Other signs of a cat in heat are a deep wailing sound to worse, a screaming sound at all hours of the day. Cats routinely have their first estrus cycle around 6 months of age and differ from dogs in that cats have multiple cycles during the year.
Also beware – indoor cats that have cared less about the outside might now try to dart out the door. These heat cycles are confusing to cats and stressful to their owners – early spaying can avoid a lot of headaches for all.