Generally, in the US at least, we use 'speechless' to mean when something bad/amazing/surprising/crazy/unusual happens and then a person is 'speechless'...i.e. so shocked that they can't express themselves with words. It is more used as a figure of speech. In your example, it would be better to say 'without speech' or 'without language' or 'they lack complex language.' Or, more truly: "Animals seemingly lack the ability to express complex thoughts to one another as they lack complex language." Of course we don't know for sure, as perhaps humans don't have the ability to understand the languages of more intelligent animals like birds and dolphins. We should then be careful to respect and care for animals because, although seemingly lacking complex language, they may possess an intelligence and capacity to feel that is greater than we may expect. Note: 'seemingly' is useful as it expresses doubt, i.e. it seems animals lack complex language, but perhaps humans do not understand its complexity. Interesting question.

Yes, the word "speechless" can be used in the sentence to mean that animals and birds are not able to communicate through human language. However, it is important to note that animals and birds do communicate with each other in various ways such as through body language, vocalizations, and scents. So while they may not speak human language, they are not completely without means of communication. The sentence could be rephrased to say, "Animals and birds are unable to speak like humans, but they communicate in other ways, so we must take care of them well."