DOI: 10.5061/dryad.2bvq83brd
title: Nonideal nest box selection by tree swallows breeding in farmlands: evidence for an ecological trap?

description: Animals are expected to select a breeding habitat using cues that should
 reflect, directly or not, the fitness outcome of the different habitat
 options. However, human-induced environmental changes can alter the
 relationships between habitat characteristics and their fitness
 consequences, leading to maladaptive habitat choices. The most severe case
 of such nonideal habitat selection is the ecological trap, which occurs
 when individuals prefer to settle in poor-quality habitats while better
 ones are available. Here we studied the adaptiveness of nest box selection
 in a tree swallow (Tachycineta bicolor) population breeding over a 10-year
 period in a network of 400 nest boxes distributed along a gradient of
 agricultural intensification in southern Québec, Canada. We first
 examined the effects of multiple environmental and social habitat
 characteristics on nest box preference to identify potential settlement
 cues. We then assessed the links between those cues and habitat quality as
 defined by the reproductive performance of individuals that settled early
 or late in nest boxes. We found that tree swallows preferred nesting in
 open habitats with high cover of perennial forage crops, high spring
 insect biomass, and high density of house sparrows (Passer domesticus),
 their main competitors for nest sites. They also preferred nesting where
 the density of breeders and their mean number of fledglings during the
 previous year were high. However, we detected mismatches between
 preference and habitat quality for several environmental variables. The
 density of competitors and conspecific social information showed severe
 mismatches, as their relationships to preference and breeding success went
 in opposite direction under certain circumstances. Spring food
 availability and agricultural landscape context, while related to
 preferences, were not related to breeding success. Overall, our study
 emphasizes the complexity of habitat selection behavior and provides
 evidence that multiple mechanisms may potentially lead to an ecological
 trap in farmlands. 
description: Study area The study area
 included 40 farms distributed along a gradient of agricultural
 intensification covering approximately 10,200 km<sup>2</sup>
 in southern Québec, Canada. Three land cover types dominated the study
 system: low-intensity agricultural fields (i.e. hay, alfalfa
 (<i>Medicago sativa</i>), clover (<i>Trifolium</i>
 spp.), and pastures, henceforth referred to as “forage crops”), intensive
 agricultural fields (i.e., annual row crops mainly composed of corn
 (<i>Zea mays</i>), soybean (<i>Glycine max</i>),
 and wheat (<i>Triticum</i> spp.)) and forest.
 Each farm included 10 identical nest boxes mostly arranged in a
 row along field margins and separated by at least 50 meters to limit intra
 and interspecific competition (see Ghilain and Bélisle 2008 for further
 details on the study system). A Thermochron iButton device was fixed on
 the outside of one nest box on each farm to record hourly ambient
 temperature (model DS1922L; Embedded Data Systems, Lawrenceburg, Kentucky,
 USA). A pluviometer collected precipitation data (millimeters of rainfall)
 on each farm. We used the mean daily temperature and mean daily rainfall
 between May 1 and May 15 to characterize spring climate. 
   Nest
 monitoring We monitored nest boxes
 every other day from 2009 to 2018 starting in the first week of May. We
 recorded the occurrence of nest materials, the laying date (first egg),
 and the number of eggs, hatchlings and fledglings. Monitoring ended when
 all nestlings had fledged on a given farm, which occurred between June 15
 and August 5 during the study. Nest boxes were cleared of any nest
 material and/or dead nestlings every year in October.
   Preference
 Preference for each nest box was estimated for each year
 according to the occurrence of a laying event (at least one egg laid) and
 settlement date (Julian date at which nesting material was first
 observed).   Because some early
 settlement dates were left-censored given that some boxes already
 contained nest material at the first visit (45% of all boxes), settlement
 dates were classified as either “early” or “late” with respect to the
 annual median settlement date. The category “early” included boxes with
 settlement dates preceding or equal to the annual median, which comprised
 nearly all (91.3%) left-censored dates. Overall, the average difference
 between the annual mean settlement dates categorized as “early” and “late”
 was 10.4 ± 2.9 days (mean ± SD). Nest boxes occupied by other species were
 excluded from analyses (<i>N</i> = 964 boxes between 2009 and
 2018). Such exclusions were made possible, even in the absence of a laying
 event, because the material and shape of nests are very species specific.
 We are thus confident that the vast majority of nests included in the
 study were initiated by tree swallows. The ordinal preference variable
 featured three categories: 1: No laying event
 2: Laying event and late settlement 3: Laying
 event and early settlement  
 Habitat quality We used two
 proxies of reproductive success: (1) the number of hatchlings produced in
 a nest box and (2) the proportion of hatchlings that successfully fledged
 in a nestbox (i.e., fledging success).  
 Landscape
 context We characterized landscape
 habitat composition by measuring the relative cover of forest, perennial
 forage crops, as well as of water bodies and wetlands, within radii of
 respectively 100 m, 5 km and 10 km. We assessed landscape habitat
 composition up to the 500-m scale on a yearly basis in the field by
 visually identifying cultures and marginal habitats and delineating them
 using orthophotos (1:40,000). Characterization beyond the 500-m scale was
 based on a mosaic of yearly georeferenced classified optical and radar
 satellite images taken between 2011 and 2018 (pixel resolution 30 m
 × 30 m;
 Agriculture and Agri-Food Canada (AAFC) 2018). Only the year 2018 was used
 to assess water cover at the above range of scales because it showed
 better accuracy than the data of previous years (AAFC, 2020), and because
 the cover of large water bodies, as those covered by the data we used, did
 not vary significantly across years (e.g., median between-year correlation
 of yearly water cover between 2011 and 2018 was 0.90 at the 10-km
 scale).   Food
 availability Two passive insect
 traps were installed on each farm around the first and second third of the
 nest box transect. Traps consisted of ~4-L yellow buckets placed 1.5 m
 above ground. They were filled with ~2 L of salty detergent solution to
 reduce surface tension and slow the growth of bacteria and fungi. Two
 transparent plexiglass screens were mounted perpendicularly to one another
 above each bucket to intercept flying insects (see Bellavance et al. 2018
 and Garrett et al. 2021a for details). We collected the content of each
 trap on every visit to a farm (i.e., every other day) and conserved
 arthropods in 70% ethanol until processing. We sorted samples by removing
 arthropods unlikely to be preyed upon by tree swallows (i.e., bumble bees
 (<i>Bombus</i> spp.: Hymenoptera), June bugs
 (<i>Phyllophaga</i> spp.: Coleoptera), large spiders (Araneae,
 &gt; 0.5 cm body width), and caterpillars (Lepidoptera); Bellavance et
 al. 2018). The rest of the sample was dried at 50ºC for at least 48 hours
 before being weighed (Adam Equipment, model AAA250L, ± 0.0001 g). The mean
 daily dry biomass of arthropods collected between May 1 and May 15 was
 used as a proxy of yearly food availability on a given farm at the time of
 nest site selection.  
 Heterospecific social information
 House sparrows (<i>Passer domesticus</i>) are tree
 swallow’s main nest-site competitors in our system and they initiate
 breeding before swallows return from their wintering grounds (Robillard,
 Garant and Bélisle, 2013). We evaluated the use of heterospecific social
 information through the number of nest boxes occupied by house sparrows on
 each farm in the current year. Nest boxes and house sparrow nests were
 visited every other day concurrently to tree swallow monitoring. Occupancy
 was determined by the presence of at least one egg, and only first
 clutches observed in each box were included since a nest box is rarely
 used by another species once house sparrows have built a nest
 therein. 
 Conspecific social information
 We defined two sources of social information regarding the future
 breeding success that an individual could expect to experience on a given
 farm: the density of tree swallows that bred on a farm during the previous
 year and the mean number of fledglings obtained by those
 breeders.