Grazie Marziotta, it is pretty clear to me now, gatte is pronounced more lightly than gatti, as if it was gatteh, while gatti I have to emphasize on the ending like gatty, again thanks a lot and have a wonderful day :-)
Referring to a known female cat in the masculine form is generally because the person thinks of the animal as an object, a bit like saying "it" for the family cat in English. Someone who cares for an animal will refer to it in the correct form.
From what I understand if the sex of a group of cats is unknown or mixed, it would be I gatti and masculine is predominately used if in doubt. Cats and dogs can be referred to as masculine or feminine. In this case they are female.
A dictionary is useful for finding out if there is a féminine form. According to mine, there is no feminine form of "uccello". Other animals do have a féminine form but if you don't know the sex of a particular animal(s) it is more normal to use the masculine form.
Because the sentence starts "le gatte". If the speaker did not know the gender of the cats he/she would most probably use the masculine plural, "i gatti". Female plural is normally only used if you know they are all female.