Characterization of Plastics Ingested by the Bioindicator Cory’s Shearwater from Tenerife Island (Canary Islands): Implications for Marine Environmental Monitoring

Abstract

Plastic pollution is a pressing environmental concern globally, especially in marine ecosystems. In this study, the evaluation of the potential ingestion of plastic, mostly in the form of microplastics (MPs), by fledglings of Cory’s shearwaters (Calonectris borealis) from the Canary Islands (Spain) was conducted. The total number of plastics found in the stomach samples was 674, primarily comprising large MPs (1–5 mm: 82%), followed by mesoplastics (>5–25 mm: 18%). The predominant morphology was threadlike (31.6%), followed by hard, irregularly shaped fragments (28.3%), microspheres (22.4%), and sheets (15.7%). Loads were found to overlap with those described for the same species in highly populated areas such as the Mediterranean Sea. Plastic counts above Cory’s threshold value may suggest poor environmental status for the Canary Current region. FTIR-ATR analysis evidenced the predominance of polyethylene (PE) (46.7%), polypropylene (PP) (24.6%) and polyamide (PA) (20.4%). This is likely linked not only to the fact that PE is the most produced plastic worldwide, but also the fact that, along with PP, it makes up the highest amount of single-use plastic products. Overall, findings provide a contamination-controlled, FTIR-verified baseline for fledglings from Tenerife; however, given the limited, single-season sample (n = 33) and opportunistic design, results are descriptive and not intended for population-level inference. Yet, the potential of Cory’s shearwater as a sentinel species to monitor plastic pollution is highlighted, emphasizing the urgent need for effective mitigation strategies to address plastic pollution in marine environments.

1. Introduction

Plastic production has witnessed a significant surge in recent times. During 2000–2022, the world produced at least 71% of all virgin fossil-fuel-based plastics ever made, with an annual output reaching over 500 million tons by 2022 [1]. Furthermore, the Organization for Economic Cooperation and Development (OECD) forecasts that global plastic production will triple by 2060 due to economic growth, a rising middle class, and population increase [2]. Most of this production consists of polyethylene (PE), comprising over 25%, followed by polypropylene (PP), and poly(vinyl chloride) (PVC) [3].

Despite their great utility, plastics pose considerable challenges due to their inherent properties such as strength, durability, and low production costs [4], which, combined with inadequate recycling methods, exacerbate plastic accumulation in landfills [5]. As landfills become saturated, plastics and their associated chemicals seep into water bodies, eventually reaching rivers, lakes, seas, and oceans [6,7,8]. Additionally, fishing activities and anthropogenic plastic waste along coastlines [9] coupled with ineffective management of marine debris, result in the transportation and deposition of plastic debris onto beaches, coastal zones, and oceanic environments. This facilitates easy access for marine organisms such as birds, fish, turtles, or whales, to name a few [9,10,11,12].

Moreover, it is well known that all plastic waste exposed to solar ultraviolet radiation undergoes photodegradation, leading to the breakdown of polymer chemical bonds through oxidation [13]. This phenomenon, when coupled with the action of wind and waves in marine or lacustrine environments, as well as abrasion, causes fragmentation, generating microplastics (MPs) and nanoplastics (NPs) [14,15]. Today, according to the European Food Safety Authority (EFSA) criterion, MPs can be defined as plastic particles in the 0.1 µm to 5 mm size range, whereas NPs are typically ~1–100 nm [16].

The shape, size and color are critical factors influencing the impact of plastics on marine biota [17]. Given their resistance to degradation, plastics can persist and accumulate in the marine ecosystem over extended periods of time. Consequently, they are readily assimilated into the marine food web, as birds and marine organisms ingest them, often mistaking them for prey. Their interaction with plastic can, sometimes, lead to entanglement and the ingestion of plastics and MPs, and potentially to the transfer and assimilation of pollutants [18,19].

Plastic ingestion is primarily observed in the digestive system, although the presence of NPs has also been detected in the liver or muscle tissue of fish and mammals [20,21], which can stem from fragments produced in the digestion of larger plastics, and which occurs in other animals such as birds. In fact, NPs, due to their smaller particle size, may have the capacity to permeate digestive membranes and migrate into the circulatory system or even other organs by crossing the highly selective blood–brain barrier [22].

In general, the potential adverse impacts that plastic, including MPs and NPs, can inflict on organisms encompass three distinct categories: physical (which may, for instance, induce suffocation or pseudo-satiation upon ingestion), chemical (arising from the presence of additives used during plastic manufacturing and/or contaminants adsorbed during environmental exposure), and biological (resulting from potential colonization by pathogenic microorganisms on the plastic surface) [5,14,23]. From a chemical perspective, it is important to note that plastic materials inherently contain a variety of chemicals, including unreacted monomers or oligomers, by-products, and, most notably, additives used in their production to confer specific properties. Moreover, plastics have a significant ability to sorb (adsorb or absorb) and accumulate hydrophobic organic chemicals present in surrounding waters partially derived from their high surface-to-volume ratio—particularly MP and NPs—and hydrophobic properties [24]. In turn, under specific conditions (pH, T, organic matter, etc.), plastics can release some of these chemical components, many of which are known to be toxic [8]. On this note, it is worth mentioning the potential role of plastics in different forms as vectors of different organic pollutants such as antibiotics, UV filters, polycyclic aromatic hydrocarbons (PAHs), brominated flame retardants (BFRs), polychlorinated biphenyls (PCBs), organochlorine pesticides (OCPs), or poly- and perfluorinated compounds (PFAS), among others [25,26].

Different research highlights the serious impact of plastic ingestion on seabirds and waterfowl, which can suffer various harmful effects from plastic pollution [10,27]. Plastic can disrupt their food chain by affecting prey populations and resulting in malnutrition due to a lack of food [28]. Moreover, both chicks and adults can be exposed to harmful substances as plastic particles contaminate nests [29]. Accidental ingestion of plastic while feeding is another significant issue faced by marine birds. Mistaking these particles for food, birds may ingest them, leading to various health problems caused by blockages, asphyxiation, or malnutrition since birds can develop a false sense of satiety [30]. Additionally, the presence of plastic in the guts of certain seabird species can disrupt their microbiota [31], and plastic-related fibrosis has been diagnosed in seabird intestines [32]. Fledglings are particularly vulnerable to plastic ingestion, as parental regurgitation during feeding can unintentionally transfer plastics to them, potentially causing long-term effects on their health and survival [31,33].

Conducting comprehensive studies to analyze and quantify the prevalence of plastics, including MPs and NPs within the gastrointestinal tracts of birds, is imperative. These studies are indispensable for quantifying the magnitude of the problem, monitoring its evolution over time, and assessing the generated environmental impact. Moreover, the identification of specific types of plastics, including the type of polymer, is key to identifying pollution sources, potentially facilitating the study and implementation of preventive future measures aimed at curtailing plastic ingress into ecosystems [9,33].

Cory’s shearwater (Calonectris borealis), as a surface feeder, is a suitable bioindicator species for monitoring the presence of floating plastic in the marine environment [34,35]. In that line, Rodríguez et al. (2024) [35] have proposed a Cory’s Threshold Value (Cory’s-TV), indicating good environmental status when no more than 20 percent of fledglings carry more than four plastic items, assessed over five-year windows with minimum annual sample sizes. Cory’s shearwater breeding population of the Canary Islands feeds in the so-called Canary Current (CC), a region of great ecological and economic significance, recognized as one of the most important fisheries in the world, and therefore holds substantial ecological and economic importance [36,37]. While color, size, and abundance of plastics and MPs in this species have been documented [35,38,39], studies addressing the chemical composition of ingested plastics in the Canary Islands—particularly in Tenerife Island—remain scarce. Working with the initial hypothesis that this species can indicate plastic pollution because of its chronic dietary exposure, here we present an integrated assessment of plastic ingestion in Cory’s shearwater fledglings from Tenerife, combining item counts, mass, morphology, color, size classes, anatomical compartment, and FTIR-ATR polymer identification. Expecting an increased degree of plastic input in the feeding grounds of this species, our primary objective is to quantify and chemically characterize the ingested plastics in fledglings from Tenerife and to derive indicator values comparable with current regional monitoring frameworks.

2. Materials and Methods

2.1. Study Species, Sample Collection and Processing

Cory’s shearwater is an abundant Procellariform seabird species breeding on the North Atlantic islands (mainly Azores, Madeira, Salvagems, and Canary Islands). Fledglings are attracted to artificial lights during their first flights from their nest to the ocean, causing mass mortality events [40]. To mitigate light-induced mortality, the local governments coordinate rescue campaigns. Here, we used birds gathered through rescue campaigns conducted on Tenerife, the largest and most populated of the Canary Islands, by the Wildlife Rehabilitation Center La Tahonilla (WRC; La Laguna, Spain). Thirty-three specimens were sampled after being found deceased or requiring euthanasia in 2022, thereby providing valuable samples for the examination of plastic ingestion during the breeding period. The use of birds stranded by artificial light offers, in principle, a homogeneous and unbiased sample consisting of specimens with similar bodily conditions, as recognized in previous studies [34,35,38].

2.2. Sample Preparation

Data on body and plastic mass were collected for the 33 specimens using a balance with an accuracy of ±0.0001 g. The stomach consists of two distinct parts: the proventriculus (PV) or glandular stomach, responsible for chemical digestion, and the ventriculus (V) or muscular stomach, commonly known as the gizzard, which is responsible for grinding the digestive contents. For treatment, the collected specimens were dissected with a clean scalpel to access the PV and the V, where plastic items accumulate, following the same procedure as in [34]. Plastics were then extracted separately from these two parts of the stomach for subsequent counting and identification. For this purpose, each PV and V were placed in a Petri dish and rinsed with ultrapure water to ensure the removal of any residual organic matter. Afterwards, PVs and Vswere digested in glass jars covered by aluminum foil to avoid airborne MP contamination. The reagent employed for digestion was a mixture of 50.0 mL of KOH:H₂O₂ solution (70:30, v/v) at room temperature for 48 h, or more if needed, to ensure all the organic material was digested. During the digestion process, jars were shaken by an orbital mixer and shaker Rotaterm (J.P. SELECTA, Barcelona, Spain). After digestion, samples were diluted with ultrapure water and filtered using a glass vacuum filtration apparatus with a vacuum control-press pump (J.P. SELECTA, Barcelona, Spain). They were filtered through a 1 µm size pore Whatman glass microfiber filter, rinsing jar walls with filtered distilled water to recover all the material stuck on the walls. The filters were placed in clean, sealed Petri dishes to dry and were subsequently examined for the identification and chemical characterization of the plastic items.

2.3. Visual Screening and Microplastics Quantification

The plastic particles deposited on glass microfiber filters from different parts of the stomach of seabirds were examined using a Motic SMZ-171 stereo microscope (MOTIC, Hong Kong, China) at magnifications ranging from 10× to 50×. To avoid contamination, the samples were strictly opened under the microscope. Several physical characteristics were considered to sort the plastics and MPs into different categories, i.e., color (off-white/clear, black, blue-purple, green, grey-silver, orange-brown, red-pink, and yellow) and shape (threadlike plastic, microspheres, hard fragments and foamed and sheet plastics) based on and adapting the criteria of [41]. In addition, plastic particles were counted and classified according to their size as large microplastics (1 to 5 mm) or mesoplastics (>5 up to 25 mm) [35]; note that, consistent with EFSA’s definition (0.1 µm–5 mm), our analyses targeted the upper end (>1 mm) given the method constraints. The results of plastic (P) content for each specimen were expressed as the percentage of plastic mass (g) per gram (body mass) (%P/BM). Additionally, the percentage of plastics associated with PV and V, in relation to the stomach, was also calculated. These variables were abbreviated as (%PPV/stomach and %PV/stomach).

2.4. Polymer Identification and Composition of Microplastics

The chemical characterization of extracted plastics (>1 mm) from the digestive systems of shearwaters was carried out by Fourier transform infrared spectroscopy in attenuated total reflectance mode (FTIR-ATR) using an FT/IR-4100 spectrometer equipped with an ATR ONE accessory (JASCO, Tokyo, Japan). Spectra analysis was performed within the range of 4000 to 500 cm⁻¹, with a data interval of 1 cm⁻¹, at a 4 cm⁻¹ resolution. After each sample, the diamond crystal interface was cleaned with methanol (prefiltered), and a background scan was performed. Spectra were identified using the Spectra Manager^® software Ver.2, based on spectral absorption bands and confirmed by comparison with the spectral library database using the equipment library. Only particles with matches above 90% were considered and accepted. To aid comparability, we report size binning and explicitly state our operational cut-off (>1 mm, FTIR-verified), noting that published Atlantic and Mediterranean studies often use different thresholds and workflows.

2.5. Quality Assurance/Quality Control in Microplastic Analyses

To ensure data quality and minimize cross-contamination, rigorous control measures were implemented at all stages of the analysis, from sample collection to processing and polymer identification. All laboratory instruments, work surfaces, and glassware were carefully cleaned before each use with ultrapure water and 96% ethanol. Work was conducted in a closed room where air circulation was minimized during critical stages such as chemical digestion, filtration, and particle counting. Several measures were taken during each stage of the analysis, including sample collection, preparation and analytical procedures to avoid contamination from external plastics or fibers. Samples were stored in glass Petri dishes that were wrapped with aluminum foil to prevent airborne contamination. The dishes were only opened at the precise moment of analysis. Furthermore, to prevent the introduction of external synthetic fibers, nitrile gloves and cotton laboratory coats were used. The air circulation in the room was minimized during sample preparation for the digestion stage, filtration, and counting. To assess and control potential sources of environmental contamination in the laboratory, blank control tests were conducted during each day of analysis. A total of 30 blank controls were performed, consisting of exposing clean filters in open Petri dishes to the laboratory environment for the same duration and under the same conditions as the experimental samples. Filters from the blank controls were analyzed following the same protocol as the samples, including stereoscopic microscopy and FTIR spectroscopy for suspected particles. None of the particles detected in the blanks matched synthetic polymers above the 90% correlation threshold used for reliable polymer identification. Therefore, laboratory environmental contamination was considered negligible, and the study results were not compromised by external contamination.

3. Results

Examination of the PV and V of seabirds revealed the presence of foreign objects, including various particles (squid beaks, algae, remains of prey, stones, etc.). A total of 700 particles were documented in the 33 shearwaters evaluated, of which 674 were identified as plastic. Specifically, 345 items were in the V and 168 in the PV, representing 75% and 25% of the plastic content in each of these stomach parts, respectively.

Table 1. Summary of the results for the analysis of P in Cory’s shearwater fledglings, including the mass of non-plastics and plastics found in the PV and V, as well as the percentage of plastic items in relation to body and stomach mass, respectively.

Sample ID	BM (g)	Item TBM (mg)	PTBM (mg)	P Specimen	PPV (mg)	PV (mg)	% P/BM	% PPV/Stomach	% PV/Stomach
458	599	124.8	9.7	4	4.1	5.6	0.002	42.2	58.8
460	649	74.8	10.5	5	0	10.5	0.002	0	100
461	704	119.8	16.7	15	9.6	7.1	0.002	57.5	42.5
462	673	93.3	31.7	19	8.1	23.6	0.005	25.6	74.4
465	673	223.1	95.9	63	10.9	85.0	0.014	11.4	88.6
466	666	47.6	11.6	13	0	11.6	0.002	0	100
467	780	197.5	14.2	14	14.2	0	0.002	100	0
468	440	118.6	25.0	32	0	25.0	0.006	0	100
469	560	405.9	23.0	33	9.2	13.8	0.004	40	60
470	633	39.5	2.9	3	1.1	1.8	<0.001	37.9	62.1
471	578	102.1	23.7	26	12.7	11.0	0.004	54	46
472	630	119.4	39.4	78	0	39.4	0.006	0	100
473	701	623.5	0	0	0	0	0	0	0
474	758	275.3	8.4	16	0	8.4	0.001	0	100
475	570	285.3	3.9	7	0	3.9	0.001	0	100
476	616	87.6	2.9	5	0	2.9	<0.001	0	100
477	862	775.2	0	0	0	0	<0.001	0	0
478	530	60.4	49.0	14	0.4	48.6	0.009	0.8	99.2
480	588	186.7	4.4	38	0.8	3.6	0.001	18.2	81.8
481	664	914.3	8.9	14	7.4	1.5	0.001	83.1	16.9
482	611	324.4	18.7	17	12.9	5.8	0.003	69.0	31.0
483	739	486.7	254.4	11	30.1	224.3	0.034	11.8	88.2
486	706	479.3	16.2	24	13.6	2.6	0.002	84.0	16.0
487	692	394.9	6.4	14	0	6.4	0.001	0	100
488	649	246.4	3.3	7	0	3.3	0.001	0	100
489	-	318.8	111.3	41	7.3	104.0	-	6.5	93.5
490	450	310.1	4.2	10	0	4.2	0.001	0	100
491	673	178.8	12.7	30	5.0	7.7	0.002	39.4	61.6
492	552	143.6	97.3	39	46.3	51.0	0.018	47.3	52.7
494	647	732.6	24.9	8	12.1	12.8	0.004	48.6	51.4
498	1063	119.2	12.9	20	0	12.9	0.001	0	100
499	724	511.7	14.4	24	11.5	2.9	0.002	79.7	20.3
504	628	447.1	3.7	6	1.7	2.0	0.001	45.9	54.1
Average	657	289.7	29.2	20	6.6	22.5	0.004	27.4	66.6
Median	649	223.1	12.9	14	1.7	7.1	0.002	11.8	74.4
Range	440–1063	39.5–914.3	0–254.4	0–78	0–46.3	0–224.3	<0.001–0.034	0–100	0–100

BM: body mass of the specimen; Item TBM: Total mass of non-plastic items found relative to the total mass of each seabird; P TBM: total mass of plastic items found relative to the total body mass of each seabird; PPV: total mass of P in proventriculus; PV: total mass of P in ventriculus; % P/BM: percentage of P mass (g) per gram (body mass); PPV/stomach: percentage of P associated with ventriculus in relation to the stomach; % PV/stomach: percentage of MPs associated with ventriculus in relation to the stomach.

summarizes the findings from the samples, detailing the mass distribution of foreign objects and plastic debris retrieved from the PV and V, along with their respective percentages relative to total body mass and stomach mass. The average percentage of P relative to shearwater’s body mass was below 0.004%, resulting in an overall average number of 20 plastic items. However, the distribution of plastic items did not follow a normal distribution (Shapiro–Wilk, W = 0.849, p < 0.001), and thus the median was also calculated, yielding a value of 0.002% and 14 plastics for the entire sample set.

Within the study dataset, Cory’s shearwaters ingested mostly large MPs (82%), followed by mesoplastics (18%). Figure 1 shows some images obtained through optical microscopy of different isolated MPs. According to the morphology, the identified plastics were classified as sheet plastics (15.7%), threadlike plastics (31.6%), foamed synthetics (2%), microspheres (22.4%) and hard fragments (28.3%) (Figure 2a). The predominant plastic morphology was thread-like fibers, followed by hard fragments with irregular shapes. Microspheres ranked third in terms of abundance.

The plastic particles found in the stomachs consisted of six different polymer types. Polyethylene (PE) was the most identified polymer (46.7%), followed by polypropylene (PP, 24.6%), polyamide (PA, 20.4%) and poly(vinyl chloride) (PVC, 7.6%). Less frequent polymers included block copolymer styrene-butadiene-styrene (SBS) (0.4%) and ethylene propylene diene monomer (EPDM) (0.3) (Figure 2b). The predominant color was off-white clear (48.7%), followed by orange-brown (14.3%), yellow (9.4%), black (9.2%), green (7.4%), and blue-purple (7.0%), with less frequent colors being grey-silver (2.5%) and red-pink (1.5%) (Figure 3).

FTIR-ATR analysis was performed to establish the polymeric composition of the different plastic particles isolated from the birds’ digestive system. The results of this study, based on the characteristic bands of different polymers, allowed for the unequivocal identification of the analyzed particles. The PE spectrum (Figure 4A) presents three characteristic absorption bands: the C-H stretching at 2850 cm⁻¹, the C-C stretching at 1500 cm⁻¹, and a CH₂ bending motion at 700 cm⁻¹ [42]. The bands observed for PP in Figure 4B correspond to asymmetric stretches of CH₃ at 2953 cm⁻¹ and CH₂ at 2914 and 2840 cm⁻¹. CH₂ deformations result in the band at 1448 cm⁻¹, while CH₃ deformations give rise to the band at 1376 cm⁻¹. The band at 964 cm⁻¹ is attributed to CH₃ rocking and C-C stretching, whereas the band at 838 cm⁻¹ is due to a combination of CH₂ rocking and C-CH₃ stretching. The band at 812 cm⁻¹ corresponds to the rocking of CH₂ and tension in both C-H and C-CH [42]. The bands for PVC (Figure 4C) are as follows: the band at 2850–2900 cm⁻¹ corresponds to C-H stretching, while the band at 1420 cm⁻¹ is assigned to CH₂ bending. Absorption bands at 1330 and 1241 cm⁻¹ result from CH bending in the CH-Cl bond, and the band at 1096 cm⁻¹ is attributed to C-C stretching vibrations. The bands at 693, 685, and 602 cm⁻¹ are assigned to C-Cl stretching [43]. For PA (Figure 4D), the bands at 3291, 1639, 1550, and 1370 cm⁻¹ correspond to NH stretching modes, amide I (C=O stretching), amide II, and amide III, respectively. The bands at 2932 and 2859 cm⁻¹ are assigned to the symmetric and asymmetric CH₂ stretching vibrations, respectively. The peaks observed at 1436 and 1463 cm⁻¹ are attributed to CH₂ scissoring. Finally, the bands between 1278 and 1169 cm⁻¹ correspond to CH₂ bending or torsional modes [43]. The EPDM spectrum (Figure 4E) shows bands at 2920 cm⁻¹ and 2850 cm⁻¹ corresponding to CH₂ stretching vibrations. A characteristic absorption band for the C=C double bond appears at 1640 cm⁻¹. The peaks at 1460 cm⁻¹ and 1380 cm⁻¹ are assigned to out-of-plane bending vibrations of the C–H bond, while the peak at 720 cm⁻¹ is attributed to out-of-plane C–H bending [44]. In Figure 4F, SBS is characterized by the following absorption peaks: the bands at 2925–2851 cm⁻¹ are attributed to the symmetric and asymmetric stretching of aliphatic methylene groups (–CH₂–); the absorption peaks between 1650 and 1436 cm⁻¹ correspond to the stretching of double bonds (C=C) present in both the aliphatic chain and the aromatic ring of SBS; and finally, the signals at 1000–970 cm⁻¹ and 698 cm⁻¹ are associated with the aromatic ring of PS [45].

4. Discussion

Given the global rise in plastic production and improper and uncontrolled disposal, it was hypothesized that plastic exposure in Cory’s shearwaters from this research would exceed levels reported for the same region and species in earlier studies. The mean number of plastic items in our study was 20, with a median of 14 (Table 1), both of which are higher values than that reported by Navarro Et Al. (2023) [39], who examined Cory’s shearwaters and yellow-legged gulls (Larus michahellis) sampled in Gran Canaria Island during 2020–2021 and found means of 7.22 and 0.55 items per bird, respectively. A broader assessment across the Azores and the Canary Islands for 2015–2022 reported lower averages for the Azores at 10 items per fledgling and higher, yet comparable values for the Canary Islands at 26.3 items per fledgling [35]. In line with expectations, an earlier work from 2009–2011 also reported lower loads in Cory’s shearwater fledglings from Tenerife Island, with a mean of 8.0 items per bird [34]. More recently, Sobrino-Montelliu Et Al. (2025) [38] documented widespread ingestion in fledglings across Tenerife, Gran Canaria, and Lanzarote Islands during the 2020 breeding season, with more than 90 percent prevalence but lower counts than ours, with 13.2, 7.2, and 7.4 items per bird, respectively. Importantly, although our dataset does not yet meet the formal Cory’s-TV requirements of five-year windows and a minimum of 40 fledglings per year [35], about 91% of fledglings carried more than four items, well above the 20% criterion. This suggests that the threshold would likely be exceeded on Tenerife, highlighting the level of plastic pollution in the Canary Current (CC), which serves as a feeding ground for the fledglings’ parents. This is consistent with the global increase in plastic production and the associated rise in marine plastic waste. According to Plastics Europe [3], in 2022, around 400 million tons were produced worldwide, and in 2023, this figure is expected to have risen to 415 million. Yet, because the sample is relatively small (n = 33), single-season and non-probabilistic, observed patterns should be considered indicative and hypothesis-generating, rather than definitive for the wider population.

When comparing this study’s findings with those reported in the Mediterranean Sea, an area recognized as a hotspot for plastic pollution due to its semi-enclosed nature and dense coastal populations [46,47], the results were somewhat unexpected. For instance, [48] reported values in the digestive tract of different seabirds in the Mediterranean Sea such as Cory’s shearwater (average of 15.3 plastic fragments and 23.4 mg of mass in 49 samples), the Balearic shearwater (Puffinus mauretanicus, average of 3.5 fragments and 5.5 mg in 46 samples), the Yelkouan Shearwater (Puffinus yelkouan, average of 7.0 fragments and 42.1 mg in 31 samples), the Audouin’s Gull (Ichthyaetus audouinii, average of 49.3 fragments and 113.6 mg in 15 samples) or Yellow-legged Gull (Larus michahellis, average of 2.7 fragments and 4.3 mg in 12 samples) between May 2003 and June 2010. These values were comparable to, or even lower than, those reported in the Northeastern Atlantic region, as previously described. Comparisons within the same species, Cory’s shearwater, are especially interesting since the number of plastic items and the average mass found were lower than those observed in this study (average of 20 items and mass of 29.2 mg in 33 samples, Table 1). Differences in sampling periods may explain part of this gap. Yet, recent Mediterranean studies also report relatively low loads in seabirds. For instance, Murano Et Al. (2024) [49] reported the presence of plastics on Razorbills (Alca torda) in the western Mediterranean Sea, finding an average of 8.2 plastics per bird, and with plastic detected in 3 out of every 5 specimens. Another research by De Pascalis Et Al. (2022) [50] studied the regurgitated plastics of 30 planktivorous seabirds in Sardinia, Italy, between July and August 2019, obtaining an average of just 1.86 microplastics per sample. Species ecology is likely to strongly contribute to these contrasts, as surface feeders and divers experience different exposure; even so, the high values in the northeastern Atlantic remain notable. This pattern, however, seems at odds with Clark Et Al. (2023) [51], who identified the major plastic exposure risk areas as the Mediterranean and Black seas, the northeast and northwest Pacific, the South Atlantic, and the southwest Indian oceans. Further investigation is needed to understand why the extent of plastic pollution in an open area like the Northeast Atlantic, around the Canary Islands, can overlap that of the semi-enclosed and densely populated Mediterranean region. Factors to consider include oceanic currents, particularly the influence of the CC, which plays an important role in the formation and maintenance of the North Atlantic Subtropical Gyre [52], as well as the activities of the local marine fishing industry, one of the most productive fishery regions in the world [53]. Additionally, cross-study contrasts should be interpreted cautiously due to methodological heterogeneity, including size cut-offs and ranges (e.g., >1 mm in this study), mesh sizes, digestion/filtration protocols, and whether polymer identity was spectroscopically verified.

Beyond the metrics regarding the number of items found in this study, it is noteworthy that some specimens (473 and 477) had fully digested stomach contents and no plastic items. In contrast, individuals with undigested material, consistent with recent feeding, showed higher plastic loads. For example, sample 465 contained 63 particles (95.9 mg), sample 472 had 78 particles (39.4 mg), and sample 483 had only 11 particles, yet a much higher plastic mass (254.4 mg), indicating fewer but heavier items. These patterns show that count and mass provide complementary information, reinforcing the concept that reporting both parameters (alongside the stomach compartment examined) offers a fuller and more comparable assessment of plastic ingestion in biota. Consistent with general findings, plastics recovered from the PV were often larger than those from the V, reflecting that digestion begins in the PV. In the V, particles are subjected to reduction through wear or fragmentation [54]. Additionally, due to the possibility of regurgitation or the passage of food into the V, plastics were found to be less prevalent in the PV compared to the V, where food remains or plastics are retained for a longer period.

Regarding the morphology of the ingested plastics, the common types were microspheres (22.4%), sheet plastics (15.7%), and hard fragments with irregular shapes (28.3%) (Figure 2a). However, the majority were filiform (31.6%), mirroring other studies on Cory’s shearwaters in the Atlantic Ocean as bioindicator species. This morphology likely originates primarily from the degradation of fishing gear, such as fishing lines, nets and ropes, highlighting the significant role of fishing activity as a major source of marine plastic pollution in the study area [35,38,39].

To date, the lack of standardization in color assignment is quite common in studies on plastic pollution in marine environments [55,56,57]. This characteristic is important as it aids in a more precise interpretation of the aging of plastics and MPs in relation to their exposure time in the environment. In addition to the abundance of plastic in the marine environment, Cory’s shearwaters may ingest plastic items of different colors due to their similarity to prey or greater visibility in the sea [58]. Their diet primarily consists of mollusks, squid, and other pelagic organisms [59,60], and plastic ingestion may be related to specific colors depending on the species [61]. In this study, the highest percentage of plastic items exhibited a white color (48.7%), followed by orange/brown (14.3%) and yellow (9.4%), as shown in Figure 2b. The predominance of these colors is thought to be explained by photoaging processes, with the white/light yellow resulting from discoloration, and the yellow/amber due to weathering that generates chromophore products (yellowing before tanning) [57]. These results are consistent with some other studies on Cory’s shearwaters, where the most frequently occurring color of plastic items was white-transparent, found in 45% of specimens from the Canary Islands [35,38] and 64% of birds sampled in the Azores [35]. In the study [39], in the five Madeiran storm-petrels examined, white was also the predominant color (25%), along with yellow (21.4%). However, for Cory’s shearwaters from Gran Canaria Island, green (19.4%) and blue (16.9%) predominated, with white/transparent next (14.8%), suggesting temporal variation in the sources of plastic entering their feeding grounds. Color plays an important role in plastics’ environmental interactions, affecting both their degradation rates and the likelihood of ingestion by aquatic organisms [62]. Yet, the factors driving color accumulation in Cory’s shearwaters or other seabirds remain unclear. Further research is needed to assess whether these patterns are influenced, and to what extent, by prey preferences, environmental availability, or other ecological and behavioral factors.

Finally, the polymer composition of the plastics ingested by Cory’s shearwaters analyzed in this study provides a perspective that is both consistent with and distinct from previous reports for this species. The most common polymer type identified by FTIR-ATR (Figure 4) in Cory’s shearwater stomachs was PE (46.7%), followed by PP (24.6%) and PA (20.4%) (Figure 2b). The large share of PE and PP was expected since it agrees with what is commonly described in Atlantic Ocean studies [63]. In turn, this reflects the global production and widespread use of these plastics in disposable products, such as packaging [63,64], which is a key aspect of the global plastic pollution issue. For instance, production data from 2022 shows that over 25% of global plastics are derived from PE, with PP accounting for 18% of global plastic production in Europe [3]. These percentages are easily increased in the marine environment, given that both polymers are buoyant, heavily used in short-lived packaging and fishing gear that can fragment into many small plastic pieces, and are concentrated and preferentially sampled at the surface. Nevertheless, previous studies on this species in the Canary ecosystem reported even higher proportions of PE as 71% [39] and 79% [35], raising the question of what factors could explain these mismatches. A plausible explanation is the comparatively high nylon PA fraction in our samples (not reported in the other studies), which would reduce the relative share of PE. This could potentially be associated, among other factors, with contributions from fishing gear at the sea surface. Differences in sampling windows and locations may also play a role, since polymer signals can vary with season, year, and colony foraging areas. Finally, methodological choices such as subsampling, FTIR match thresholds, and size cutoffs may further shift proportions. Taken together, our data show PE and PP as the dominant polymers, while revealing a stronger PA signal in the Tenerife fledglings studied, pointing to specific source effects that should be considered when comparing different colonies/breeding populations and time series.

5. Conclusions

This study provides a chemically verified, contamination-controlled baseline of ingested plastics in Cory’s shearwater fledglings from Tenerife Island. Given the limited sample size (n = 33) and single-season coverage, extrapolation to a broader region or to interannual patterns is beyond the scope of this dataset. Instead, findings should be viewed as current baseline patterns to be tested in future larger, multi-year cohorts. Yet, the detection and characterization of plastic particles in the samples studied using FTIR-ATR emphasizes the role of Cory’s shearwater, particularly fledglings, as effective biomonitors, shedding light on the extent of plastic pollution impacting the Canary Current region within the Northeast Atlantic, which might overlap with findings reported on Mediterranean seabirds, specifically in Cory’s shearwaters. However, methodological variability (e.g., differing size cut-offs and workflows) and temporal variability require caution when making direct comparisons.

Plastic fragments of various types and sizes were detected, composed mainly of PE, followed by PP and PA, with PVC, EPDM, and butadiene block copolymer found at a lower proportion. Given that PE is the most produced plastic worldwide, and that PE and PP make up most single-use plastic products, it reinforces the concept that addressing plastic pollution requires prioritizing a reduction in single-use plastics, both in production and consumption. Encouraging the adoption of reusable materials, exploring biodegradable alternatives, and implementing stricter regulations on disposable items are vital steps toward mitigating their significant impact on environmental and human health.

Continued research is essential for more comprehensive studies. Investigating the potential assimilation of organic pollutants, such as PAHs, PCBs, or PFAS, from plastic items in organs and tissues like the liver, adipose tissue, or intestines is highly advisable to increase our knowledge on the full impact of plastic contamination.