen and I have asserted, BMR should be treated as a parameter (I think we called it a statistic), not a constant. That said, measurement of BMR in the daytime is reasonable, as long as it is identified as such (I make measurements in both the day and night).

Also, Michal suggests you make measurements in the daytime. Daytime measurements may have more ecological relevance, especially when you pair them to simultaneous measurements of EWL. The reason is that EWL, converted to evaporative heat loss, is surely more important to thermoregulation at higher temperatures.

Hugh, that what I suggested as an explanation for the differences was guessing. But, it would not surprise me if it would be so - EWL as a mean to get rid of excess heat. Speaking about EWL. I was thinking rather about cuteneous water loss then about the respiratory WL. After all in birds it may be a significant component of total water loss.

Dear Michal and Hugh, thank-you both for your comments. How long would I need to measure daytime MR for? Maybe 4 hours at a time? I have video cameras on my birds, and the video along with the VO2 trace would tell me if the birds were resting or not, so I could give it a go.

Lindy, as long as the bird will be postabsorptive at the time of measurement, 4h should be enough.

OK, thanks very much Michal, I will try it out.

Should daytime measurements be done with a light on, or off? Or does it not matter? One of my birds is very jumpy and just won't settle with the light on.

Here is how I do it. I typically find that it takes a bird about an hour to settle down in a chamber. Sometimes, birds that do not settle down easily need a bigger chamber. I don't know what species you are working with, but chamber size does have to be matched to flow rate. Larger chambers require faster flow rates for adequate mixing, but faster flow rates cannot be used if a bird is too small (there is a limit to how sensitive most O2 analyzers are). I like to do multiple runs at a given temperature with different flow rates. That tells me if I am getting sufficient mixing (there is if the calculated VO2 is always the same). I keep it in dim light, occasionally in the dark (especially if it is a nighttime measurement). I have never had a video feed, but I can hear a bird move around in a chamber. I like to know they are awake, even if they have settled down. Although I am monitoring/recording VO2 the whole time, I usually start paying close attention after an hour. A calm bird may be done an hour later, so two hours total. An excitable bird can take much longer, and 4 h is not out of the question. I record mass and Tb immediately before and after each run and am careful to get an exiting Tb within about a minute of the end of the run.

Michal is right that the bird be postabsorptive. He also is right in pointing out that cutaneous water loss can be important. There have been relatively few papers that partitioned cutaneous and respiratory water loss though and the relationship may differ in different species. Also, I suspect that at much warmer temperatures, respiratory water loss (passive, panting, gular flutter, etc.) are more important. Somewhere I must have a copy of what may have been the first partitioning paper. I recollect it was by Richard Smith on pigeons. He told me about it in the early 1980s, so he probably did it in the late 60s or 70s.

Finally, although you are probably in Durban (it's been a while since I was in South Africa), and Pretoria is not exactly close, Andrew McKechnie is at the University of Pretoria and has tons of experience with all this. If you are not already in touch, you should contact him. I no longer know who else is active in this field near you.