My friend found a bunch of dead flies by the window in her apt. Window has screen and the are all by a lap

Oh for pity's sake! I live in a rural area, and dead flies around windows are a fact of life. They get in as baby flies through tiny openings, grow quickly, and die from lack of food and water. Or just one female adult can produce hundreds of eggs inside who also hatch and die trying to get out.

To answer the question in your title, they all flew towards the warmest, brightest spot they could see, hoping to get out into the sunshine where there would be food and breeding.

I apologize for saying 'Oh for pity's sake.' Terrible way to say 'Don't worry.'

Pardon me, but some education is due about the biology and taxonomy of flies. Flies are holometabolous, which means that they undergo a complete metamorphosis to grow. Most flies start as eggs, which hatch into larva. Growth is in stages, called instars. Some commonly known species of fly larva are in the form known as maggots.

A maggot does not have wings. A larva could essentially be called a "baby fly". A larval instar will expand as it feeds and when it is ready, a larva will shed its skin (molt) and become a bigger larva. A species of fly will usually spend a few instars as larvae. After they complete their pre-determined larval stages, they will molt into the resting pupal stage where they change into the adult.

The adult fly that emerges from the pupa will have wings. This is the fully formed, reproductive adult fly that is also the final stage in a fly's life. There is no further growth.

Technically, "fly" is a common term that applies to an entire order of insects know as Diptera. There are many types of flies - flesh flies, horse flies, flower flies, house flies, and even mosquitoes are flies - with many types of life styles. Some are parasitic, some are predaceous, some are carrion feeders, some are vegetarian, some are terrestrial and some are aquatic.

To be sure you would want to submit your sample to a knowledgeable person, but I suspect that an unusually large amount of a single species of fly, is due to the simultaneous eclosion (emergence) of flesh flies or bottle flies from a dead animal that is stuck somewhere in your house or garden.

Blow Flies and Flesh Flies | Iowa Insect Information Notes (http://www.ipm.iastate.edu/ipm/iiin/fblowfli.html)

It is true that many insects are phototactic and would be attracted to a window. Phototaxis - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Phototaxis)