Tree decay and vegetation cover: a possible role for temperature
We found a higher than expected number of GFWO nests within live trees. Previous literature on woodpecker nesting ecology has indicated a preference for excavating cavities in partially to fully decayed trees, which require less energy and time than dense, live wood (Conner, Miller, & Adkisson, 1976; Cockle et al., 2011; Blanc & Martin, 2012). However, these studies have focused on temperate regions such as northwestern, northeastern United States, Canada, and European countries where breeding season temperature rarely exceeds 35° C and occasionally reach freezing during the early spring (Conner et al., 1976;Blanc & Martin, 2012;Seavy, Burnett, & Taille, 2012). In contrast, the mean breeding season temperature at our study site in southern Texas was 27.8° C and daytime temperatures frequently reached over 42.2° C Currently, there is little information on how cavity nesting birds regulate nest temperature, though some species may modulate incubation initiation and duration in relation to temperature (Coe, Beck, Chin, Jachowski, & Hopkins, 2015; Simmonds, Sheldon, Coulson, & Cole, 2017) and there are reports of GFWO clinging to the sides of the cavity which could be an attempt to reduce heat transfer (Skutch, 1969). Nest temperature is also affected by nest site location and cavity design (although not always) (Butler, Whitman & Dufty, 2009; Zingg, Arlettaz & Schaub, 2010; Sonnenberg, Branch, Benedict, Pitera & Pravosudov, 2020).
Tree decay, in particular, affects thermoregulation of the nest cavity, in that live trees -with higher water content- provide greater insulation against high and low temperature extremes (Grüebler, Widmer, Korner-Nievergelt & Naef- Daenzer, 2014). However, the same trait that makes live trees good insulators also makes them more costly to excavate; on average, live trees are denser than partially dead or decaying trees. Therefore, these birds may be facing an energetic trade-off; whether to put additional effort into excavating a dense live tree- which has higher water content and is better able to thermoregulate eggs and nestlings- or save time and energy by excavating a less stable decayed tree and risk eggs and nestlings overheating.
This possible role for temperature in nest site selection and structure is further strengthened by the trend we observed in vegetation cover, with cavity nesters like the GFWO and the BEWR having higher success in cavities with increased vegetation cover. While the effect size for vegetation (β ranged from 0.02 to 0.05) seems small at first, across the large range of possibilities for cover (1-100) this variable showed a strong effect. For example, with a 15 percent increase in vegetation cover, the effect size for the BEWR grew to 0.30 and the same increase in vegetation cover for the GFWO resulted in an effect size of 0.75, rivaling that of stronger predictors such as decay and cavity cover. Again, these results contrast with previous literature on cavity nesters which indicated a preference for exposed cavities due to increased visibility of approaching predators (Mannan et al., 1980; Li & Martin, 1991; Loye & Carroll, 1998; Newlon, 2005; Jusino et al., 2016). Vegetated cavities in this region may provide increased shade and thus reduced internal temperatures, resulting in another tradeoff, one between temperature regulation and predation.