Testing for assortative mating based on migratory phenotypes in the Common Tern (Sterna hirundo)

1.   Introduction

Positive assortative mating is a widespread phenomenon in long-lived, monogamous seabirds (Coulter, 1986; Stern and Jarvis, 1991; Helfenstein et al., 2004; Ludwig and Becker, 2008; Wojczulanis-Jakubas et al., 2017; Visalli et al., 2023). It is defined as a form of mate choice, where individuals select mates on the basis of phenotypic similarity to themselves (Crespi, 1989; Harari et al., 1999; Shine et al., 2001), while negative assortative (or disassortative) mating means the selection of mates based on phenotypic differences (Jiang et al., 2023). Positive assortative mating is often found in relation to morphological traits (e.g., Coulter, 1986; Stern and Jarvis, 1991; Helfenstein et al., 2004; Wojczulanis-Jakubas et al., 2017; Visalli et al., 2023), but also in regards to migratory phenology: in Blackcaps (Sylvia atricapilla; Bearhop et al., 2005; Rolshausen et al., 2010) and Swainson's Thrushes (Catharus ustulatus; Ruegg et al., 2012) positive assortative mating has been found in relation to migratory direction and wintering area, as a result of differences in arrival date of birds showing different migratory phenotypes.

In the Common Tern (Sterna hirundo), positive assortative mating has been found for bill size (Coulter, 1986), age (Nisbet et al., 1984) and arrival date at the colony (Ludwig and Becker, 2008). This species is a long distant migrant, breeding in inland as well as coastal habitats in Europe, and wintering along the African coast as far south as South Africa. Common Terns show a high degree of mate fidelity (González-Solís et al., 1999; Rebke et al., 2017) and nest site tenacity (Austin, 1949; González-Solís et al., 1999). Females tend to start their autumn migration earlier than males (Nisbet et al., 2010; Kürthen et al., 2022) and males therefore probably provide the majority of post-fledging care (Nisbet et al., 2010, 2011). Several ringing studies on Common Terns showed their use of the eastern Atlantic flyway and identified two main wintering areas along the African coast: while birds from western Europe, including birds from southern and western Germany, mainly spent the winter at the north western African coast between western Sahara and Nigeria (Nebelsiek, 1966; Neubauer, 1982; Bairlein et al., 2014), Scandinavian as well as Baltic and eastern German inland populations have their main wintering areas further south on the coasts of Namibia and South Africa (Neubauer, 1982; Bairlein et al., 2014; Heinicke et al., 2016). Using tracking data, Kürthen et al. (2022) demonstrated that the wintering areas of Common Terns are highly repeatable. Recent tracking studies also revealed the use of the eastern African flyway and the use of wintering areas on the eastern African coast by Common Terns breeding in Croatia and Hungary (Kralj et al., 2020), as well as at the German Baltic coast (Piro and Schmitz Ornés, 2022). In this last study we also showed that birds breeding in the same colony on Riether Werder island at the eastern coast of Germany use both flyways and winter in all three of the described main wintering areas (Piro and Schmitz Ornés, 2022), making this colony the ideal place to study the effects of migratory phenotypes on mate choice. Accordingly, this study aims to investigate, if Common Terns show assortative mating in regards to their migratory phenotype in terms of their migratory route and in terms of their wintering area. Assortative mating might be expressed positively, if the Common Terns breeding on Riether Werder tend to mate with individuals using the same route and wintering on the same site, or negatively if the birds tend to mate with individuals showing a different migratory phenotype, hence use a different migratory route or wintering site. However, migratory behavior may play no role at all in the mate choice of Common Terns, resulting in neither positive nor negative assortative mating. As assortative mating regarding migratory phenology seems to be mainly driven by arrival dates (Bearhop et al., 2005; Rolshausen et al., 2010; Ruegg et al., 2012), we also tested for differences in arrival date between birds using different migratory routes and wintering in different areas.

Genetic analyses of short-lived species such as songbirds, where juveniles usually are solitary nocturnal migrants who reach their wintering areas without the guidance of their parents or conspecifics (Åkesson et al., 2021), showed that variation in migratory behavior is largely due to genetic differences (Liedvogel et al., 2011). Crossbreeding experiments of Blackcaps for example suggest that the direction of migration is largely heritable and that there is little genetic dominance, as hybrids show intermediate migratory directions (Helbig, 1991; Berthold and Helbig, 1992). However, identifying genes determining migratory traits and understanding their regulation are still in their infancy (e.g., Liedvogel et al., 2011; Delmore and Liedvogel, 2016; Lundberg et al., 2017, 2023; Delmore et al., 2020, 2023; Sokolovskis et al., 2023; Louder et al., 2024). This is especially the case in long-lived bird species, in which the types of genes involved may differ from short-lived species, and where the genetic program might be complemented by or modified via social learning, for example from parental guided migration (Méndez et al., 2021; Byholm et al., 2022). The results of this study might provide hints about the degree of genetic fixation of the migratory phenotype in Common Terns: while assortative mating might indicate a genetic fixation of migratory direction or wintering areas, the absence of assortative mating might be an indication for either a low degree of genetic fixation or a dominance of a certain phenotype.

2.   Materials and methods

Riether Werder (54°42ʹ N, 014°16ʹ E), part of the special protected area "Kleines Haff, Neuwarper See und Riether Werder" (DE_2250-471), is an 82 ha Island off the German coast in the Szczecin Lagoon. With 10, 000–12, 000 pairs, the island hosts the largest Black-headed Gull colony in Germany (Herrmann, 2018). Common Terns use the shelter of this colony and form a smaller colony of 100–150 pairs within the gull colony, using the sandy areas on the dike as nesting sites.

For the study, a total of 120 breeding Common Terns were equipped with light level geolocators (Intigeo-W65A9-SEA-NOT, Migrate Technology Ltd) between 2019 and 2023 (2019: n = 40, 2020: n = 23, 2022: n = 40, 2023: n = 17). For details about programming and deployment of loggers, as well as light level analyses conducted in R Studio (V4.0.2., RStudio Team, 2020) using the packages BAStag (Wotherspoon et al., 2016) for twilight determination and FLightR (v0.5.1; Rakhimberdiev et al., 2015) for the analysis see methods and script already provided in Piro and Schmitz Ornés (2022). While only one individual of a breeding pair was fitted with a logger in the first study year, from 2020 onwards, both individuals of a breeding pair, or partners of previously tracked individuals, were fitted with loggers. To trace pairing, all nests in the colony were marked with a number, and breeding birds were caught using cage traps placed above their nests (for details see Piro and Schmitz Ornés, 2022). As every caught individual was marked with a coded ring, and could be assigned to the nest it was caught on, they could also be assigned to its respective partner in every study year.

A total of 77 birds tagged with a logger were recaptured. As two loggers failed before the bird started migration, and six birds returned without loggers, a total of 69 individuals could be tracked, one of them for two subsequent years. While 68 of the 77 birds were recaptured at Riether Werder, 7 were recaptured in the colony of Śmięcka, a polish Island in the Szczecin Lagoon, 16 km west of Riether Werder (53°45ʹ N, 14°13ʹ E). The bird tracked for two subsequent years was recaptured in the Riether Werder colony after the first year of tracking, and in the Śmięcka colony after the second year. Another bird was recaptured 23 km south of Riether Werder in the colony on a small artificial island at Lake Krugsdorf (53°32ʹ N, 14°5ʹ E).

Wintering areas were defined as the areas where the birds stayed between December and February. Birds wintering in north-western Africa (north of the equator) were defined as the west group, while birds wintering south of the equator were split into the south group, including all birds wintering in Namibia and South Africa, and the east group, including birds wintering at the eastern African coast (mainly in the Mozambique Channel). As Kürthen et al. (2022) demonstrated a high repeatability for the wintering areas of Common Terns, we also assumed a high repeatability of the use of the eastern or western flyway.

Pairing data were analyzed for pairs with known flyway for both individuals, although the information about flyway use and wintering area must not have been collected in the same year. Pairing data therefore include all pairs where both partners have been tracked with light-level geolocators, as well as pairs where one of the birds was tracked while the information about the partner's flyway could be obtained from ringing data. As birds do not necessarily pair with the same mates every year, one individual could appear several times in the pair-dataset, if it had different partners in different years and the flyways of these partners are known.

Pearson's χ² tests were used to compare observed and expected numbers of pairs where the partners used the same or a different migration route, and pairs where partners used the same or a different wintering area. Expected numbers of pairs with same or different migratory routes were calculated as the probabilities of birds mating with a partner using the same or different route assuming random mating between birds using a respective migration route according to the percentage of tracked birds that used that respective route.

Expected numbers of pairs using the same or different wintering areas were calculated as the probability of birds mating with a partner using the same or different wintering area, assuming random mating between birds using a respective wintering area according to the percentage of tracked birds that used each of the respective wintering areas. Pairs where the information about a bird's migration route is based on ringing data were excluded for the test in regards to the wintering area, as this information cannot be reconstructed from ringing data alone.

Arrival dates of all individuals were obtained from the tracking data with an accuracy to the day, and were converted into day of the year. A t-test was used to compare arrival dates between bird using the eastern and birds using the western route, while a one-way ANOVA was used to compare arrival dates of birds wintering in the three different wintering areas.

3.   Results

3.1   Tracking

Of the 69 birds with usable tracking data, 56 individuals (81.16%) used the western, and 13 individuals (18.84%) used the eastern migration route. As one bird track ended in October in Ghana, it could be assigned to the western migration route, its wintering area however could not be identified. Of the 55 birds with known wintering area using the western route, 11 (20.00%) spent the winter in north western Africa, while 43 (78.18%) wintered on the south western African coast. The last one (1.82%) crossed Cape Agulhas and was assigned to the birds wintering at the eastern African coast as it wintered between Durban (South Africa) and Maputo (Mozambique). Similarly, one (7.69%) of the 13 birds using the eastern route crossed the Cape and was assigned to the birds wintering at the south-western African coast as it wintered in the area around Cape Town (South Africa). The other 12 birds (92.31%) using the eastern route wintered in the Mozambique Channel.

In total, of the 68 tracked birds with known wintering area, 11 (16.18%) wintered at the north western African coast, 44 (64.70%) at the south western African coast, and 13 (19.12%) at the east African coast.

3.2   Pairing

As pairing had been traced in all years between 2017 and 2024, a total of 33 pairs where both partners have been tracked could be identified. Three additional pairs with known flyways of both birds were identified using ringing data. One individual was assigned to the western flyway as it was ringed in Spain during autumn migration in 2011. A second individual which has been paired to two different tracked individuals had lost its logger when recaptured at Riether Werder. However, it was assigned to the eastern flyway as it was recaptured in Israel during spring migration in 2023. Information about all pairs is shown in Table 1.

Table  1.  Pairing data.

First partner				Second partner				Paired in:	Same route	Same wintering area
ID	Source of information	Migration route	Wintering area	ID	Source of information	Migration route	Wintering area
DEH NA132326	Tracked (2022/2023)	Western	SW-Africa	DEH NA108996	Tracked (2022/2023)	Western	SW-Africa	2022, 2023	Yes	Yes
DEH NA132359	Tracked (2022/2023)	Western	SW-Africa	DEH NA132210	Tracked (2022/2023)	Eastern	SE-Africa	2020	No	No
DEH NA132359	Tracked (2022/2023)	Western	SW-Africa	DEH NA176466	Tracked (2022/2023)	Western	SW-Africa	2022, 2023	Yes	Yes
DEH NA171576	Tracked (2022/2023)	Western	NW-Africa	DEH NA163168	Tracked (2019/2020)	Western	SW-Africa	2022, 2023	Yes	No
DEH NA038700	Tracked (2022/2023)	Western	SW-Africa	DEH NA121680	Tracked (2022/2023)	Western	NW-Africa	2022, 2023	Yes	No
DEH NA163426	Tracked (2022/2023)	Western	SW-Africa	DEH NA132224	Tracked (2019/2020)	Eastern	SE-Africa	2023	No	No
DEH NA121822	Tracked (2022/2023)	Western	SW-Africa	DEH NA176437	Tracked (2022/2023)	Western	SW-Africa	2022, 2023	Yes	Yes
DEH NA163466	Tracked (2022/2023)	Western	SW-Africa	ESI 1V015497	Ringing data (ringed 27.08.2011 in Spain)	Western	Na	2020	Yes	No
DEH NA171636	Tracked (2019/2020)	Western	SW-Africa	DEH NA163466	Tracked (2022/2023)	Western	SW-Africa	2019, 2022, 2023	Yes	Yes
DEH NA190118	Tracked (2022/2023)	Western	NW-Africa	DEH NA172966	Tracked (2020/2021)	Western	NW-Africa	2023	Yes	Yes
DEH NA172971	Tracked (2022/2023)	Western	SW-Africa	DEH NA171574	Tracked (2020/2021)	Western	SW-Africa	2017, 2020, 2021, 2022, 2023	Yes	Yes
DEH NA108996	Tracked (2022/2023)	Western	SW-Africa	DEHNA163205	Tracked (2019/2020)	Western	SW-Africa	2020, 2021	Yes	Yes
DEH NA028195	Tracked (2022/2023, 2023/2024)	Western	SW-Africa	DEH NA163477	Tracked (2020/2021)	Western	SW-Africa	2022, 2024	Yes	Yes
DEH NA132217	Tracked (2022/2023)	Eastern	SE-Africa	DEH NA132313	Tracked (2019/2020)	Western	SW-Africa	2020, 2021, 2022, 2023	No	No
DEH NA098476	Tracked (2022/2023)	Western	SW-Africa	DEH NA163470	Tracked (2020/2021)	Western	NW-Africa	2022	Yes	No
DEH NA163477	Tracked (2020/2021)	Western	SW-Africa	DEH NA163137	Tracked (2019/2020)	Western	SW-Africa	2020, 2021	Yes	Yes
DEH NA132355	Ringing data (recaptured 11.04.2023 in Israel)	Eastern	Na	DEH NA163202	Tracked (2019/2020)	Western	SW-Africa	2020	No	Na
DEH NA172966	Tracked (2020/2021)	Western	NW-Africa	DEH NA171504	Tracked (2019/2020)	Western	NW-Africa	2019, 2020, 2021, 2022	Yes	Yes
DEH NA190054	Tracked (2019/2020)	Eastern	SE-Africa	DEH NA204734	Tracked (2020/2021)	Western	SW-Africa	2020	No	No
DEH NA190054	Tracked (2019/2020)	Eastern	SE-Africa	DEH NA163262	Tracked (2020/2021)	Western	SW-Africa	2021, 2022	No	No
DEH NA190054	Tracked (2019/2020)	Eastern	SE-Africa	DEH NA132377	Tracked (2020/2021)	Western	SW-Africa	2023	No	No
DEH NA163247	Tracked (2020/2021)	Eastern	SE-Africa	DEH NA163470	Tracked (2020/2021)	Western	NW-Africa	2023	No	No
DEH NA132355	Ringing data (recaptured 11.04.2023 in Israel)	Eastern	Na	DEH NA171631	Tracked (2022*)	Western	Na*	2021, 2022	No	Na
DEH NA171512	Tracked (2023/2024)	Western	SW-Africa	DEH NA171564	Tracked (2023/2024)	Western	SO-Africa	2021, 2023, 2024	Yes	No
DEH NA132379	Tracked (2020/2021)	Eastern	SE-Africa	DEH NA171564	Tracked (2023/2024)	Western	SO-Africa	2020	No	Yes
DEH NA163466	Tracked (2022/2023)	Western	SW-Africa	DEH NA132332	Tracked (2022/2023)	Western	SW-Africa	2024	Yes	Yes
DEH NA190280	Tracked (2020/2021)	Western	SW-Africa	DEH NA132354	Tracked (2023/2024)	Western	NW-Africa	2024	Yes	No
DEH NA160339	Tracked (2020/2021)	Western	SW-Africa	DEH NA163175	Tracked (2019/2020)	Western	SW-Africa	2021	Yes	Yes
DEH NA163262	Tracked (2020/2021)	Western	SW-Africa	DEH NA028195	Tracked (2022/2023)	Western	SW-Africa	2017	Yes	Yes
DEH NA132379	Tracked (2020/2021)	Eastern	SE-Africa	DEH NA098476	Tracked (2022/2023)	Western	SW-Africa	2019	No	No
DEH NA163236	Tracked (2022/2023)	Eastern	SE-Africa	DEH NA121680	Tracked (2022/2023)	Western	NW-Africa	2020	No	No
DEH NA163236	Tracked (2022/2023)	Eastern	SE-Africa	DEH NA171576	Tracked (2022/2023)	Western	NW-Africa	2017	No	No
DEH NA038700	Tracked (2022/2023)	Western	SW-Africa	DEH NA028195	Tracked (2022/2023)	Western	SW-Africa	2019	Yes	Yes
DEH NA028195	Tracked (2022/2023)	Western	SW-Africa	DEH NA098476	Tracked (2022/2023)	Western	SW-Africa	2020	Yes	Yes
DEH NA038608	Tracked (2022/2023)	Western	NW-Africa	DEH NA163470	Tracked (2020/2021)	Western	NW-Africa	2021	Yes	Yes
DEH NA038608	Tracked (2022/2023)	Western	NW-Africa	DEH NA132332	Tracked (2022/2023)	Western	SW-Africa	2014	Yes	No
Note: Na = not available, and loggers that stopped working are marked with *.

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Of the 36 pairs with known migratory phenotypes, 23 pairs (63.89%) consist of individuals both using the western route, while 13 pairs (36.11%) consist of one bird using the eastern and one bird using the western route. A pair where both partners used the eastern route could not be documented.

In 13 (56.52%) of the 23 pairs where both individuals used the same route, both spent the winter in south-western Africa, while in three pairs (13.04%) both partners wintered in north-western Africa. In five pairs (21.74%) one individual wintered in south-western Africa while the partner wintered in north-western Africa. In one pair (4.35%) one individual wintered in south-western Africa, while the other circumnavigated the Cape Agulhas and spent the winter on the south-eastern African coast in the area between Durban (South Africa) and Maputo (Mozambique). For the remaining pair, the wintering area for one partner is unknown as the assignment to the western route is based on the bird being ringed during spring migration in Spain.

The 13 birds using the eastern route whose partner's flyway is known all wintered at the south-eastern African coast. Eight of their partners (61.54%) wintered in south-western Africa, three (23.08%) wintered in north-western Africa, and one (7.69%) wintered at the eastern African coast as well, as it is the bird that circumnavigated Cape Agulhas (mentioned above). The wintering location of the last partner is unknown as it is the one whose logger stopped working in October in Ghana, and the bird might not have reached its final wintering area by then.

The Chi-square tests showed no significant differences between the observed pairing and expected random pairing in accordance to either migratory route (χ² = 0.51845, n = 36, df = 1, p = 0.4715) or wintering area (χ² = 0.015057, n = 33, df = 1, p = 0.698). Several individuals for which the migratory phenotype of more than one partner is known were documented to breed with partners using different routes (NA132359, NA171576, NA098476, NA171564, NA163470, NA163262 see Table 1) or wintering in different areas (NA132359, NA171576, NA038700, NA098476, NA163470, NA16326 see Table 1).

3.3   Arrival date

When tested for migration route, mean arrival date was April 22nd ± 5.7 days for birds using the western route, and April 23rd ± 4.1 days for birds using the eastern migration route with no significant difference between the two routes (t_21.696 = 0.404, p = 0.69, n = 66). When tested for wintering area, mean arrival date was April 22nd ± 5.7 for birds wintering in north-western Africa, April 22nd ± 5.8 days for birds wintering in south-western Africa and April 23rd ± 4.0 days for birds wintering in east Africa, with no significant difference between the three groups (F_2,63 = 0.2028, p = 0.817).

4.   Discussion

Positive assortative mating has been found in Common Terns, when considering age (Nisbet et al., 1984) and arrival date at the colony (Ludwig and Becker, 2008), as well as morphological traits such as bill size (Coulter, 1986). Additionally, courtship-feeding rates (Nisbet, 1973; Wiggins and Morris, 1986) and wing molt (Bridge and Nisbet, 2004) have been identified as quality markers used in mate choice by Common Terns. On the contrary, our results show that migratory route and wintering area play no role when it comes to mate choice, as birds using both migratory routes and all three wintering sites mate randomly in the colony. As some birds were documented to pair with several individuals using different migratory routes or wintering sites, our results indicate that Common Terns show no individual preference to a partner's migration route or wintering area. So considering migration route and wintering area, neither positive nor negative assortative mating was found in the Common Terns breeding at Riether Werder. Our results regarding to wintering areas are consistent with the ones of Kürthen et al. (2022), who also reported pairing of birds wintering in different areas of western and southern Africa.

The absence of assortative mating in this species in relation to migratory direction and wintering areas might be explained by the lack of differences in arrival times, as this difference has been identified as a main driver of assortative mating in populations across migratory divides of songbirds such as Blackcaps (Bearhop et al., 2005; Rolshausen et al., 2010) and Swainson's Thrushes (Ruegg et al., 2012). Ludwig and Becker (2008) indicated that Common Terns show assortative mating in regards to age and arrival time, which are directly related, as the older and more experienced the Common Terns are, the earlier they arrive at the colony (Ludwigs and Becker, 2002; Dittmann and Becker, 2003). Our results showing an absence of assortative mating in regards to migratory phenotypes might give us hints about the degree of genetic fixation of migratory directions in the Common Tern. Successful migration relies on many traits which have been shown to possess heritable components, such as accurate timing as well as precise navigational and physiological abilities (Merlin and Liedvogel, 2019; Åkesson et al., 2021; Moiron et al., 2023). Heritable traits may be particularly striking in short-lived songbirds, where juveniles usually are solitary nocturnal migrants that reach their wintering areas without the guidance of their parents or conspecifics (Åkesson et al., 2021). In these species, variation in migratory behavior is largely due to genetic differences (Liedvogel et al., 2011). Crossbreeding experiments of Blackcaps suggest a heritability of migratory direction and little genetic dominance, as hybrids show intermediate migratory directions (Helbig, 1991; Berthold and Helbig, 1992). Such intermediate migratory routes most likely can be excluded for Common Terns, as such "hybrids" would migrate directly in southern direction, which not only would lead to the problems of crossing the Alps and Sahara desert, but would also result in recoveries of Common Terns ringed in NE Germany in the central Mediterranean, especially Italy, which is not the case (Heinicke et al., 2016).

The detection of potential heritable components for migratory direction and wintering sites in long-living birds such as the Common Tern is certainly not as simple and straight-forward as in short-lived passerines. A dominance might explain the absence of intermediate migratory directions, but social learning, which might supplement or modify a genetic program might be a much more important component in the migration of long living species than in short-living ones. Some long-living species such as Cranes (Alonso et al., 2008) and Geese (Gupte et al., 2019) are known to migrate as whole families. Byholm et al. (2022) showed that in the Caspian Tern (Hydroprogne caspia), a tern species expressing different migratory routes as well, breeding partners do not migrate together but that male parents carry most of the responsibility for guiding naïve young during their first migration, as they migrate together and the bond between father and young only gradually breaks down upon arrival at the wintering grounds. As Caspian Terns show a high fidelity to migratory routes and stopover sites, migratory phenotypes seem to be passed from father to offspring via social learning (Byholm et al., 2022).

In other bird species however, migratory flocks are not affected by family bonds, in which case the decision of a migratory direction might simply be dependent on the flock a naïve individual might join. Migrating in flocks may be especially beneficial for long distant migrants, as it might reduce the energetic demands of long migration, increase navigational accuracy and favour group foraging at migratory halts (Beauchamp, 2011). As they have to decide about schedule, route, and destination without previous experience (Aikens et al., 2022), young individuals might benefit most of migrating in company with experienced conspecifics.

As the Szczecin Lagoon is an important roosting area with large roosting assemblages in Świnoujście and Berndshof (Heinicke et al., 2016), young terns from the Riether Werder colony would have the opportunity to join birds breeding in German and Polish coastal and inland colonies, which are known to use both flyways. This would be consistent with a recent tracking study on American Common Terns, which indicated that large proportions of a breeding population migrate together, especially during the initial part of migration (Bracey et al., 2024). Although based on tracks of one single family, this study also indicates that juveniles may not always migrate with relatives but may rely on social interactions with other non-related individuals. However, similar as in Caspian Terns (Byholm et al., 2022) it has been shown in the Common Tern that females start their autumn migration earlier than males (Nisbet et al., 2011; Kürthen et al., 2022) and that males probably provide the majority of post-fledging care (Nisbet et al., 2010, 2011). Hume (1993) reported that Common Terns are fed by parents for long periods in some instances, and often stay with their parents on their first migration south and may still be fed by them some thousands of miles from the breeding colony. This might indicate, that similar to the Caspian Tern (Byholm et al., 2022), migratory phenotypes in Common Terns could be passed from the father to the offspring via social learning. The phenomenon of early deserting females and males providing parental care for longer times is widespread in monogamous, single-clutch shorebirds (Méndez et al., 2021), and shown for different tern species (Ledwoń and Neubauer, 2017; Byholm et al., 2022). It possibly originates in a sexual conflict where females gain in terms of residual fitness over time (Arnqvist and Rowe, 2005; Byholm et al., 2022). This often results in males taking the responsibilities to guide the offspring during its first migration, hence passing their migratory phenotype to the next generation, as it has been shown for tracked Oystercatchers (Haematopus ostralegus; Méndez et al., 2021) and Caspian Terns (Byholm et al., 2022). To investigate parental guidance and effects migrating in flocks, tracking of many birds and especially of families (both parents as well as the young) would be necessary. Due to high mortality and breeding dispersion, retrieval rates of loggers put on nestlings would be very low. Live tracking with GPS-trackers would be more helpful here, as soon as tracking devices will become small and lightweight enough to be safely used on Common Terns.

Although our results regarding mate choice indicate that the degree of genetic fixation of migratory directions might be low in the Common Tern, genetic studies of birds with known migratory routes may reveal interesting differences between birds with different migratory phenotypes. That would be especially possible due to recent success in identifying genetic markers associated with migration in songbirds (Lundberg et al., 2023; Sokolovskis et al., 2023). The Riether Werder colony, as a place where birds using both migratory routes and wintering in all three wintering areas breed together, will be the perfect place for future studies focusing on genetics, especially as the migratory phenotypes of a large number of individuals of the long-living birds in this colony are known.

CRediT authorship contribution statement

Simon Piro: Writing – review & editing, Writing – original draft, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization. Angela Schmitz Ornés: Writing – review & editing, Supervision, Project administration, Methodology, Investigation, Funding acquisition, Data curation, Conceptualization.

Ethics statement

All field work and experiments done in this study comply with the current laws of the country in which they were performed, and have been authorized by the local authorities for nature protection and animal welfare. Tracking of terns was implemented in an authorized long-term ringing project on Riether Werder. The use of geolocators was authorized by the local authorities for nature protection (Untere Naturschutzbehörde Landkreis Vorpommern-Greifswald, permit: 60.5/BR, VG 19–028) and animal welfare (Landesamt für Landwirtschaft, Lebensmittelsicherheit und Fischerei Mecklenburg-Vorpommern, permit: 7221.3-2-007/19).

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We want to thank Frank Joisten, caretaker of Riether Werder, for the possibility to work in this colony and for all his assistance during field work. Additionally, we want to thank Dominik Marchowski and Zbigniew Kajzer as well as their whole ringing team for their help in recapturing the terns resettling in the Polish colony on Śmięcka, without their help retrieving these loggers would not have been possible.