Over time flowers have developed characteristics that attract various pollinators.  Among them are flower shape, color, scent, nectar amount, pollen amount and presence of nectar guides.

Some flowers have lips that provide a perch for an insect to sit on while it picks up pollen on its head.  More open, bowl shaped flowers attract birds and insects that hover.

Specific pollinators are attracted to certain colors.  For example, bees tend to like bright blue and violet, hummingbirds go for shades of red and purple, butterflies tend to like bright colors.

Flowers are most likely to have their strongest scent when they are ready for pollination and their preferred pollinators are active.  Plants pollinated by moths will have their strongest scent after dark; those pollinated by bees will have their greatest odor during the day.  Flowers that are pollinated by flies are likely to have a scent we would consider bad—they are attracted to rotten meat as a place to lay their eggs.

Nectar is the reward flowers offer to the animals that pollinate them.  The more nectar they produce, the more likely pollinators are to visit.

Some plants produce large amounts of pollen; think about the pine pollen blowing around in the spring.  These plants are often pollinated by the wind blowing pollen from one flower to another.

Nectar guides can be patterns or markings on the flower that guide the pollinator to the nectar source.  Some, however, are invisible to the human eye and can only be seen by pollinators that see ultraviolet light.