If a bird spider gets enough food and if the atmospheric humidity is ideal, a bird spider rarely if ever needs to drink.
If the atmospheric humidity (RH) is too low the spider will drink to compensate dehydration. In that case it is important
to place a little water bowl in the terrarium, because the bird spider will die when it is deficient in bodily fluid.
The air in a terrarium is comparable to a mini–atmosphere when it comes to temperature and humidity. Water vapour is created by evaporation in the air (atmosphere). The amount of fluid that the air can absorb depends on the temperature. The higher the temperature the more fluid the air can absorb.
This is called Relative Humidity (RH). If the RH of 100% is reached, we speak of saturated air and no water vapour can be absorbed anymore. The point of condensation is then reached and the steam will condense on the windows of the terrarium. When the humidity is too high in the terrarium, this can be solved by more ventilation. Between 80% and 90% the air can be called humid and with a RH of 50% (living rooms) we speak of dry air.
The humidity in the terrarium can be measured with a hydrometer. For example in terrariums that are too dry, bird spiders will have problems shedding their old skin. Therefore it is necessary to increase the humidity in terrariums. In most terrariums a big water bowl is sufficient and the presence of real plants increases the RH. However we do need to spray water once in a while. We can spray young bird spiders (spiderlings) just daily, this way we do not have to provide them with a water bowl. This is to prevent death by drowning.
The humidity that is brought into the atmosphere of the terrarium disappears (escapes) after a while out of that atmosphere. That is why it is necessary to humidify the terrarium regularly. In terrariums that are too humid, fungus growth will arise. In terrariums that are too dry, the presence of a full water bowl is sufficient. If the spider is always near the water bowl, the RH needs to rise. The simplest and cheapest way is to spray by means of a plant spray (also for the more humid terrariums). Although a lot of owners of spiders do this, it is not advisable to use cold water to spray terrariums. This because of the following reasons.
1. A sudden drop in temperature will arise, which is bad for the spider and the microclimate in the terrarium. And we do want to disturb this microclimate as little as possible. Furthermore the cold jet of water can frighten the spider so much that it can become stressed up or aggressive.
2.When cold water is sprayed into the terrarium, the temperature will drop strongly, so that the point of condensation will be reached and part of the steam will condense on the windows.
By this we will increase the absolute humidity instead of the RH of the microclimate in the terrarium, what is actually the intention. It is much better to use water that has about the same or a slightly higher temperature. This will have no temperature drops in the terrarium as result. The mini–atmosphere will use the thermal energy present in this warmer water to raise the temperature slightly, so that the RH will increase anyway.
When the temperature drops again afterwards the air will contain more than enough fluid. Furthermore the drop in temperature to a "normal" level will cause an increase in RH. Nevertheless you may not go too far with the high temperature, because when the fluctuations in high temperature are too strong, this is also not healthy for the spider. Besides a jet of water that is too warm can also frighten the spider so that it gets stressed or aggressive.
Source: S.I.T. number 45 – Gerda Achtergaele