1. INTRODUCTION
Ecosystem engineers alter the availability of resources for other species by causing physical state changes in biotic or abiotic materials (Jones, Lawton & Shachak, 1994; Wright, Jones, & Flecker, 2002; Buse et al., 2008). Given the important role they play in local environments, the literature surrounding ecosystem engineers is historically focused on how their actions affect other species (Jones et al., 1994;Robles & Martin, 2013;Tarbill, Manley, & White, 2015;Wiebe, 2017), but little research has been published on what external factors influence the engineers themselves (seeMikusinski, 2006;Jusino, Lindner, Banik, & Walters, 2015). Importantly, little has been done to investigate how ecosystem engineers choose breeding and young rearing grounds (Nilsson, Johnsson, & Tjernberg, 1991; Garmendia, Cárcamo, & Schwendtner, 2006). Understanding these driving factors is essential to understanding the ecology of not only the ecosystem engineers themselves, but the organisms that rely on them for their own breeding and nesting grounds as well.
The modifications made by ecosystem engineers have far-reaching consequences and directly impact not only ecological associations, but also the behavior of animals within an ecosystem. For example, animal movement and community composition may be altered by the actions of local ecosystem engineers (Lill & Marquis, 2003; Bangert & Slobodchikoff, 2004). In this way, ecosystem engineers can indirectly influence local trophic levels through multi-level environmental modifications, such as altering local invertebrate diversity and abundance, which in turn may increase foraging opportunities for other vertebrates (Lill & Marquis, 2003; Bangert & Slobodchikoff, 2004), or by providing more suitable species specific habitat for nesting (Showalter & Whitmore, 2002)
Although insects themselves can act as ecosystem engineers (Bell & Whitmore, 1997; Lill & Marquis, 2003; Bangert & Slobodchikoff, 2004), they can also act as crucial resources for other ecosystem engineers at higher trophic levels (Hess & James, 1998;Pechacek & Kristin, 2004). For example, declines in insect richness and abundance have been reported with parallel declines in a number of insectivorous ecosystem engineers, such as woodpeckers (Lister & Garcia, 2018,Møller, 2019,Karr, 1976;Benton, Bryant, Cole, & Crick, 2002;Rioux Paquette, Pelletier, Garant & Bélisle, 2014;Narango, Tallamy, & Marra, 2017;Bowler, Heldbjerg, Fox, Jong, & Böhning‐Gaese, 2019). Therefore, ecosystem engineering activities may be better understood by looking at the distribution and abundance of their food resources.
Woodpeckers are avian ecosystem engineers that have a large proportion of insects in their diet (Jones et al., 1994;Tarbill et al., 2015), and control the location, construction, and availability of nesting cavities, a limiting resource for secondary cavity nesting birds (SCB; i.e. species that require a cavity to nest in but cannot create the cavity themselves). Woodpeckers are primary excavators of nesting cavities, often creating multiple cavities within their home range each year to avoid predation, external parasite buildup, and cavity wood degradation (Loye & Carroll 1998; Husak & Husak, 2002; Wiebe, 2017). Once abandoned, these cavities are used by a variety of secondary cavity nesting species (Martin & Eadie, 1999,Pakkala, Tiainen, Piha, & Kouki, 2019). Woodpeckers select nesting sites based on characteristics that protect their eggs and nestlings from predation, tending to nest high in moderately to heavily decayed trees with wide diameters at breast height (DBH), and with limited vegetation covering the cavity entrance (vegetation cover, Mannan, Meslow, & Wight, 1980; Li & Martin, 1991; Loye & Carroll, 1998; Newlon, 2005; Jusino et al., 2016). Additionally, the shape of woodpecker cavities functions to exclude nest predators by having small entrance holes and deep depths (Sedgwick & Knopf, 1990; Li and Martin, 1991; Martin, Aitken, & Wiebe, 2004; Rhodes, O’donnell, & Jamieson, 2009). Given the nest construction preferences of woodpeckers, the cavities they leave behind are often superior nesting spaces when compared to naturally occurring cavities, both of which are used by SCB (Martin & Li, 1992; Maziarz, Broughton, & Wesolowski, 2017).
Woodpecker resources can be defined both in terms of food (mainly wood burrowing insects, largely in the order Coleoptera) and in the number of trees suitable for excavation (Bonnot, Millspaugh, & Rumble, 2009 ;Rota, Rumble, Lehman, Kesler, & Millspaugh, 2015). These resources have been shown to be directly linked to woodpecker nest site location and home range sizes (e.g. the area used by a bird in its daily movements) (Worton, 1989; Powell, 2000;Wiktander, Olsson, & Nilsson, 2001;Pasinelli, 2007). For example, the Black-backed woodpecker (Picoides arcticus ) selects nesting sites based on infestations of the mountain pine beetles (Dendroctonus ponderosae ) (Rota et al., 2015), and the Three-toed woodpecker’s (Picoides dorsalis ) home range size is negatively correlated with the number of trees with suitable DBH for cavity excavation (Pechacek & d’Oleire-Oltmanns, 2004). However, no studies to date have looked at the impact of food resources on both the nest site location and home range sizes of woodpeckers, which in turn directly impacts neighboring SCB.
The Golden-fronted woodpecker (GFWO, Melanerpes aurifrons ), is a poorly studied, medium sized bird, whose range extends from Central America to Texas (Wetmore, 1948; Sauer, Link, Failon, Pardieck, & Ziolkowski, 2013; Schroeder, Boal, & Glasscock, 2013). GFWO numbers are in decline across their Texas distribution, and are considered a species of concern in the Texas Wildlife Action Plan (Bender, 2007). As with other woodpecker species, GFWO act as ecosystem engineers, providing nesting cavities for SCB throughout their range (Husak & Maxwell, 1998). Determining the factors that influence the nest site location and construction of cavities is crucial to not only understand the conservation needs of GFWO, but also for the conservation and basic ecology of SCB that may rely on the cavities created by GFWO.
To investigate relationships between the GFWO and local SCB nesting successes, we conducted an observational study on GFWO nesting success (≥ 1 fledgling) in relation to nesting site locations, home range sizes, local insect biomass, and cavity construction, along with the nesting success of the four most common SCB in our study area, the Black-crested Titmouse (BCTI; Baeolophus atricristatus ), Ash-throated Flycatcher (ATFL; Myiarchus cinerascens ), Brown-crested Flycatcher (BCFL; Myiarchus tyrannulus ), and Bewick’s Wren (BEWR;Thryomanes bewickii ) in the southern Texas Tamaulipan Brushlands (Baumgardt, Morrison, Brennan, Pierce, & Campbell, 2019).
The objectives of our study were to determine 1) the role of insect availability in nest site location and home range size of GFWO, 2) the role of nest metrics (e.g. DBH, vegetation cover) in the nesting success of GFWO and the four species of SCB, and 3) if SCB cavity selection and nesting success differed between abandoned woodpecker cavities and natural cavities. We predicted 1) insect abundance would be greater at GFWO occupied sites versus GFWO unoccupied sites and that home range size would be negatively correlated with the availability of insect orders commonly eaten by birds, 2) the same cavity metrics would influence nest success in both GFWO and SCB species and 3) that SCB would tend to nest in, and have higher nest success in abandoned woodpecker cavities compared to natural cavities, and that abandoned woodpecker cavities would share characteristics making them more suitable for nesting birds, compared to natural cavities.