Tree Swallows are a valuable native swallow that often competes with Purple Martins for housing. They are smaller than martins but can be very aggressive. If allowed to nest in housing intended for martins, they will often discourage cavity-hunting martins. A strategy for dealing with Tree Swallows can be found at http://www.drugfreeworkplace.com/~Dan/TRIHABITATION/TSEMERGENCY.html.
