Monday, May 12, 2014

Why do we need Artificial Nest Boxes for Cavity-nesting Seabirds?

I have always been fascinated with the nesting process in birds. Nesting is such an obvious limitation to an egg-laying species, and birds have evolved fascinating behaviors and preferences that seem arbitrary and sometimes bone-headed. Many birds make their own nests or just lay their eggs on the ground in the open and sit on them until they hatch. Others depend on natural cavities, and it works great when cavities are plentiful, like for forest birds when dead trees are left standing and hollowed out by woodpeckers.

In landscapes that we have converted and managed for human use, the nest cavities are usually limiting resources. What should be a plentiful resource becomes a bottleneck for populations in altered lanscapes. The Bluebird Nest Box movement in the continental United States is credited with restoring the population. For seabirds, we are hoping to get a similar project going that can have population-level impacts for several species.

Nest boxes provide better nesting sites for White-tailed Tropicbirds in Bermuda
In Bermuda, David Wingate, Jeremy Madeiros, and many others have been installing nests for Bermuda Petrels and White-tailed Tropicibirds. Both species were formerly very common on the mainland of Bermuda, but have been devastated by dogs, cats, pigs, rats, and other invasive mammals. For tens of millions of year, these species nested in cavities on islands devoid of terrestrial predators. They have had a hard time adjusting to this new geological era where people and their commensal mammalian community invaded those islands, and their populations have plummeted.

Here is the case for artificial nests for Tropicbirds. You rarely see this type of data in print. But this paper shows - by a simple Wilcoxon Matched Pairs Signed Ranks Test (W+ = 16, W- = 5, N = 6, p <= 0) - that Tropicbirds that nested in artificial nests from 2006 to 2011 had significantly more fledging success than those in natural cavities . The difference was 75% success in artificial nests to 67% in natural nests. Thus, not only are the artificial nests acceptable to tropicbirds, they increase fledging success by 12% on average.

My First Forays into Nest Building
The fact that cavity-nesting birds are limited by nest sites is easy to demonstrate; just put up nest boxes in a suburban area. As my obsession with birds developed in high school, I began building nest boxes for different birds. The first nest I ever set up - a "bluebird" house - was quickly taken up by a pair of Tufted Titmice at my suburban Atlanta yard. Next, I made my own Screech Owl box. Sure enough, there was a Screech Owl pair using it that same year.

The piece de resistance of my nest-box days was the Barred Owl house. It measured about 3 ft in height, 18 inches in width and 15 inches in depth (something like this). I hope my mother has a picture of this monstrosity. I'll post it if I can find it. I had my good friend JR, who worked for a tree-cutting company, use his harness to climb up into an old red oak in my parents' front yard. He tied the box on the main trunk about 20 feet up the tree. Again, within a year, the box was occupied. Not by Barred Owls, but by their bigger, meaner cousins, a pair of Great Horned Owls.

And 20 years later, the Road Goes on Forever

Today, I find myself in possession of 25 nests for cavity-nesting seabirds (see below). About 15 are spoken for so far. Please let me know if you would like to try some in your part of the Caribbean.

All or almost all of the world's Bermuda Petrels currently nest in artificial nest sites, and White-tailed Tropicbirds that use nests provided for them are doing better than those in natural cavities. I've made Audubon's Shearwater nests and tropicbird nests in the Bahamas using cement and had good success, but the fragility and inconsistent nature of such nests makes them less than ideal. These nests are stackable, affordable (about $75 - less than an owl house like the one I made back in 1993), and they can help stop the decline of seabirds in the Caribbean.