Life-history theory is structured around the idea that the evolution of suites of traits meant to maximize an individual’s fitness constrain each other and consequently generate trade-offs (Stearns, 1992; Reznick et al., 2000; Roff and Fairbairn, 2007). With time and energy being limited, behavioral trade-offs occur when animals must strike a balance among behaviors that are energetically costly or require substantial time. The habitat in which an individual lives has been shown to modulate trade-offs between behaviors, because the environmental context determines an animal’s time and energy budget, and the extent to which behaviors constrain each other (Gunnarsson et al., 2005; Holtmann et al., 2017). Resource availability, conspecific density, predator presence, and heterospecific interactions are among the contextual factors that determine an individual’s capacity to maximize fitness linked traits (Linden and Møller, 1989; Martin, 1995), all of which are being altered by global anthropogenic change (McKinney, 2008; Grimm et al., 2008b). Supporting the expansion of species to urban habitats can maintain urban biodiversity as human expansion continues (McKinney, 2002; Grimm et al., 2008b). However, urban habitats present wildlife with an altered abiotic and biotic landscape in which to live and reproduce (Chace and Walsh, 2006; Grimm et al., 2008a) that may modify the trade-offs among life-history traits.

Many species of songbirds have readily adjusted to cities, but breeding adults likely face new or exaggerated challenges not present in their native habitats. One way that animals can adjust to novel conditions is by shifting how they resolve life-history trade-offs. Urban habitats contain anthropogenic predators (e.g., domestic cats) (Loss et al., 2013; Rosenberg et al., 2019) that influence survival rates of both offspring and adults (Baker et al., 2008; van Heezik et al., 2010). These habitats are more fragmented than native habitats with different resource distribution and availability (Saunders et al., 1991; Chace and Walsh, 2006; McKinney, 2006). Finally, obligate brood parasites tend to follow fragmentation and human disturbance (Brittingham and Temple, 1983; Robinson et al., 1995; Chace et al., 2003; Rodewald, 2009; Faaborg et al., 2010) and may be more prevalent in urban communities. Comparing the behaviors of species in which individuals maintain breeding territories in both urban and rural habitats (termed ‘urban adapters’; Blair, 1996; McKinney, 2002; Shochat et al., 2006; Bonier, 2012), can reveal how individuals re-allocate effort into different life-history behaviors in response to novel urban habitats (Blair, 1996; McKinney, 2002; Shochat et al., 2006; Bonier, 2012).

One trade-off that could be shifted in urban habitats is the balance between territoriality and paternal care in male songbirds. During the breeding season, male songbirds choose and defend territories with quality nesting substrates and resources needed to survive and rear young (Stamps and Krishnan, 1997; Garcia and Arroyo, 2002). In theory, the highest quality males should be the most aggressive and best able to defend the highest quality breeding territories (Otter and Ratcliffe, 1996; Garcia and Arroyo, 2002; Scales et al., 2013) leading to increased fitness (Otter and Ratcliffe, 1996). Urban habitats often have fewer available breeding territories and resources compared to rural habitats (Juárez et al., 2020), and urban habitats likely favor a more aggressive phenotype (Duckworth, 2008; Foltz et al., 2015; Juárez et al., 2020). Indeed, one behavioral pattern found in many urban populations (Lowry et al., 2013; Renthlei et al., 2017; Dammhahn et al., 2020) but especially in songbirds, is increased male conspecific aggression (Evans et al., 2010; Fokidis et al., 2011; Scales et al., 2011; Atwell et al., 2014; Davies and Sewall, 2016). While an exaggerated aggressive phenotype may facilitate territory acquisition in urban habitats, investment in aggressive behavior could result in a trade-off with other energetically costly behaviors, such as parental care (Ketterson and Nolan, 1994; Ketterson and Nolan, 1999; Duckworth, 2006; McGlothlin et al., 2007). For socially monogamous urban songbirds, any trade-offs in breeding behaviors (i.e., territoriality and parental care) can have consequences for social partners because mates cooperate in rearing young (Gunnarsson et al., 2005; Holtmann et al., 2017). Thus, if urban habitats drive shifts in how male songbirds resolve life-history trade-offs, there may also be consequences for their female partners and the condition and survival of their offspring (Marzluff, 2001; Duckworth, 2006; Grimm et al., 2008a; Scolozzi and Geneletti, 2012).

Many territorial songbird species establish social partnerships during the breeding season to provide biparental care to their offspring (Westneat, 1987; Arcese, 1989; Griffith et al., 2002). This social monogamy allows partners to split the energetic demands of reproduction, so the burden of care does not fall on one individual (Møller and Birkhead, 1993; Griffith et al., 2002). However, female’s higher energetic investment, and guaranteed genetic relatedness to offspring, means they often show increased reproductive investment compared to males (Westneat, 1987; Arcese, 1989; Griffith et al., 2002). Indeed, prior studies of the consequences of male aggression have highlighted impacts on mates and offspring, including compensatory behaviors and reduced fledging success (Whittingham et al., 1994; Saino and Møller, 1995; Paredes et al., 2005). More aggressive male house finches, for instance, provide less paternal care and females compensate by providing more care (Badyaev and Hill, 2002). While females can often compensate for reduced care by their male partners, harsh environmental conditions that make compensation impossible can result in smaller offspring or reduced fledging success. If increased aggression in urban males constrains their ability to provide paternal care, this could increase the demands on their social partner to provide greater maternal care.

Here, we investigated the effects of urbanization and the associated increase in male aggression on the parental care provided by males, the care provided by their mates, and the condition and fledging success of their offspring by comparing adult behavior and nestling outcomes among wild song sparrows (Melospiza melodia) in urban and rural habitats. Song sparrows are a commonly studied urban adapter that, like many songbird species, are open cup nesters with altricial young, for which biparental parental care is especially vital (Nice, 1943; Smith and Roff, 1980; Roff et al., 2005; Martin and Briskie, 2009). We hypothesized that urban males, which are reliably more aggressive toward conspecifics, may resolve trade-offs between territoriality and paternal care differently than their rural counterparts and provide less care to offspring. We expected that their social partners would compensate to maintain offspring fitness or, if they could not, nestling growth and survival would be reduced. Specifically, we predicted that urban males would provide less care compared to rural males. We also predicted that maternal care would be positively correlated with social partner aggression, and negatively correlated with paternal care. Finally, we predicted urban nesting success would be lower, and urban offspring would be underdeveloped near fledging compared to rural birds.

Time and energy are limiting resources for seasonal breeding species that have a narrow window to mate and raise their offspring, and decisions on how individuals allocate these resources can be influenced by the environment. For example, greater male aggression may shift an existing trade-off between paternal care and territorial defense, such that more aggressive males provide less paternal care (Saino and Møller, 1995; Badyaev and Hill, 2002; Pryke and Griffith, 2009). We hypothesized that this trade-off could result in urban male song sparrows, which are reliably more aggressive, providing less paternal care. Decreased male care could, in turn, force females to increase maternal investment. However, if females are unable to compensate for reduced paternal care, offspring condition and survival could decrease. Contrary to the described theoretical framework, we found no evidence that urban male song sparrows sacrificed parental visitation for territorial aggression. Urban males did not have lower visitation rates than rural males at any nestling age or time of day ( Figure 2 ). In fact, we found that the more aggressive urban males visited the nest more often than their rural counterparts and began feeding earlier in the day ( Figure 2 ). Additionally, there was no correlation between male aggression and parental visitation rates for males or females within or across habitats ( Figure 6 ; Lane and Sewall, 2022). This refutes both the idea that urban males are facing a trade-off between territorial defense and parental care, and that urban habitats compromise male parental care. Previously, researchers found that urban male songbirds in this population had better body condition and access to greater fat reserves compared to rural (Goodchild et al., 2022), suggesting that birds in urban habitats are of higher quality than their rural counterparts and are therefore released from constraints experienced by other birds (Reznick et al., 2000). Additionally, urban males likely have different time and energy budgets compared to rural. Our previous research has shown that urban song sparrows have decreased conspecific density and increased territory size compared to rural ones (McKinney, 2006; Davies and Sewall, 2016; Juárez et al., 2020). With larger territories to defend, but decreased conspecific density, urban male song sparrows may be more aggressive yet have fewer territorial encounters with neighbors each day. Though future studies of time budgets for urban and rural birds would be required to evaluate this possibility, we can conclude that neither rural nor urban song sparrows seem to be under sufficient constraint to force a behavioral trade-off between our measures of aggression and paternal care. However, it is important to point out that some of the most aggressive males in the population provided the least amount of care ( Figure 1 ). It may be possible that with an even larger sample size the extremes in behavior would be detected and a trade-off between male aggression and parental care could be established.

Female visitation rates were not significantly different across habitats, but there was a nonsignificant trend for urban females to visit the nest at a higher frequency ( Figure 4 ). Specifically, urban females increased visitation rates during the day compared to rural, while rural females are more active at night compared to urban females ( Figure 4 ). We were unable to confirm this night behavior was not an artifact of the RFID systems through video recordings because we did not have access to night vision equipment. However, we would expect to see similar patterns of behavior at night in both habitat types if the RFID systems were causing an artifact. Understanding nocturnal time budgets around species with different life-history strategies, such as cavity or open-cup nesting species, is a current gap in our understanding of animal behavior (Graham et al., 2017; Hau et al., 2017). Future studies examining daily patterns of behavior across urban and rural habitats in species with different life-history traits could determine which behaviors are adaptive under different environmental conditions and confirm or refute our surprising observation of nocturnal activity in rural song sparrows.

Since urban male song sparrows provide as much, if not more, paternal care as their rural counterparts we found no evidence that females were compensating for their mates. In fact, we found a strong positive relationship between parental care provided by social partners ( Figure 5 ). Females that visited the nest more often each day were paired with males that did the same. Collectively, these parental care findings also refute the hypothesis that urban females are compensating for their social partners; rather, song sparrows seem to match their reproductive effort with that of their partner. This behavioral matching raises the hypothesis that song sparrow social pairs express assortative mating and/or matching of parental behaviors within the brooding periods (Wolfenbarger, 1999; Westneat et al., 2011). Our finding contrasts with a previous study in a cavity nesting species that found that urban birds showed decreased coordination in provisioning behavior compared to rural (Baldan and Ouyang, 2020). Rather, male song sparrows at our urban study sites maintain higher territorial aggression and high parental visitation, presumably because time and energy resources allow them to do so.

Song sparrow nestlings in urban and rural habitats had similar body size (tarsus and mass; Figure 6A ) before fledging, suggesting that urban habitats are not negatively affecting offspring condition before fledging. However, offspring body mass ( Figure 6B ) and tarsus length near fledge were significantly smaller in parasitized nests compared to unparasitized nests. Additionally, in the subset of nests that avoided predation, hatching and fledging success of song sparrow nestlings was significantly reduced in parasitized nests compared to unparasitized nests. Obligate brood parasites, such as brown-headed cowbirds, lay their eggs in the nest of a host species, forcing the host to raise the nestling, often to the detriment of the parent and their offspring (Ortega et al., 2005; Faaborg et al., 2010). Nestlings that hatch later often die of malnutrition in nests with brood parasites, and only the largest, most robust host nestlings survive to fledge (Payne, 1977; Remeŝ, 2006). This may explain why nestling body size and mass at fledge was reduced, and why the number of host offspring that were able to hatch and fledge from the nest were significantly lower in nests with brown-headed cowbirds. Collectively, this suggests that urban parental care is adequate, if not elevated, compared to that of rural, but the elevated rates of brood parasitism in urban habitats still compromise their ability to fledge a full clutch of offspring, and reduce nestling body mass and size at fledge.

We found that urban nests had higher daily and annual survival rates across all five years of our study compared to rural ( Table 1 ). Additionally, we found that urban song sparrows had greater nesting, hatching, and fledging success, and began nesting significantly earlier (16 days ± 2.27 days) compared to rural. This higher nestling survival was driven by significantly lower nest predation rates compared to rural. Our findings add to a growing body of evidence that, despite many studies demonstrating increased predator presence in urban habitats, nest predation rates decrease in urban settings compared to rural habitats (Eötvös et al., 2018; Fischer et al., 2012; Gering and Blair, 1999; Shochat et al., 2006). Nest predation may be lower in urban environments because predators can exploit anthropogenic food sources such as trash cans and human food supplementation (i.e., the Predator Paradox hypothesis; Fischer et al., 2020; Seress and Liker, 2015). Alternatively, or additionally, snakes are often important nest predators, and their conspecific density decreases in urban habitats (Klug and Jackrel, 2010). The difference we found in nest predation rates between habitat types could also influence the differences in urban parental visitation rates (Lyon and Montgomerie, 1987; Martin and Ghalambor, 1999; Eggers et al., 2006). Previous research has experimentally shown that increased nest predation pressure decreased nest visitation rates (Ghalambor and Martin, 2002; Mouton et al., 2020). Therefore, it is possible that rural birds limit their visitation rates to their nests to reduce the risk of attracting nest predators. Collectively, these results suggest that some urban generalist species, such as song sparrows, can have greater nesting success in urban habitats compared to rural ones. Resolving the impacts of abiotic aspects of urbanization, nest predation, and nest parasitism risk on parental behavior in songbirds would require future studies that experimentally manipulate these variables using brood size manipulations or manipulations of parental effort.

There is an underlying assumption in many urban ecology studies that urban habitats are detrimental to animal fitness compared to the rural, less disturbed environments. Our data join an increasing number of studies showing this is not the case for all species, and that urban habitats provide benefits to species that are able to invade these novel environments (McKinney, 2002; McKinney, 2008; Chamberlain et al., 2009). We found that more aggressive urban male song sparrows did not sacrifice parental care for territorial aggression. Consequently, their social partners are not suffering increased energetic demands to care for young. This indicates that either song sparrows do not face such a trade-off, or they are released from it in urban habitats. In fact, urban males visited the nest more often than their rural counterparts, a pattern also observed in females though the relationship was not significant. These behavioral differences could be the result of decreased nest predation pressure in urban habitats. Few studies have examined the effects of urbanization on parental care, and these studies have focused on incubation behavior in cavity nesting species (Baldan and Ouyang, 2020; Heppner and Ouyang, 2021; Hope et al., 2022). Hope et al., 2022 found that urban great tits (Parus major) increased overall incubation, yet decreased incubation bout rates compared to rural. Heppner and Ouyang (2021) found the opposite, with urban house wrens (Troglodytes aedon) decreasing incubation time and increasing bout rate. The limited number of studies in this area and their disparate results further illustrates the need for research on the effects of urbanization on parental care, and the importance of including species with diverse life history traits.

We observed a negative effect of brood parasitism on the hatching and fledging success of parasitized nests compared to unparasitized. However, this impact is minimal relative to the much higher nest success in urban habitats and the high nest predation in rural habitats. Understanding the effects of urbanization requires understanding community dynamics, including the interactions between brood parasites, predators, and their hosts. Previous studies found that urban adapters, such as the songbirds in this study, receive benefits such as increased food, water, maintained breeding substrates, and respite from predators (McKinney, 2002; Chamberlain et al., 2009; Shochat et al., 2010; Stracey and Robinson, 2012; Soulsbury and White, 2016; Reynolds et al., 2019). Collectively, our findings align with these studies, and suggest that, if given the space and opportunity to breed and reproduce, some generalist urban adapters and the species that rely on them can flourish in urban environments.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The animal study was reviewed and approved by Institutional Animal Care and Use Committee of Virginia Tech.

SL and KS conceived the ideas and designed methodology. SL, IV, VB, CL, and TF collected the data. SL and CG analyzed the data. SL, CG, and KS led the writing of the manuscript. All authors contributed critically to the drafts and gave final approval for publication.