Traits that converge in appearance under similar environmental conditions among phylogenetically independent lineages are thought to represent adaptations to local environments. We tested for convergence in nest morphology and composition of birds breeding in two ecologically different locations in Canada: Churchill in northern Manitoba and Elgin in southeastern Ontario. We examined nests from four families of passerine birds (Turdidae: Turdus, Parulidae: Dendroica, Emberizidae: Passerculus and Fringillidae: Carduelis) where closely related populations or species breed in both locations. Nests of American Robins, Yellow Warblers, and Carduelis finches had heavier nest masses, and tended to have thicker nest-walls, in northern Manitoba compared with conspecifics or congenerics breeding in southeastern Ontario. Together, all species showed evidence for wider internal and external nest-cup diameters in northern Manitoba, while individual species showed varying patterns for internal nest-cup and external nest depths. American Robins, Yellow Warblers, and Carduelisfinches in northern Manitoba achieved heavier nest masses in different ways. American Robins increased all materials in similar proportions, and Yellow Warblers and Common Redpolls used greater amounts of select materials. While changes in nest composition vary uniquely for each species, the pattern of larger nests in northern Manitoba compared to southeastern Ontario in three of our four phylogenetically-independent comparisons suggests that birds are adapting to similar selective pressures between locations.
We found nests by observing females carrying nesting material or making repeated trips to a single site, and by flushing incubating females while walking through appropriate habitat. At both study sites, we found five nests of each species and all nests were collected after natural predation events or after fledging.
Once a nest either fledged young or failed, we carefully removed the nest from its substrate and immediately placed it into a bag to prevent the loss of nesting materials. We left bags open to allow nests to air dry in the same ambient lab conditions for 1–2 months and determined the dry mass of all nests using an electronic balance.
In addition to nest mass, we measured five aspects of nest morphology: 1) external nest diameter, 2) internal nest-cup diameter, 3) external nest depth, 4) internal nest-cup depth, and 5) nest-wall thickness. Measures of the external and internal nest-cup diameter are the average of the maximum and minimum diameters of the outer and inner nest-cup, respectively. External nest depth is the distance from the top rim of the nest walls to the bottom of the nest's exterior. Internal nest-cup depth is the distance from the top rim of the nest walls to the base of the interior nest cup (where eggs are placed). Measures of nest-wall thickness represent the average of eight evenly spaced measurements of the nest wall to help account for variation in nest-wall size and shape. All measures of nest morphology that we used in our analyses were made by CAC to control for inter-observer variation, and all measures were made on nests prior to hatching or the presence of large nestlings because nestlings can distort the shape of the nest . All nest dimensions were measured to the nearest 0.5 mm with a manual caliper and all nest measures for each species are summarized in Table 1. To account for possible changes in nest morphology throughout the breeding season, we only used nests constructed early in the breeding season at each study site (southeastern Ontario: all nests collected 4–28 May 2009, except American Goldfinch nests collected 26–30 July 2009, northern Manitoba: all nest collected 17 June–25 July 2009). While breeding in northern Manitoba can start in late May and early June, the spring of 2009 was one of the coldest on record, delaying breeding by nearly a month .