Tree Swallows (Tachycineta bicolor) are breeding earlier at Creamer s Field Migratory Waterfowl Refuge, Fairbanks, AK

Transcription

1 Tree Swallows (Tachycineta bicolor) are breeding earlier at Creamer s Field Migratory Waterfowl Refuge, Fairbanks, AK Abstract: We examined the average annual lay, hatch, and fledge dates of tree swallows (Tachycineta bicolor) breeding in Creamer s Field Migratory Waterfowl Refuge in Fairbanks, AK from 2000 through From 2000 to 2008, lay and hatch date exhibited a significant negative relationship, indicating that the timing of swallow reproduction has advanced by approximately a week during this period. We also compared the timing of laying in May, hatching in June, and fledging in July to three weather variables (monthly high, average temperature and total precipitation). We found a nearly significant relationship between lay date and the high temperature for May from 2000 through Our results are consistent with other long-term studies of the timing of tree swallow reproduction and suggest that climate change may be responsible for shifts in breeding dates. Introduction: Data from long term nest box studies of tree swallows (Tachycineta bicolor) have provided biologists with an opportunity to understand the effect of global climate change on avian nesting behavior. Dunn and Winkler (1999) discovered that tree swallows across North America were breeding, on average, nine days earlier in 1991 than in Furthermore, the authors found that earlier breeding was associated with increasing surface temperatures. Ardia et al (2006) have also reported that temperature affects swallow reproduction. Namely, the proportion of daily temperatures above physiological zero during laying was a strong predictor of early incubation. Ardia et al (2006) also reported that early laying and higher temperatures led to shorter incubation periods and increased hatching asynchrony. We studied a ten year dataset ( ) for the tree swallow population in Creamer s Field Migratory Waterfowl Refuge in Fairbanks, AK. The effect of global climate change is thought to be accelerated in arctic environments (Wormworth and Mallon 2008). Therefore, this data provides us with the unusual opportunity to compare any changes over time with those reported across North America (Dunn and Winkler 1999). We analyzed whether tree swallows in Creamer s Field were laying, hatching, and fledging earlier each year due to an annual increase in temperature during the months of May, June or July from 2000 through As in past studies, we can posit three predictions if rising temperatures associated with climate change are causing tree swallows to breed earlier in Creamer s Field. First, we predict that if swallows are breeding earlier each year, a linear regression would reveal a negative relationship between relevant breeding dates, such as lay, hatch, and fledge date, and year. Second, if a global warming effect is being felt in Fairbanks, we would expect a positive relationship between surface temperatures and year. Third, if there is an association between these increasing surface temperatures and earlier breeding dates, we can expect to see a negative relationship between breeding date and temperature. Materials and Methods:

2 Since 1999, the Alaska Bird Observatory has monitored tree swallow nestboxes in Creamer s Field Migratory Waterfowl Refuge in Fairbanks, AK (latitude 64, longitude 147). During the 2008 breeding season, individually-labeled nest boxes were available for birds to use. All boxes were located in grassy farm fields and surrounded by mixed boreal forest, with access to nearby water sources (e.g. artificial ponds for waterfowl). During a typical breeding season, clutches were initiated in late May, hatching occurred in mid June, and fledging in early July. While there has been some variation in the number of boxes monitored from year to year, the general protocol has remained consistent. Each year, boxes were monitored every 1-4 days from May through July. Observers typically began tracking boxes at the first appearance of nest materials and followed the fate of the nest through fledging. During each scheduled nest check, observers recorded the number of eggs, hatched chicks, or successful fledges. Nestlings were also banded and measured. We used the nest check data from each box to calculate the mean annual lay date, hatch date, and fledge date for the population. As swallows generally lay one egg per day (Robertson et al 1992), the date for which egg laying was initiated in a nestbox was calculated by back-dating the clutch (subtracting one day for each egg found in the nest). Hatch and fledge dates were based on the date at which nestlings were first discovered, or when older chicks were missing from the nest with no evidence of mortality. Given that boxes were only checked every 1-4 days, our estimated dates of hatching and fledging are less exact than lay date. Using data from , we ran a series of linear (least-squares) regressions to determine whether the mean annual lay date, hatch date, or fledge date showed any significant pattern relative to year. The prediction for global warming is that reproductive behavior should occur earlier each year. We next examined the pattern of monthly high, average surface temperature, and total monthly precipitation over the ten year study to quantify any positive associations indicative of climate change (e.g. monthly high increased across years). For significant relationships, we then compared lay, hatch or fledge dates to three different weather variables (monthly high, average surface temperature and total monthly precipitation) to determine if climate was related to reproduction. Lay, hatch, and fledge dates were compared to climate data for May, June, and July respectively. All regressions were run using Microsoft Excel The dataset from 1999 did not include lay, hatch, and fledge dates and consequently was excluded. Fledge data from 2007 was unavailable and fledge data from 2008 was excluded as an unusually cool, wet spring may have delayed the development and subsequent fledging of nestlings. Results: Our results indicate that tree swallows have been laying, hatching and fledging earlier in Creamer s Field from (Figure 1a-c). From , we found a significant negative correlation between lay date and year (n=9, F 1,7 = , r 2 = , P= ) as seen in Figure 1a. During this period, average lay date advanced eight days. A significant negative correlation was also found between hatch date and year from (n=9, F 1,7 = , r 2 = , P= ), shown in Figure 1b, and fledge date and year from , shown in Figure 1c (n=7,

3 F 1,5 = , r 2 = , P= ). Average hatch date advanced five days and average fledge date, seven days. As seen in Figure 2a, monthly high in May was positively correlated with year (n=10, F 1,8 = , r 2 = , P= ). While the relationship, shown in Figure 2b between year and average surface temperature in the month of May approached significance (n=10, F 1,8 = , r 2 = , P= ), Figure 2c shows that average May surface temperature and May high showed a strongly significant positive correlation from (n=10, F 1,8 = , r 2 = , P= ). As seen in Figure 3, we found a nearly significant negative correlation between lay date and monthly high temperature in May (n=9, F 1,7 = , r 2 = , P= ). When 2002 and 2004 were excluded from comparison of lay date and May high (due to sample sizes below 3 nestboxes), a significant negative relationship existed (n=7, F 1,5 =6.676, r 2 =0.5718, P=0.0492). A complete list of all comparisons made can be seen in Table 1. No other comparison of climate data and hatching or fledging dates were statistically significant (7<n<10; <F< ; <P< ). Discussion: Our results demonstrate that the tree swallows in Creamer s Field are laying, hatching and fledging earlier each year (Figure 1a-c). This change in breeding behavior has coincided with an increase in temperature (approximately.9deg/yr) between 1999 and 2008 (Figures 3, 2c). We also detected a nearly significant correlation between lay date and May high temperatures, a pattern generally similar to other investigations (Dunn and Winkler 1999). The lack of a negative correlation between hatch date and fledge date with June and July temperatures may reflect the relatively higher degree of measurement error in these parameters. Differences in the nest box checking protocol over the past 10 years resulted in estimates for hatching and fledging dates as high as ±4 days, compared to lay date, which could be back-calculated based on the number of eggs in the nest. There is growing evidence that climate change is being felt more intensely at arctic locations (Wormworth and Mallon 2008), such as interior Alaska. Using the available tree swallow data, it is possible to compare the magnitude of reproductive advancement between more temperate locations, such as the continental US, and Interior Alaska. For example, Dunn and Winkler (1999) reported an advancement in lay date of five days in thirty-two years (or 0.16 days per year) in a study area spanning N and W. By contrast, we have detected a change of eight days in nine years (at approximately 1.2 days per year) in interior Alaska (64 N, 147 W), a region not included in Dunn and Winkler s study. This difference may reflect a greater impact of climate change on our interior field site. This comparison is clearly an oversimplification, as Dunn and Winkler s dataset only included dates through 1999, whereas our analyses began with Another possibility is that both locations have experienced relatively rapid warming since Dunn and Winkler (1999) found a significant relationship between lay date and average surface temperature. In our study, we found a significant relationship between lay date and May high temperature. In interior Alaska, May high showed a more predictable rise across years than average surface temperature (Figures 2a, 2b), although both May high and surface temperature were strongly correlated (Fig 2c). The lack of

4 significant correlation between lay date and average surface temperature may reflect latitudinal differences with regard to how temperature variables affect nesting behavior. Our study made use of data collected by volunteers through an education program, demonstrating the usefulness of such a program in long-term studies of avian breeding behavior. Investigations of the effect of rising temperatures on reproduction are imperative given trends in global climate change. Changing in the timing of reproduction is considered one of the first signs of major climate change, preceding shifts in habitat and community (Wormworth and Mallon 2008). Continued monitoring of the population and comparison with other long-term datasets for tree swallow populations promises to reveal not only the general impact of climate change, but also how this impact varies in specific regions. Acknowledgements: The Alaska Bird Observatory in Fairbanks, AK Swarthmore College, in Swarthmore, PA. J. Hagelin, S. Sharbaugh T. Blake, S. Guers, A. Harding D. Ardia, D. Winkler, P. Dunn 2008 volunteers: S. Bristow, K. Stickel, Q. Evenson, T. Peter, R. Troyer, M. Eager, R. Roos, S. Fish, L. Devaney, and T. Dodge E. Stevens (weather data) Literature cited: Ardia, D.R., C.B. Cooper, and A.A. Dhondt Warm temperatures lead to early onset of incubation, shorter incubation periods and greater hatching asynchrony in tree swallows Tachycineta bicolor at the extremes of their range. Journal of Avian Biology 37: Dunn, P.O., and D.W. Winkler Climate change has affected the breeding date of tree swallows throughout North America. Proceedings of the Royal Society B 266: Robertson, R.J., Stutchbury, B.J., and R.R. Cohen Tree Swallow. In The Birds of North America, No. 11 (A. Poole, P. Stottenheim, and F. Gill, Eds.) Philadelphia: The Academy of Natural Sciences; Washington, D.C.; The American Ornithologists Union. Wormworth, J. and K. Mallon Bird Species and Climate Change: The Global Status Report version 1.0. Climate Risk Pty Limited, Fairlight.

5 y = x r 2 = Lay Date Year Figure 1a. Lay date (represented as Julian Date) and year were negatively correlated between (n=9, F 1,7 = , r 2 = , P= ). The slope of the equation indicates that swallows bred, on average, 1.2 days earlier days earlier each year. Numbers beside each data point represent sample sizes.

6 Hatch Date y = -1.05x r 2 = Year Figure 1b. Hatch date (represented as Julian Date) and year were negatively correlated between (n=9, F 1,7 = , r 2 = , P= ). The slope of the equation indicates that swallows hatched, on average, 1.05 days earlier each year. Numbers above each data point represent sample sizes.

7 191 Fledge Date y = x r 2 = Year Figure 1c. Fledge date (represented as Julian Date) and year were negatively correlated between (n=7, F 1,5 = , r 2 = , P= ). The slope of the equation indicates that swallows fledged, on average, 1.07 days earlier each year. Numbers above each data point represent sample sizes.

8 May High Temperature y = 0.923x r 2 = Year Figure 2a. Monthly high in May (in degrees Celsius) was positively correlated with year from (n=10, F 1,8 = , r 2 = , P= ). The slope of the equation suggests that monthly high has increased at a rate of.9 degrees each year.

9 60 Average May Surface Temperature y = x r 2 = Year Figure 2b. Average surface temperature (in degrees Celsius) in May and year approached significance between 1999 and 2008 (n=10, F 1,8 = , r 2 = , P= ).

10 Average May Surface. Temperature y = x r 2 = May High Temperature Figure 2c. Average May surface temperature and May high (both in degrees Celsius) showed a strongly significant positive correlation from (n=10, F 1,8 = , r 2 = , P= ).

11 Lay Date y = x r 2 = May High Temperature Figure 3. The relationship between May high temperature (in degrees Celsius) and average lay date (represented as Julian date) approached significance (n=9, F 1,7 = , r 2 = , P= ).

12 Table 1. This table enumerates all linear regressions explored in this study. The data are grouped by lay date, hatch date and fledge date. Figures provided in this report are given in bold. Two years (2002 and 2004) were excluded in analyses of lay date, as fewer than 3 nest boxes of data were available, making the estimates for these years unreliable. Some hatch and fledge comparisons excluded 2008 data, as unusually cold, rainy weather delayed reproduction once eggs had been laid. Comparison (y, x) N df NUM df DEN r 2 F P LAY DATE: Year, Lay Date Year, Lay Date excluding 02, 04* Lay Date, May High excluding 02, 04* Lay Date, May High Year, May High Year, May Average Surface Temperature Lay Date, May Average Surface Temperature Lay Date, May Precipitation Average May Temp, May High E-07 HATCH DATE: Year, Hatch Date Year, Hatch Date Excluding 08* Hatch Date, June High Year, June High Year, June Average Surface Temperature Hatch Date, June Average Surface Temperature Hatch Date, June Precipitation FLEDGE DATE: Year, Fledge Date Year, Fledge Date Excluding 08* Fledge Date, July High Year, July High Year, July Average Surface Temperature Fledge Date, July Average Surface Temperature Fledge Date, July

13 Precipitation