Diet and Prey Selection of Barn Swallows ( Hirundo rustica ) at Vancouver International Airport AuDrey A. L Aw 1, 6 , MIrAnDA e. T hreLfALL 1, 2 , BrenDon A. T IjMAn 1, 3 , erIc M. A nDerSon 1, SeAn MccAnn 4, GAry SeArInG 5, and DAVID BrADBeer 5 1British columbia Institute of Technology, 3700 willingdon Avenue, Burnaby, British columbia V5G 3h2 canada 2current address: 5651 chester Street, Vancouver, British columbia V5w 3B3 canada 3current address: 8851 Ash Street, richmond, B ritish columbia V6y 3B4 canada 4Simon fraser university, 8888 university Drive, Burnaby, British columbia V5A 1S6 canada 5Vancouver International Airport, 3211 Grant Mcconachie way, richmond, British columbia V7B 0A4 canada 6corresponding author: [email protected] Law, Audrey A., Miranda e. Threlfall, Brendon A. Tijman, eric M. Anderson, Sean Mccann, Gary Searing, and David Bradbeer. 2017. Diet and prey selection of Barn Swallows ( Hirundo rustica ) at Vancouver International Airport. canadian field- naturalist 131(1): 26 –31. https://doi.org/10.22621/cfn. v131i1. 1777 The Barn Swallow ( Hirundo rustica ) is the most widely distributed aerial insectivore in north America, but has declined appreciably in recent decades. reasons for these declines are largely unknown, though presumably relate mainly to changes in prey availability. To help inform conservation priorities for this species, we assessed their diet and prey selection using birds lethally struck by aircraft at Vancouver International Airport (yVr). esophagi and gizzards of 31 Barn Swallows collected from june 2013 to october 2013 contained insects mainly from the orders hymenoptera (mean across birds = 40% of insect numbers), Diptera (31%), hemiptera (15%), and coleoptera (12%). To assess prey selection, we compared the esophagi and gizzard contents of 20 swallows collected from july 2013 to September 2013 to populations of aerial insects we sampled during the same period using Malaise traps. Barn Swallows selected strongly for insects in the order hymenoptera (mainly formicidae, which comprised 29% of diet), and selected against insects in the orders coleoptera, Diptera, and Lepidoptera. for all prey taxa combined, Barn Swallows displayed strong selection for insects of length 4−8 mm (body length excluding appendages). conversely, they selected against smaller and larger insects, despite the fact that smaller insects comprised about 80% of all insects sampled in Malaise traps. combined with past studies, our results suggest that Barn Swallows select among available aerial insects within local feeding sites for taxa that (i) are of intermediate size, (ii) occur at relatively high density, and (iii) have poor flight performance. Key words: Barn Swallow; Hirundo rustica ; aerial insectivore; diet; prey selection; Vancouver International Airport; British columbia Introduction Very few detailed diet studies have been completed Populations of aerial insectivores in canada have for Barn Swallows, particularly for the north American declined by 70% since 1970, constituting the most pro - subspecies ( H. r. erythrogaster ) that differs in behaviour nounced decline among all categories of canadian birds and morphology from the eurasian subspecies ( H. r. (north American Bird conservation Initiative canada rustica ; Brown and Brown 1999). Limited results for 2012). The Barn Swallow ( Hirundo rustica ) is the most both subspecies suggest that prey size is an important widely distributed aerial insectivore in north America factor affecting prey selection; although small prey and has declined appreciably throughout canada, with items occurred in the diet, Barn Swallows fed mainly Breeding Bird Survey data for 1970‒2011 showing a on relatively large prey (≥ 1.5 mg dry mass; Turner mean annual trend of −4.01% (environment canada 1982; holroyd 1983). Adult Barn Swallows also tend 2013). Barn Swallows were assessed as Threatened in to feed larger prey to their young than they eat them - 2011 by the committee on the Status of endangered selves (waugh 1978). A sample of 467 Barn Swallows wildlife in canada (coSewIc), but have yet to be list - collected throughout the united States and canada con - ed under the Species at Risk Act (coSewIc 2011; tained mainly Diptera (39.5%), hymenoptera (22.7%), British columbia conservation Data centre 2013; Gov - coleoptera (15.6%), and hemiptera (15.1%; method ernment of canada 2017). The causes of Barn Swallow of analysis not specified; Beal 1918). declines are unknown, but likely include a combination The purpose of this study was to assess the diet and of the following threats on breeding grounds: loss of prey selection of an at-risk aerial insectivore, the Barn nesting and foraging sites due to agricultural changes, Swallow. To study this sensitive species, we took ad - decreased abundance of aerial insects, climate change, vantage of birds lethally struck by aircraft at Vancouver competition for nest sites with house Sparrows ( Pass - International Airport (yVr) on Sea Island in rich - er domesticus ), ectoparasites, and nest removal by hu - mond, British columbia, canada (49.2°n, 123.2°w). mans (coSewIc 2011). Barn Swallows are a concern at yVr because since 26 ©The Ottawa Field-Naturalists’ Club (2017) 2017 LAw ET AL .: B Arn SwALLow DIeT AnD Prey SeLecTIon 27 2000 they have accounted for about 35% of identified that using rotary, glass-barrier, or sticky traps (juillet birds struck by aircraft annually (D.B., un published 1963; Pickering 1994). Malaise traps were distributed data). To assess prey selection, we compared the gut evenly throughout the airfield in areas that would not contents of Barn Swallows to populations of aerial interfere with airport operations. Malaise traps were insects we sampled throughout yVr airfields. These left open from 4 july 2013 to 12 September 2013, and results may help re duce the frequency of Barn Swal - samples were gathered weekly from each trap ( n = 50 low strikes at yVr if the abundance of key prey items total samples). Samples were stored in jars of 95% can be managed. ethanol at room temperature until they were processed. Insect identification Methods Insect specimens from the Malaise traps and from Barn Swallow carcass collection and dissection Barn Swallow esophagi and gizzards were identified we obtained Barn Swallows ( n = 31) from the yVr using dissecting microscopes (8‒32×) and taxonomic airfield after they had been lethally struck by aircraft keys (Triplehorn and johnson 2005). Specimens in between 2 june 2013 and 1 october 2013 (specific Barn Swallow guts were identified to family, and then collection dates were unknown for two swallows further sorted into three size classes: small (0‒4 mm obtained during this period). Swallows were not ran - body length excluding appendages), medium (4‒8 mm), domly sampled and therefore are not necessarily rep - and large (>8 mm). In two cases we sorted insects only resentative of all Barn Swallows at yVr. carcasses to the level of infraorder (Pentatomomorpha, culico - were collected from the runway < 10 min after they morpha) and in one case to the level of superfamily were struck, and then stored at −20˚c until they were (Muscoidea) because many individuals in these groups dissected. for dissections, birds were opened ventrally could not reliably be identified to family. Barn Swal - and the gastrointestinal tract was removed. An incision low consumption of many insect families observed in was made down the entire length of the esophagus and Malaise traps was too low to reliably assess swallow the gizzard and all their contents were flushed and pre - prey selection at the level of family. Thus, prey selec - served in 95% ethanol until they were processed. The tion and identification of Malaise trap specimens were contents of intestines were discarded because diges - conducted only at the level of order. All non-aerial in - tion rendered them unidentifiable. sects, such as spiders, were excluded from Malaise trap Availability of insect prey samples because Barn Swallows are aerial insectivores. we collected samples of insect relative abundance only one spider was found in the 31 Barn Swallow on the yVr airfield from five Malaise traps (156 cm esophagi and gizzards. width × 170 cm height; figure 1). Malaise traps pro - Analysis of Barn Swallow prey selection we calculated selection ratios separately for insect orders and size classes. however, to avoid biased esti - mates we did not calculate selection ratios for several orders that were rare (i.e., mean values of < 1%) in either Barn Swallow esophagi and gizzards or Malaise traps. Selection ratios were calculated by dividing the mean percentage of insects that Barn Swallows had consumed by the mean percentage sampled in the Malaise traps (Manly et al. 2002). Selection ratios > 1 indicate selection for a prey category, while selection ratios < 1 indicate selection against a prey category. Separately for selection ratios based on insect orders and size classes, we also calculated 95% simultaneous confidence intervals using the Bonferroni inequality as follows: Sr ± [z α/(2 I) × Se(Sr)], fIGure 1. Locations of sampling sites for the insect prey of Barn Swallows ( Hirundo rustica ) at Vancouver Inter - where Sr is a selection ratio, Se is its standard error, national Airport (yVr). At each of the five sites, zα is the value that is exceeded with probability α = 0.05 Malaise traps were left open from 4 july 2013 to 12 by a standard normal random variable, and I is the num - September 2013 during which weekly samples were ber of selection ratios considered (Manly et al. 2002). obtained of insect prey availability ( n = 50 total sam - collection dates for nine of the 31 Barn Swallows oc - ples). Photo: esri, redlands, california, uSA, 2009. curred either 32 days before or 6–19 days after the peri - od of Malaise trapping, and exact collection dates for vide a reliable method of capturing a diverse taxonom - two Barn Swallows were unknown. Thus, for prey ic range of flying insects. An exception is coleoptera selection analysis, we used only the 20 Barn Swallows that are only captured in Malaise traps at a rate 25% of that were gathered during Malaise trap sampling; col - 28 The cAnADIAn fIeLD -n ATurALIST Vol.