Experimental test of nest‐site limitation in mature mixed forests of central British Columbia, Canada

Abstract

Nest‐site availability limits cavity‐using populations in many harvested forests; however, little is known about the extent of nest‐site limitation in mature forests with a full complement of excavator species and intact processes of cavity creation and loss. To examine the role of nest‐site availability in limiting cavity‐using populations in mature mixed conifer forests in central British Columbia, Canada, we conducted an 11‐year before‐after control‐impact experiment in which we increased nest‐site availability via nest box addition. Our 7 sites (3 treatments, 4 controls) had low cavity densities (<2/ha) prior to treatment and cavity occupation rates were also low (<10%/yr), which is a relationship often cited in the literature as evidence of non‐limitation in cavity‐nesting populations. Following nest box addition at our treatment sites, which tripled the availability of cavities, total density of bird and mammal nests more than tripled. Density of mountain chickadee (Poecile gambeli) nests increased 9‐fold on treatment sites and returned to pre‐treatment levels following box removal, suggesting that chickadee populations were limited by cavity availability at our study sites. Density of red squirrel (Tamiasciurus hudsonicus) and northern flying squirrel (Glaucomys sabrinus) nests and roosts also increased significantly at treatment sites following box addition and declined following box removal. We noted little change in chickadee or squirrel nest density at control sites monitored concurrently. Squirrels preferred large‐sized over small‐sized boxes, and significantly enlarged the entrance areas of small boxes by chewing, suggesting that there may have been a shortage of suitable nest and roost sites for them in our study area. We contend that low cavity occupancy rates may not accurately reflect nest‐site availability for cavity nesters in mature forests, and that cavity size may influence the true availability of cavities on the landscape. © 2011 The Wildlife Society.

Responses to experimental reduction and increase of cavities by a secondary cavity-nesting bird community in cavity-rich Pyrenean oak forests

“…Caine and Marion, 1991;Bock and Fleck, 1995;Pöysä and Pöysä, 2002;Lõhmus and Remm, 2005;Loman, 2006;Sánchez et al, 2007;Lindenmayer et al, 2009;Cockle et al, 2010;Miller, 2010;Aitken and Martin, 2012), whereas few studies have manipulated the abundance of natural cavities, mostly by reduction of cavity numbers through cavity-blocking (Brush, 1983;Waters et al, 1990;Aitken and Martin, 2008;Blanc and Walters, 2008;Strubbe and Matthysen, 2009). Only Brush (1983) has previously performed simultaneous cavity-blocking and nest-box addition experiments, but only in one replicate per treatment.…”

Patterns of co-occurrence in woodpeckers and nocturnal cavity-nesting owls within an Idaho forest

“…However, even when the number of available cavities is high, the proportion of high quality cavities may limit occupancy (Cockle et al 2008). Although cavities can indeed be an important limiting factor for forest dwelling birds (Cockle et al 2008, Aitken andMartin 2012), sometimes there is also no clear relationship between the number of cavities and the density of breeding birds (Bonar 2000, Wiebe 2011). …”

Interspecific and intraspecific spatial separation by birds breeding in nest boxes

“…Generally, it can be assumed that most sites provide nest boxes in excess, which when combined with possible natural cavities, means that nest site availability is not limiting. A study in Canada showed that provision of nest boxes in a mature woodland habitat that tripled the availability of potential nest sites and produced a ninefold increase in density of Mountain Chickadee (Poecile gambeli) nests (Aitken and Martin 2012). Control sites showed no change over the same period and once the boxes were removed from test sites then density returned to previous levels.…”