(Lagenaria siceraria) 95 days. For over 50 years, Dr. David Bradshaw has been selecting the strongest and best-shaped gourds for Purple Martin (Progne subis subis) birdhouses. This species, the largest American swallow, suffered a population crash in the 20th Century due to the spread of European Starlings, and the birds now rely on humans for nesting houses. It is thought that without our assistance, Purple Martins would be in danger of extinction. Monitor your birdhouses to prevent take-over by aggressive, non-native birds. Purple Martins eat mosquitoes and many other insect pests. The sprawling plants produce up to 12 gourds each.