Human impacts on nature have increased dramatically in the last 20 years, with over two thirds of global land area now facing moderate or intense human pressure (Williams et al., 2020). These pressures (e.g. hunting), along with related habitat changes have vastly altered wildlife communities through extirpations, extinctions, and changes to ecosystem functioning (Amir et al., 2022a; Dirzo et al., 2014; Haddad et al., 2015). These direct impacts are also accompanied by indirect impacts that change the behaviour of remaining wildlife, posing a significant threat to species fitness and interactions as well as overall biodiversity and ecosystem functioning (Gaynor et al., 2018; Sih et al., 2011; Zanette and Clinchy, 2019).

Human presence and habitat disturbance (together ‘human pressure’ hereafter) can generate a deep fear in animals and this “landscape of fear” can influence their diel activity patterns and use of space (Laundre et al., 2010; Zanette and Clinchy, 2019). While there are some studies showing little impact from human pressure on animal behaviour (Pardo et al., 2021), in general medium and large mammals tend to avoid humans by increasing their nocturnality (Gaynor et al., 2018; Mendes et al., 2020) or by reducing their home range closer to urban environments (O’Donnell and Delbarco-Trillo, 2020). Bird behavioural responses to humans, on the other hand, are less understood and may differ from those of mammals, especially in the tropics (Chace and Walsh, 2006; Zanette and Clinchy, 2019). A study on Chilean forest birds found that several species shifted their diel activity peaks from early morning in old-growth forests to noon in plantations, and to the afternoon in logged forests (Fontúrbel et al., 2021) and another study on Hawaiian forest birds showed increased diurnality in fragmented landscapes (Smetzer et al., 2022). Moreover, a global review of the impacts of hunting and degradation on mammal and bird populations found consistent declines in mammal abundance as degradation and hunting increased, while birds showed the opposite trend (Osuri et al., 2020). Another global review of hunting in tropical forests found that birds and mammals both showed significant declines in hunting-accessible areas, although 25% less for birds than for mammals (Benítez-López et al., 2017).

There are different potential reasons why the ecology of fear may manifest itself differently across taxa (Zanette and Clinchy, 2019). Fearful animals may perceive safety differently depending on their ability to hide versus escape, with most birds preferring higher daytime visibility and ease of escape via flight, while terrestrial mammals may often view nocturnal stealthy foraging as safer (Chace and Walsh, 2006; Zanette and Clinchy, 2019). Body size and diet are other factors that can influence species responses to human pressure (Dehaudt et al., 2022; Dunn et al., 2022; Hendry et al., 2023; Osuri et al., 2020). Species with larger body sizes are more susceptible to human pressures, particularly hunting (Benítez-López et al., 2017; Dirzo et al., 2014; Osuri et al., 2020), so these species could be more prone to change their activity patterns in areas with human presence (Benítez-López et al., 2017; Osuri et al., 2020). Habitat degradation and conversion can also drive changes in species behaviour by altering resource availability for some dietary guilds (Osuri et al., 2020). In particular, changes in diurnality patterns may be the consequence of altered foraging patterns due to changes in food availability in degraded habitats [e.g. due to logging, edges, fragmentation (Fontúrbel et al., 2021; Norris et al., 2010; Smetzer et al., 2022)].

Understanding behavioural responses of birds and mammals to human pressure is an important knowledge gap. Synchronous or opposing guild-level shifts among mammals and birds can shape consequences for food-web dynamics and ecosystem functioning (Gaynor et al., 2018; Zanette and Clinchy, 2019), and can also have repercussions for species interactions through changes in predatorial behaviour and increased competition between ecologically similar species (Amir et al., 2022b; Mendes et al., 2020; Zanette and Clinchy, 2019). Here, we investigate the impacts of human pressure on mammal and bird behaviour in Colombia, a megadiverse country that has seen a doubling of human pressure levels in the last 50 years (Correa Ayram et al., 2020). We used camera trapping along a gradient of human pressure in the Colombian Llanos region to measure changes in diel activity patterns of understory birds and mammals. We expected to find an increase in nocturnality in mammals and in diurnality for birds, a more marked shift in diel activity patterns for larger species or those more susceptible to hunting, and consistent responses from species with similar dietary characteristics.

We found evidence of significant activity shifts in response to human pressure for five of the 11 species of understory birds (45%) and for five of 14 species of the understory mammals (36%) that had enough records to be assessed. For mammals, all five species became more nocturnal in disturbed areas (Table 1, Fig. 3a & 4a), while in birds four became more diurnal while one became more nocturnal (Table 1, Fig. 3b & 4b). The average increase in nocturnality for mammals was 11.3% while the average increase in diurnality for birds was 7%. While these shifts are considerable, the statements “becoming more nocturnal” or “becoming more diurnal” do not mean that a species shifted its peak of activity from a period to another. Instead, it means that although the species main activity is still occurring during its natural activity period (i.e., night for nocturnal species and day for diurnal species), the proportion of day/night records has changed.

Fig. 3.

Shifts in diurnality of (a) five understory mammals and (b) five understory bird species in response to human pressure in three sites in the Colombian Llanos, from the most preserved sites (black point) to the most disturbed sites (arrow point) in which each species was recorded. Arrows to the right represent shifts toward diurnality, while arrows to the left represent shifts toward nocturnality. The species depicted are the ones that presented statistically significant changes in diurnality. Mammal and bird silhouettes were obtained from the PhyloPic public domain database (http://phylopic.org/).

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Fig. 4.

Changes in diurnal activity for (a) five understory mammals and (b) five understory bird species with increases in human pressure. We used generalized linear mixed models (GLMM) to evaluate the effect of human pressure on understory birds’ and mammals’ diel activity patterns, with the day/night records for each species as a binomial response variable and the Legacy-adjusted Human Footprint Index (HFI) for 2018 (Correa Ayram et al., 2019) as the predictor (x-axis). When the species was detected in a single site or the low sample size at a given site caused singular fit problems, we used generalized linear models (GLM) instead. The species depicted are the ones that presented statistically significant changes in diurnality. The blue circles are the independent day or night captures and the doted lines in the GLM show the 95% confidence intervals. Mammal and bird silhouettes were obtained from the PhyloPic public domain database (http://phylopic.org/).

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The four understory bird species which responded to human pressure by becoming more diurnal were the swainson’s thrush (Catharus ustulatus), the little tinamou (Crypturellus soui), the white-tipped dove (Leptotila verreauxi) and the amazonian motmot (Momotus momota) (Table 1). None of the species weigh more than a kilogram, all of them have omnivorous diets and one is opportunistically hunted (the little tinamou; SI Table 2). The pauraque (Nyctidromus albicollis), which weighs less than a kilogram, is an insectivorous species and is considered crepuscular, was the only bird that responded to human pressure by becoming more nocturnal. The eight other bird species did not present statistically significant activity shifts in response to human pressure.

The five mammal species which responded to human pressure by becoming more nocturnal were the paca (Cuniculus paca), the agouti (Dasyprocta fuliginosa), the common opossum (Didelphis marsupialis), the grey four-eyed opossum (Philander opossum) and the collared anteater (Tamandua tetradactyla) (Table 1). These species’ body weight ranged from less than 1 kg–13 kg. Two of the species have an herbivorous diet (the agouti and the paca), two have an omnivorous diet (the common opossum and the gray four-eyed opossum), and one is mainly insectivorous (the collared anteater). Three of these species - the paca, the aguti and the common opossum - are hunted (SI Table 2). The other nine species did not present statistically significant activity shifts in response to human pressure.

We found evidence of significant activity shifts in response to human pressure for 45% of the understory bird species assessed, with four of five shifts being towards more diurnal activity patterns. This is one of the first studies to our knowledge that reports such a behavioural change for tropical understory birds, and our findings align with emerging evidence of increased diurnality patterns for birds in disturbed subtropical landscapes (Fontúrbel et al., 2021; Smetzer et al., 2022).

None of the bird species that showed a change in activity patterns had a body mass higher than 500 g, and only one was opportunistically persecuted (the little tinamou). This suggests that the increases in diurnality for these bird species may not be strongly associated with direct human pressures like hunting, and instead may be more related to habitat disturbance. This may be because degraded landscapes have a higher variation in microclimate and increased reach of sunlight to the understory, which can affect birds circadian clocks and activity patterns (Fontúrbel et al., 2021; Lehmann et al., 2012). Additionally, degraded landscapes tend to have altered resource availability, including lower insect abundance (Vergara et al., 2021) and decreased fruit production (Jansen et al., 2021), which can result in increased daytime foraging in diurnal bird species, specially at times with better sunlight, to compensate for decreased food availability (Fontúrbel et al., 2021; Smetzer et al., 2022). Increased diurnality could also be associated to the increased nocturnality of certain mammal predators (or other animals, e.g. snakes) not investigated in this study. The pauraque was the only bird species that showed a significant increase in nocturnality in landscapes with high human pressure. This species has activity peaks at dusk and dawn, when natural light intensity is low, meaning light pollution could affect this species as has been reported for other nightjar species (Sierro and Erhardt, 2019) and mammals (Mendes et al., 2020).

For mammals we found a divergent behavioural response to human pressure from that of birds, with all significantly impacted species becoming more nocturnal. These results align with other studies that show increased nocturnality for mammals in areas with high human pressure (Gaynor et al., 2018; Mendes et al., 2020). One of the factors that can influence mammal diel activity is hunting, as three of the five species with significant activity shifts are hunted or persecuted. Species with diel activity change varied in body size from less than 1 kg–13 kg. These suggest that the increases in nocturnality may not be more prevalent in larger mammal species in the study area.

Our results show that human pressure goes beyond impacting species richness and abundance and can also have a marked impact in the activity pattern of multiple vertebrate groups. Bird species had a general shift towards diurnality, while mammal species towards nocturnality, revealing that different vertebrate groups can react distinctly to increased human presence and activity. Changes in activity patterns can increase activity overlap with certain predators and ecologically similar species. This could increase predation risk and competition for resources (Reilly et al., 2017). On the other hand, predators can be affected by the decrease of prey availability due to shifts on the preys daily activity pattern (Ordiz et al., 2017; Reilly et al., 2017). Several of the understory species that presented significant activity shifts in our study area are prey of other vertebrates like ocelots and pumas. These shifts in activity patterns can potentially affect the predator’s fitness and survival.

There is a need for more research on the impact of human pressure on vertebrates’ behaviour and the causes of such shifts, as they could result in marked changes away from natural patterns, with consequences for fitness, population persistence, community interactions, and evolution (Chace and Walsh, 2006; Gaynor et al., 2018), as well as for conservation planning (Caro, 2016; Gaynor et al., 2018). A better understanding of these interactions would help identify which species are particularly susceptible to behavioural changes, which human activities are the most detrimental and at which times of the day this overlap is happening. This information can be then incorporated into conservation planning, for example through the implementation of restrictions to certain human activities during times of the day when species of conservation concern are most active or when the likelihood of negative human-wildlife encounters is highest (Gaynor et al., 2018).

The use of other types of surveying techniques including sound recordings, transects and field sampling will be important to assess the potential changes in diel activity patterns for mammals and bird species that prefer to use the canopy or midstory levels. The availability of camera trap information is increasing exponentially, but usually the only information reported is in relation to species’ community composition and population densities (Antunes et al., 2022). Our analysis highlights the fact that impacts of human pressure on behavioural characteristics may vary between species, individuals and sites, and that this variation can have impacts on the ecology and conservation of these species. Future studies should focus on the impact of human pressure on the behavioural patterns of vertebrates (Gaynor et al., 2018; Sih et al., 2011), especially for birds and other groups for which information in this regard is scarce (Chace and Walsh, 2006; Fontúrbel et al., 2021; Smetzer et al., 2022) and for biodiverse regions that are poorly surveyed.

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.

P.J.N and B.G.V conceived the idea; P.J.N and C.M designed the study; A.D.P and L.H.R collected the data; P.J.N and C.M performed the analysis; P.J.N wrote the original draft of the manuscript; All authors contributed comments and edits to the final paper.