Incorporation of anthropogenic debris into double-crested cormorant nests, Toronto, Ontario

Abstract

Prior research suggested using double-crested cormorants (Phalacrocorax auritus) as an indicator species to track anthropogenic debris in inland aquatic environments such as the Great Lakes. In this study, we sampled cormorant nests on the ground (n = 50) for the presence and type of debris at the Leslie Street Spit, a human built peninsula in Toronto, Ontario. We also compared the proportion of debris type to the Great Canadian Shoreline Cleanup data (2011–2018) for this site. All of the nests sampled had anthropogenic debris present, including plastic items, metal bars and clothing (average ± SD: 28.7 ± 22.0 g debris per nest). Out of four broad categories of debris type (plastic, building materials, miscellaneous and mixed materials), plastic had the highest proportion for nests (53.6%) and shoreline cleanup (63.9%), though plastic type differed. Nests (32.4%) had proportionally more building materials specific to the site, such as electrical wires compared to the shoreline cleanup data (4.7%). Because the Leslie Street Spit is a human-constructed site and cormorants accessed local construction material for their nests, the site should not be used to track changes in debris over time. We recommend that other ground-nesting colonies be used to: monitor changes in debris type and quantity; determine the proportion of nest material from terrestrial and aquatic environments; and estimate the distance cormorants will travel to gather nest material.

Introduction

Anthropogenic debris pollution is ubiquitous in the natural environment (Lippiatt et al., 2013, Rochman et al., 2016), and much of the debris in marine and freshwater environments is plastic (Barnes et al., 2009). The detrimental effects of plastic pollution to wildlife is widely documented (Gall and Thompson, 2015, Gregory, 2009), yet the majority of studies are on marine birds (Provencher et al., 2015). Corcoran et al. (2015) noted the only data available on the type and quantity of plastic pollution in the Great Lakes (Lake Ontario) were produced by volunteer-led initiatives such as the Great Canadian Shoreline Cleanup and the Adopt-a-Beach Program (a U.S.-based program from the Alliance for the Great Lakes). But, evidence is mounting that suggests that freshwater birds may also be exposed to plastic pollution, particularly near major cities and industrial sites (Zbyszewski et al., 2014). For example, Holland et al. (2016) reported ingestion rates in freshwater waterfowl were similar to historical trends in marine birds and called for a monitoring program (see also Provencher et al., 2015).

Indicator species can be used to determine the impacts of anthropogenic debris pollution in aquatic environments because they are assumed to reflect exposure or response to a stressor thus providing useful information of the health condition of the ecosystem they inhabit (Canterbury et al., 2000, Kushlan, 1993, Siddig et al., 2016, Durant et al., 2009). Colonial waterbirds are considered effective indicator species because of ease of data gathering, they gather in a few locations in large numbers, and can be used to detect trends on coexisting species across food webs (Piatt et al., 2007; see also Hebert et al., 2011).

In the Great Lakes, colonial-nesting waterbirds have been used for decades to track contaminants (Bishop et al., 2016, Yamashita et al., 1993). Double-crested cormorants (Phalacrocorax auritus, hereinafter cormorants) are widely distributed across North America nesting adjacent to both freshwater and marine environments (Dorr et al., 2014). Cormorants forage in both benthic and pelagic zones (King et al., 2017), and their diet consists almost entirely of fish which makes them prone to exposure to bio-accumulative substances (e.g., Bishop et al., 2016) and to microplastics (Brookson et al., 2019). Because colonies exist throughout the Great Lakes (see Chip Weseloh et al., 2002), Provencher et al. (2015) recommended using cormorant nests as part of an inland plastics monitoring program.

One of the most common interactions between birds and anthropogenic macro debris (>2.5 cm; Lippiatt et al., 2013) is the use of these items in nest building. Incorporation of synthetic materials in birds’ nests can cause entanglement and ingestion of debris (Lavers et al., 2013, Votier et al., 2011) and negatively impact nesting and fledging success (Wang et al., 2009). In a review on wildlife entanglement, Ryan (2018) suggested that incidence of plastic in nests correlated with its local availability. Yet birds may choose anthropogenic over natural debris even when natural nesting material is available because items may: resemble natural nest material or prey color (Lavers et al., 2013, Tavares et al., 2016); be used to enhance mate attraction (Verlis et al., 2014); or be based on sexually selected colour preferences. In marine environments, debris incidence in nests appears to be primarily related to fishing efforts (Bond et al., 2012, O'Hanlon et al., 2019). While the study of anthropogenic debris in bird nests is relatively new (Battisti et al., 2019), documentation exists primarily for marine birds (black-legged kittiwakes Rissa tridactyla, Hartwig et al., 2007; Australasian gannets Morus serrator, Norman et al., 1995; northern gannets Morus bassanus, Montevecchi, 1991, Votier et al., 2011, Bond et al., 2012; kelp gulls Larus dominicanus, Witteveen et al., 2017; brown bobbies Sula leucogaster, Verlis et al., 2014, Tavares et al., 2016; and double-crested cormorants, Gulf of Maine; Podolsky and Kress, 1989) and research is lacking for freshwater systems (Holland et al., 2016, Jagiello et al., 2018).

In this study, we quantified the frequency and type of debris in cormorant nests at a colony in Lake Ontario. We compared debris proportion and type in cormorant nests to debris recorded in the Great Canadian Shoreline Cleanup (GCSC; shorelinecleanup.ca). GCSC is a citizen science program used to monitor the abundance and distribution of anthropogenic debris (see Driedger et al., 2015) and existing surveys at the study site permitted a comparison. Finally, following Provencher et al.’s (2015) recommendation, we consider if cormorant nests should be used as an indicator to monitor anthropogenic debris pollution.