The Western purple martin is a blue-listed, secondary cavity nester, found in British Columbia. Blue-listed means that this bird is declining in numbers and is at risk of becoming an endangered species. Secondary cavity nester means that, in nature, the purple martin will only choose to nest within holes in trees which have already been carved out and previously nested in by birds such as the hairy woodpecker and red-breasted sapsucker.
The problem is that due to factors such as habitat loss, the number of secondary cavities available to these birds is dwindling. This has led to the purple martin almost exclusively nesting in artificial housing (nest boxes created by humans).
Previous research points to identifying the ideal dimensions, entrance size and shape, building materials and proximity to other structures and habitats. What hasn’t been widely studied is the internal microclimate of the artificial nest boxes. So how do we know that they are suitable?
What did we do?
The long-running Western Purple Martin Recovery Project by Nature Squamish, currently led by Davina Dubé, provided an excellent opportunity to test our question. Before the 2025 breeding season, we headed to the Squamish Estuary to install temperature loggers in seven different types of nest boxes. Most of the nest boxes were already present on the existing structures in the Squamish Estuary (see photo above). The “Aluminum Condo” was erected next to the existing structures, and the two “Snags” are dead trees with existing nest cavities found nearby. Two of the seven box designs we installed are popular with purple martin “landlords” (people who put nest boxes up in their yard for this species) and have become widely available for purchase: the “Aluminum Condo” and the “Gourd”. The “Control Box” is a design by Darren Copley from 1997, related to their project with the Ministry of Environment. The remaining four box designs are by our dedicated volunteer and conservationist, John Buchanan.
What did we find?
We found that, while the differences in temperature were not as large as hypothesized, the level of insulation inherent in the design did make a difference to the internal temperature of a nest box. The “Aluminum Condo”, a hexagonal structure of 18 nest compartments which is made of aluminum sheeting, showed the largest range in temperature. This means that the temperature fluctuated between very hot during the day to very cool during the night. The “Control Box”, which is a basic design made from wood, also showed a large range in temperature. The boxes with more insulation, such as “Box A”, “B”, “C”, and “R1”, demonstrated a smaller range in temperature.
We also found that maintaining temperature stability may be an important factor in the ideal nest structure. The larger, more insulated of the two natural cavities (“Snag 2”) demonstrated internal temperatures that rose and cooled slowly. This could reduces the nesting birds’ energy expenditure in comparison to a less-insulated nest, because the birds would not have to spend as much time tending to their young or deal with temperature-stress.
What about the temperature extremes?
It was interesting to observe conditions in the nest boxes during times of temperature ‘extremes’. During 24-hour periods with lower than average temperatures, some boxes maintained a warmer internal temperature than the “Aluminum Condo” and “Gourd”. In slightly higher than average temperatures, those same boxes remained cooler inside than the others. However, when external temperatures increased, the ability for the nest boxes to remain cool inside was pushed to the limit and the temperature in most boxes increased to almost the ambient external temperature.
In 2026, we plan to do all this again, taking what we’ve learned and testing out a revised nest box design as well as another commonly used nest box.
Check out the full report here!