Be Merciful to animals

The Quran describes that animals form communities, just as humans do: "There is not an animal that lives on the earth, nor a being that flies on its wings, but they form communities like you. Nothing have we omitted from the Book, and they all shall be gathered to their Lord in the end"
(Quran 6:38).

The Quran further describes animals, and all living things, as muslim - in the sense that they live in the way that Allah created them to live, and obey Allah's laws in the natural world. Although animals do not have free will, they follow their natural, God-given instincts - and in that sense they "submit to God's will," which is Islam. “Seest thou not that it is Allah Whose praise all beings in the heavens and on earth do celebrate, and the birds (of the air) with wings outspread? Each one knows its own (mode of) prayer and praise, and Allah knows well all that they do.” (Quran 24:41)

•Reward for mercy of animals: It is related from Abu Umama that the Messenger of Allah, may Allah bless him and grant him peace, said, "Whoever is merciful even to a sparrow, Allah will be merciful to him on the Day of Judgment."

•Animals are like humans: “A good deed done to an animal is like a good deed done to a human being, while an act of cruelty to an animal is as bad as cruelty to a human being."

•Animals cannot speak up for themselves: It is related from Sahl ibn Al-Handhaliyya that the Messenger of Allah, may Allah bless him and grant him peace, once passed by a camel that was so emaciated that its back had almost reached its stomach. He said, "Fear Allah in these beasts who cannot speak." (Abu Dawud)

•Mental cruelty also forbidden: It is related from AbdulRahman bin Abdullah that a group of Companions were once on a journey with the Prophet, may Allah bless him and grant him peace, and he left them for a while. During his absence, they saw a bird with its two young, and they took the young ones from the nest. The mother bird was circling above in the air, beating its wings in grief, when the Prophet came back. He said, "Who has hurt the feelings of this bird by taking its young? Return them to her." (Muslim)
•Forgiveness of sins: It is related from Abu Hurayra, from the Prophet, may Allah bless him and grant him peace, that a prostitute once saw a dog on a very hot day going round and round a well, lolling its tongue because of its thirst. She drew some water for it using her shoe, and for this action all her sins were forgiven her. (Muslim)
•Mistreatment is a sin: It is related from Jabir that the Messenger of Allah, once saw a donkey which had been branded on its face and he said, "May Allah curse the one who branded it." (Muslim)
•Give rest to beasts of burden: It is related from Abu Hurayra that the Prophet, may Allah bless him and grant him peace, said, "Do not use the backs of your animals as chairs. Allah has made them subject to you, so that by them you can reach places that you would not otherwise be able to reach except with great fatigue." (Abu Dawud)

Source: http://islam.about.com/od/islamsays/a/animalwelfare.htm