Introduction

Historical land transformation for cultivation gave way to an explosion of urbanisation after the Industrial Revolution. The United States has led this trend, growing from 257.52 km² of urban area in 1760 to 3,883.88 km² in 1840 at the end of the Revolution, reaching 224,456.27 km² in 2023 [1]. By 2050, one in seven people are expected to live in an urban area, primarily driven by socioeconomic factors. Combined with a growing global population, it is likely that urban areas will continue to expand in the future, especially if countries such as New Zealand persist in growing horizontally rather than vertically. With shrinking natural habitats, more species will likely have to adapt to urbanisation. This is particularly true for birds, as an estimated 20% of their global population already resides in urban areas [2]. Birds that successfully live in cities have adapted to this very different environment. Their niches may become very distinct from those of their rural counterparts. It begs the question of whether the bird is the same as it once was. Do birds still forage for the same food as they did in the wild? Are they still singing the same song? Do the birds still interact with the same neighbours, or have species they interact with been unable to acclimatise to this urban habitat?

Research has been conducted on the impacts of urbanisation on bird populations, including species composition [2], feeding [3], nesting habits [4], migration patterns [5], and bird song [6]. A relatively unexplored area of research is how urban stressors—such as artificial lights, vehicular noise, and pests—impact avian sleep. This could be because the function of sleep in birds is still not well understood despite being expressed in many genera. Sleep is needed for essential functions such as long-term memory, attention, cognition, and communication [7-8]. There is still a lot of uncertainty about the biological processes that occur during sleep that allow for these functions and why nonresponsiveness is necessary. Kamya Patel’s PhD thesis addresses how sleep disturbance caused by urban stressors such as light and mechanical disruption in adult and juvenile zebra finches (Taeniopygia guttata) impacts activity levels, vocalisations, and hormones. This paper will focus on the effects of chronic sleep disturbance by artificial light at night (ALAN) on the activity levels of zebra finches.

Methods

Male zebra finches (n=7) were recorded over 13 days to investigate the effects of artificial yellow light on their behaviour and vocalisations. This paper utilises a third of the annotations, resulting in missing data. The yellow light experiment was from the 22nd of August to the 4th of September, 2024. The days annotated were the 22nd (baseline), 25th (treatment), 28th (treatment), 1st (treatment), and 4th (recovery). The birds had a microphone pack attached to them and were filmed by a camera during the two experiments. The birds were housed in a separate cage from the others, equipped with a large overhead light, in a room with time-controlled lights to simulate a 12-hour light-dark cycle. The pretreatment and post-treatment phases were the three days directly before and after the experiment and were recorded to observe baseline and recovery. The overhead light was turned off during these periods, and only the room light-dark cycle occurred. The treatment phase was seven days long, and the overhead light was turned on with yellow light during the dark period of the 12 hour light-dark cycle.

Five-minute clips were taken at four time points (0:30, 6:30, 12:30, and 18:30) for every day of each 13-day experiment. The clips were selected to be as close to the start of the time block as possible, but a one-hour window was permitted to allow later observations if birds were not visible or visitors had come into the room. These observations were then grouped into daytime (06:30 and 12:30) and nighttime (00:30 and 18:30) to compare differences in activity levels. Their behaviour was categorised into activity, resting, mild activity, and preening. Birds were categorised as active if they were moving, flying, feeding, drinking, or preening. The birds were resting if they were sitting or perching. Mild activity and preening accounted for uncertainty due to poor film quality and were removed from the results. The annotations were completed on BORIS [9] with data and then transferred to R [10] to create graphs and use a Tukey HSD post-hoc test to generate statistics.

Results

Yellow light experiment
Changes in the average activity levels were seen across the experiment. During baseline observations, birds were active for an average of 172.4 s (Q1 = 14.1 s, Q3 = 221.1 s) during the day and 0 s (Q1 & Q3 = 0 s) at night. Activity during the day decreased from baseline to treatment (mean = 125.8 s, Q1 = 43.1 s, Q3 = 183.6 s) and increased during the night (mean = 74.4 s, Q1 = 0 s, Q3 = 131.3 s). There was also more variation in daytime activity levels during treatment (Figure 1). These changes did not persist into recovery; the recovery activity levels were like those of the baseline. The birds were active for an average of 194 s (Q1 = 149 s, Q3 =245.6 s) during the day and 0.4 s (Q1 = 0, Q3 = 0.6 s) at night. The IQR for recovery was also smaller than the treatment.

The activity levels were inconsistent across days within the treatment period (Figure 2). Day and night activity levels were lower on the 1st day of treatment than on the 4th day of treatment. There was a drastic drop in activity levels for both day and night on the last day of the treatment, the 7th day.

The F-value for the Tukey HSD post-hoc pairwise comparisons is 12.833, and the p-value was 0.000009. There were three statistically significant differences between phases in the experiment. The difference between nighttime activity between treatment and baseline was 74.4 s (CI: 1.1- 147.6 s; Table 1) and had a p-value of 0.04. The average daytime activity for recovery was 68.5 s more than treatment (CI: 5-131.9 s; Table 1), with a p-value of 0.02. The difference between recovery and treatment during the nighttime was also significant (p = 0.03; Figure 2), with recovery being 70.7 s (CI: -139 - -2.36 s) less than treatment.

Discussion

Impact of yellow light on activity and sleep
The statistical analysis suggests that yellow light impacts the birds’ activity. The birds were significantly more active during the night in treatment than in baseline or recovery (Table 1). This coincides with other studies showing that ALAN increases activity at night [11-15]. This means the birds are spending less time resting and, consequently, sleeping. This may explain why they were resting more during the day in treatment than in recovery as they were trying to make up for lost sleep. As diurnal species, zebra finches exhibit sleeping patterns similar to humans. They sleep most of the night, experiencing rapid eye movement (REM) and non-REM (NREM) sleep [16]. REM sleep in zebra finches is characterised by eye and low-amplitude head movements and is shorter and less frequent compared to mammalian REM sleep. NREM sleep in zebra finches can be broken down further into slow wave sleep (SWS) and an intermediate stage (IS) between SWS and REM. Studies show that ALAN increases the fragmentation of REM sleep and causes an overall decline [15]. This is significant because sleep is important for daytime performance, including cognition, motivation, attention, and song output [8].

Other experiments that measured brain activity for Australian magpies (Cracticus tibicen trannica) and domestic pigeons (Columbia livia) showed a decline in NREM and REM sleep when exposed to amber light at night [17]. However, this decline in NREM sleep was quickly recovered during the recovery period. This was not seen in pigeons, where sleep continued to be impacted during recovery. This suggests that the impact of amber light is species-specific but acute in some species. The response of the magpies corresponded to the zebra finches, who demonstrated a change in activity levels during the yellow light treatment but a fast recovery back to baseline activity levels (Figure 1). However, the magpies were only exposed to 4 hours of artificial light, unlike this experiment’s birds, who were exposed to 12 hours.

Impact of other colours on sleep
The PhD project also contains experiments on disruption using different colours of artificial light, such as white and blue. Responses to ALAN likely occur due to deep brain photoreceptors analysing light levels and regulating circadian rhythms [18]. These receptors contain a pigment called melanopsin, which primarily responds to small light wavelengths found in the blue light section of the visual light spectrum [17]. This may explain why the zebra finches recovered quickly after the treatment period to baseline activity levels (Figure 1), as yellow light is too large of a wavelength to interact with melanopsin significantly. When exposed to blue-rich light, zebra finches exhibit higher nighttime activity than mild blue [19]. Research done on ALAN and sleep shows an impact on sleeping behaviour. However, the colour of the light will impact certain species differently. White light is also known to be more disruptive for other species than yellow light, like Australian magpies (C. trannica). However, there was no difference in nighttime sleep disruption in domestic pigeons (C. livia) [15]. There appears to be a lot of variability in responses to different light colours based on species, indicating that research done on the effects of yellow and white light on other species may not generalise to zebra finches. However, as white light contains all wavelengths of the visual light spectrum, we might observe similar responses in the blue and white light experiment as in the previous zebra finch study [19].

Variation in responses
There was much more variation in active duration across the test subjects during treatment than in baseline or recovery across all time blocks (Figure 1). The confidence intervals for the statistical analysis indicated significant differences that could be as small as 1.1 s or as large as 147.6 s (Table 1). This could be because all the treatment observations were grouped, and the changes were observed over time. This is suggested by the time series graph (Figure 2), where the active duration at each time slot for the treatment days was inconsistent. However, there is not enough data to confirm this. Another explanation is that the yellow light is disturbing the birds’ sleep; however, their response to sleep disturbance varies between individuals. There could be some individuals that are more resilient to the treatment. Test subjects were received from many different sources, and many individuals’ ages and health could not be determined, which could have impacted the variation in responses observed. It could be that the time they choose to recover lost sleep varies between birds, which was not captured in the chosen time blocks for analysis.

Limitations
The data used for this report is mostly incomplete, meaning the results are likely unreliable. More time blocks could help address high variability caused by unobserved rest recovery. The use of film also resulted in a few issues. Nighttime observations were quite pixelated, meaning that some individuals were not visible, or their behaviour was indecipherable. This was helped by adding the mild activity category, which accounted for uncertain behaviour. However, human bias and error by the observer are still prevalent.

Conclusion

The preliminary results indicate that yellow artificial light at night impacts male zebra finches’ activity levels. Whilst this single report will not have any impact, the larger PhD project will be able to inform the effects of urban stressors on adult male zebra finches in their activity levels, hormones, and song output. By analysing their song, hormones, and activity, this project will be able to fill in knowledge gaps in the relationship between disruptive factors, sleep, internal function, and performance. Investigation into how the birds respond to different light colours may be able to suggest alternative light colours for public lighting, such as street lamps that cause light pollution in urban parks where many birds reside. This research will be relevant for cities like Auckland, which has high levels of urban sprawl and a large endemic urban bird population.