The reuse of Song Thrush (Turdus philomelos) nests by the Red-backed Shrike (Lanius collurio) in an intensive agricultural landscape: a coincidence or a new solution?

Abstract Nest site availability plays an important role in the ecology of the birds inhabiting intensive agricultural landscapes. The removal of trees and snags due to logging is one of the main threats and the cause of the observed decline in many birds living in an agricultural landscape. When nests are lost, nesting passerines typically rebuild them for a new clutch. Additionally, relocating nests after a brood loss is a prevalent strategy used to avoid predators and brood parasites. Such a strategy is common to the Red-backed Shrike, which builds successive nests for each brood. Interestingly, clutch replacement may occur in unusual forms. This study describe three cases of Song Thrush nests reused by Red-backed Shrikes during one breeding season. The time and energy-saving behaviour or possible deterrence of nest parasitism could explain why the nests have been reused. However, due to the limited observations as compared to the total number of nests, further research is necessary to clarify this phenomenon.


Introduction
Reproduction is an integral part of any species' ecology and the nest plays an essential role in avian reproduction by keeping eggs and nestlings contained, insulated from adverse environmental conditions, and protected from predators (Mainwaring et al. 2014;Deeming & Mainwaring 2015;Healy et al. 2015).The construction of a nest is a highly energyintensive process that has to be completed before the beginning of the incubation, a phase that is critical for the nest survival of most bird species (Mainwaring & Hartley 2013).The reuse of nests in the same breeding season can result from the economy of expenditure on the part of parents, but also from the lack of other adequate nesting sites (Mainwaring et al. 2015).
If the primary reason for nest reuse in the same breeding season is the economy of energy expenditure, then it can be assumed the time interval between successive breeding attempts of pairs that reuse their own nests or the nests of others would be decreased.However, if the reason for nest reuse is the scarcity of safe nesting sites (predator avoidance hypothesis), more frequent reuse of nests that produced fledglings would be expected, leading to lower breeding success in newly built nests compared to reused nests.Moreover, in areas of intense predation, nest reuse after successful breeding should be more frequent than in locations with lower predation pressure (Wysocki 2004;Hafstad et al. 2005;Chalfoun & Martin 2010).
Anyway, individuals with successful breeding outcomes often exhibit nest-site fidelity (Hoover 2003), whereas nesting failure can lead to a change in nesting site (Haas 1998).As a result, individuals can display site-fidelity after successful breeding or territory-fidelity, maintaining the same territory for breeding purposes, even if the specific nesting site within that territory changes (Chalfoun & Martin 2010;Batisteli et al. 2021;Kloskowski 2021).
The highest density of the Red-backed Shrike breeding pairs in Europe can be found on farmlands, edge forests, grasslands, dense hedges along roadsides, and open areas covered with bushes or small patches of trees, as well as in parks and open gardens (Tryjanowski et al. 2000;Morelli 2012;Kwieciński & Pawlak 2015).In Europe, several cases of double breeding of the Red-backed Shrike in one season have been recorded (Lefranc 2023).In cases, when eggs or nestlings are lost in early development, birds build new nests for successive clutch (Kuźniak & Tryjanowski 2003;Lefranc 2023).
Moreover, due to the low philopatry in the Redbacked Shrike, it was observed that in large areas rich in suitable habitats birds can change breeding places from year to year (Tryjanowski et al. 2007).
The lack of philopatry is not related to the breeding success of local birds and/or the size of the study area.Consequently, it was then suggested that nesting in the same breeding area in successive years is an effect of an adaptation to a relatively high population density at the landscape scale (Yosef 1994;Tryjanowski et al. 2000Tryjanowski et al. , 2007;;Antczak et al. 2009;Polak 2016;Kwieciński et al. 2016;Morelli et al. 2019;Pustkowiak et al. 2021).This short note describes our observations of three instances where the Song Thrush nest was reused by the Red-backed Shrike in an intensive agricultural landscape in Poland.

Material and methods
Field research was carried out in the breeding season (May-August) in western Poland in the agricultural landscape of Wielkopolska near Poznań (52°41′N, 16°94′E): one in the area of Kromolice (the total length of midfield avenues − 7.65 km; N 52.27123°,E 17.19362°) near the town of Środa Wielkopolska; the second in Srocko (the total length of midfield avenues − 8.75 km; N 52.1765°,E 16.74905°) near the village of Srocko Wielkie and the third in Piechanin (the total length of midfield avenues − 5.7 km; N 52.15476°,E 16.72905°) near the town of Czempiń.Midfield afforestation (midfield avenues) were at least 500 meters away from human settlements and 1000 m from compact forest complexes over 0.5 ha and were surrounded in 100% by agricultural fields, where cereals, maize, potatoes, and beets were cultivated.The research was conducted at five-day intervals.Over the course of the study from 2019 to 2022, we found and monitored a total of 88 Red-backed Shrike nests (more details are described in Pustkowiak et al. 2021).For the purposes of this short article for each nest, we calculated breeding success (percentage of occupied nests with at least one young reared in relation to the number of occupied nests with known breeding history) and the rate of nest failures (due to brood parasitism, predation, and desertion) that was calculated as the ratio of the failed nests divided by the total number of nests (Johnson & Shaffer 1990).

Results
The mean breeding success of all monitored nests (n = 88) was 58%, the predation rate was 43%, while failure due to the brood parasitism rate was zero.For the Kromolice research area (n = 18) nesting success was 56%, predation rate was 44%, for the Srocko (n = 56) was 64%, predation rate was 36% and for Piechanin research area (n = 15) was 53% and 47%, respectively.The brood parasitism rate and desertion was zero.
The first Song Thrush nest with a brood of the Red-backed Shrike was discovered near Kromolice on June 14, 2019.Unfortunately, all four eggs were destroyed by a predator (Figure 1b).It was built on the Willow Salix sp. and had a standard lining, similar to natural the Red-backed Shrike nests.
The second nest was discovered near Srocko on June 26, 2021.The Red-backed Shrike must have tried to adopt this nest (Figure 1c) as there were

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Z. Kwiecinski et al.The reuse of Song Thrush (Turdus philomelos) nests by the Red-backed Shrike (Lanius collurio) strings at its bottom, which are often used by the Red-backed Shrike in this area for building nests (see Figure 1d).The nest was built on a domestic apple tree Malus domestica.
The third Song Thrush nest, was found for the first time near the area of Piechanin on May 13, 2021.The nest was built on a large Wild Rose, Rosa sp., which was tangled with the Common hop, Humulus lupulus, and the Elderberry, Sambucus nigra (see Figure 1a).On July 5, we observed in this nest with an incubating the Redbacked Shrike female.Then, on July 21, we found 2-3-day-old the Red-backed Shrike chicks (see Figure 1e).On August 2, the Red-backed Shrike chicks successfully left the Song Thrush nest.(see Figure 1f).

Discussion
Breeding or breeding attempts by the Red-backed Shrike in old Song Thrush nests are extreme cases of nest reuse (Lefranc 2023).So far, this phenomenon has rarely been described in the literature with regard to open-cup nesting passerines.Therefore, such cases of nest reuse are difficult to explain (Lefranc 2023).Cohen (1952) recorded a Blackbird taking over a nest built by a Song Thrush after it had stopped using the nest, and Wysocki (2004) observed a case of a Blackbird using a Fieldfare Turdus pilaris nest abandoned after the loss of the Fieldfare clutch.Currently, common explanations of this phenomenon include lack of suitable nesting sites, reduction of the nest building cost, predator avoidance strategies, or minimising the likelihood of brood parasitism (Wysocki 2004;Batisteli et al. 2021;Mérő et al. 2022;Mikula et al. 2022).
However, in the case of the Red-backed Shrike, the deficit of breeding sites is unlikely, because many breeding areas and potential nesting sites still remain vacant in western Poland in the agricultural landscape (Tryjanowski et al. 2000(Tryjanowski et al. , 2007;;Antczak et al. 2009;Kwieciński & Pawlak 2015;Lefranc 2023).Apart from that, this behavior cannot be attributed to brood parasitism, as the Red-backed Shrikes have effectively minimized cuckoo parasitism (Tryjanowski & Golawski 2004;Adamík et al. 2009;Polak 2016;Tryjanowski et al. 2021).
Another aspect is that the absence of secure nesting sites can force birds to reuse nests, as reported for the Blackbird (Wysocki 2004).Nest reuse may be more frequent when nests are effectively concealed, implying that concealment is a key factor in selecting a nesting site (Wysocki 2004).However, we lack sufficient data to support this hypothesis in the case of Red-backed Shrike.Our findings reveal breeding success ranging from 53% to 64% and a predation rate from 36% to 47%.It appears that the exclusive explanation for nest reuse cannot be attributed to breeding success alone, especially considering the year-to-year variation.Our results are consistent with the for this species also from other places Poland (Europe) (Tryjanowski et al. 2000(Tryjanowski et al. , 2007;;Antczak et al. 2009;Polak 2016;Kwieciński et al. 2016;Lefranc 2023).In addition, thrushes build larger nests compared to shrikes (although the nests are generally similar), making thrush nests potentially more detectable by predators.Therefore, the safety of the nest is an unlikely explanation (Wysocki 2004).
Moreover, we had to speculate why the Redbacked Shrike did not reuse nests of other Redbacked Shrike pairs after their successful breeding or the brood loss.Firstly, the degree of the nest damage after the nestling have left it was so high that their reuse would be unprofitable and could result in the loss of the clutch (Deeming & Mainwaring 2015;Kwieciński & Pawlak 2015).Secondly, parents could have avoided laying new eggs in nests that had been depredated earlier (Wysocki 2004;Batisteli et al. 2021;Mérő et al. 2022).
In favourable conditions, the Red-backed Shrike can build its nest within 3-4 days, but generally it takes 6 days (Kuźniak & Tryjanowski 2003;Lefranc 2023).It seems likely that the Red-backed Shrike reduced the time required for selecting a site and building a nest by reusing a Song Thrush nest (Wysocki 2004;Batisteli et al. 2021;Mérő et al. 2022).
Second clutches replace broods in case of the first clutch failure.The three broods of the Red-backed Shrike found in thrush nests were likely replacement clutches (as evident from the provided dates).The time interval between clutch loss and re-nesting spans a minimum of 2 weeks.Consequently even a small reduction in the time required for nest construction can carry significant and sometimes even crucial in the decision to nest again (Tryjanowski et al. 2000(Tryjanowski et al. , 2007;;Tryjanowski & Golawski 2004;Antczak et al. 2009;Morelli 2012;Mainwaring & Hartley 2013;Mainwaring et al. 2015;Polak 2016;Lefranc 2023).This is perhaps why the shrikes 78 Z. Kwiecinski et al. choose this nesting approach -to save time and energy.
Finally, we also wonder why the Red-backed Shrike did not reuse nests of other bird species, such as the Blackbird, even though they were available in all three cases reported in this study.In the area of Kromolice there were 12 Blackbird nests available simultaneously in 2019, while in Srocko there were 6 in 2021, and 7 in Piechanin.We suppose that the reasons might be related to the structure and composition of the nest, specifically the inner layer of the nest cup, because the Blackbird constructs its nest cups using mud, whereas the Song Thrush uses a mixture of rot, saliva, and soil, which dries to form a harder surface.As a result, Blackbird nests retain more water during prolonged rainfall, while Song Thrush nests remove moisture more quickly (Deeming & Mainwaring 2015).It is possible that the Red-backed Shrike, being a thermophilic species, prefers drier the Song Thrush nests and avoids wetter nests of the Blackbird (Deeming 2011;Portugal et al. 2014).

Figure 1 .
Figure 1.(a) The third Song Thrush nest was built on a large Wild Rose.(b) The first Song Thrush nest with a brood of the Red-backed Shrike.(c) The second nest, and attempt to adoption the nest of the Song Thrush.(d) Strings used by the Red-backed Shrike to build a nest.(e) The Red-backed Shrike nestlings in the Song Thrush's nest.(f) Young the Red-backed Shrike just before leaving the nest.