That could be half of my problem this year.... I've been blaming the weather and fact that I've been using alot of really young birds this year. Temps have been really cold yet and I've trimmed a little but I didn't want to clip vents too bare either. I used Frontline on all my birds late last October. Checked over everything in early February and still not a bug to be found on any of them. I know wild sparrows can get into their pens so the possibility of catching bugs is there. Fertility has been running less than 50% and actual hatch percent is usually 10-15% so far this year.