Temporal Patterns in Passerine Productivity Index in Fairbanks, Alaska.

12/11/2015

Amanda Spencer

RH: Spencer × Productivity Index from Creamer's Field Migratory Waterfowl Refuge

Temporal Patterns in Passerine Productivity Index in Fairbanks, Alaska.

AMANDA SPENCER University of Alaska Fairbanks

ABSTRACT Passerines, Dark-eyed Juncos (Junco hyemalis), Lincoln Sparrows (Melospiza lincolnii), and Orange-crowned Warblers (Vermivora celata), breed in interior Alaska’s boreal forest. Climate change is dramatically affecting the boreal forest (Hinzman 2005). Bird abundance can indicate changes in vegetation and changes in weather (Tauzer 2013). This study used the Creamer’s Field Migration Station (CFMS) in Fairbanks, Alaska banding bird data from 1992. These data were used to determine temporal patterns for productivity index and relate the productivity index to species first arrival and weather patterns. The annual productivity indices of these passerines, number of juveniles per 100 mist net hours, were related by linear regression to first arrival and weather of the Fairbanks area. This study found the indices’ related over time and have a negative relationship to mean annual temperature. Other explanatory variables’ linear regression relationship had various and magnitudes and directions. The passerine’s productivity various productivity responses are likely due to the diversity of foraging techniques. Understanding passerine’s productivity responses to weather can predict future passerine abundance. This is important for managers in areas where range shift may occur in the future.

KEYWORDS Alaska, boreal forest, climate change, passerine, productivity index.

Climate change is dramatically and disproportionately affecting polar regions globally (Hinzman 2005). The state of Alaska, which is comprised of arctic and sub-arctic regions, is among areas experiencing rapid change, especially in the vegetation composition of habitats, and the bird species that rely on them (Butler 2003, Markus 2004, and Tauzer 2013,). The interior Alaska is primarily composed of boreal forest with massive expanses of spruce (Picea spp.) forest (Boreal 2012). Bird abundance can indicate changes in vegetation and changes in weather (Tauzer 2013). Productivity is an effective indicator of changes in abundance.

As a relatively high proportion of autumn caught birds are juveniles, using a metric consisting of the number of juveniles caught in relation to mist net catch effort can represent a productivity index (Benson 2012). Many variables have been found to affect this productivity index including first arrival of the species (Butler 2003, Huppop 2003, Markus 2004), annual snowfall (DeSante 1987, Dybala 2013), annual mean temperature (Dickey 2008), and annual precipitation (Dickey 2008). This study considers the annual number of juveniles caught to catch effort (mist net hours) as it relates to each of these factors to explain temporal patterns. These relationships can help to understand the productivity index and find likely models to explain temporal patterns. Boreal nesting passerines have a variety of migration patterns and diets. To account for the various life histories, three representative migratory passerine species were chosen from the watch list of the Neotropical Migratory Bird Conservation Act. These species were also chosen because of their various migration patterns, diets and high abundances in Interior Alaska. The species of choice are Dark-eyed Juncos (Junco hyemalis), Lincoln Sparrows (Melospiza lincolnii), and Orange-crowned Warblers (Vermivora celata).

This study aims to improve understanding of the major factors impacting productivity of passerines in the boreal forest. The results of this study will benefit Alaska Department of Fish and Game, Creamer’s Field Waterfowl Migration Refuge, land managers of boreal forest (Department of Natural Resources, Borough of Land Management, US Fish and Wildlife Service), ornithologists, and fellow volunteer bird banders.

Using the proportion of juveniles captured in autumn at CFMS, the objectives of this study are to 1) determine temporal patterns for productivity index and 2) relate the productivity index to species first arrival and local weather.

STUDY AREA

This study took place in interior Alaska’s Creamer’s Field Migratory Waterfowl Refuge, located in Fairbanks, Alaska (Fig. 1, 2).  The 1,800 acre land functioned as a dairy during the Alaska gold rush in the early 1900s. It was purchased by the community in 1966 to preserve the migratory birds to create Creamer’s Migratory Waterfowl Refuge managed by Alaska Department of Fish and Game for human use and recreation (Creamer’s 2015).  The long-term migration bird banding station started in the autumn of 1992 at Creamer’s Field Migration Station (CFMS), the farthest north migration station (64 50’N, 147 50’W). Bird banding takes place in the spring and autumn to catch migrating passerines.

Fairbanks has an annual temperature of -2.45C with variation from 3.3C to -8.2C. The average precipitation is 275mm with 112 rain days. The average annual snowfall is 165mm (U.S. Climate Data 2015).

CFMS consistently maintained mist nets in the same locations beginning in 1992 with little variation. These mist nets are in old growth spruce forest, deciduous birch forests, shrub lands dominated by willow and alder, edge habitats and seasonal wetlands (Fig. 3).

The refuge is used and managed for human experiences with wild spaces. There are numerous dog walking trails, dog mushing trails, hiking and snow machining trails. Hunting and trapping is permitted in the area.

METHODS

To determine temporal patterns for productivity index, juveniles were captured at CFMS for 24 years. This study used years 1992-2004, 2006-2011. Methods for banding the songbirds were established for CFMS and are written in the Creamers Field Migration Station Operations Manual (Guers 2002). CFMS adheres to this manual to reduce variability among observers over many years. All nets are open for six hours each day from approximately sunrise or 5:30 am (whichever is later). Many nets in the seasonal wetlands are not opened if the flooding is above 1.5 meters which occurs in spring. Nets are closed early in heavy or constant rain, snow, intense wind or excessively hot or cold weather. Daily records of open net duration are used to calculate total net hours for each season. Nets are checked at 45 minute intervals to collect birds from the nets and the following information is recorded: species, age, date, and morphometric data are collected.

Age is determined by ‘skulling’ (determining ossification of skull) and by plumages, and presence of breeding characteristics (i.e. brooding patches and cloacal protuberances). In this study young of the year and fledglings are considered juveniles. The productivity index is calculated using the number of juveniles caught divided by 100 mist net hours of effort.

To relate the productivity index to species first arrival and weather patterns, the average annual productivity is compared to weather data collected from Fairbanks International Airport.  The productivity can be related using linear regression to first arrival of the species, annual snowfall, annual mean temperature, annual brooding season temperature and annual precipitation to find relationships between arrival, local weather and productivity.

Weather data was collected the Fairbanks International Airport. The maximum daily temperature, minimum daily temperature, amount of snowfall and amount of precipitation was recorded each day. The mean annual temperature was an average of daily maximum and minimums averaged over the course of the year. The annual brooding season temperature was an average of daily maximums and minimums for June through August. The total annual snowfall was the total sum of all fallen snow for the calendar year. The total annual precipitation was the total sum of all rainfall and snowfall over the course of a calendar year (Fairbanks 2015).

Microsoft Excel was used to make linear regressions of the annual productivity index to weather data and arrival data for each year and for each species to best explain productivity temporal patterns. Each explanatory variable was graphed against each species productivity index. A linear regression, best supported by the data, gave the change in the explanatory variable for each unit of productivity, one juvenile per 100 mist net hours. This linear regression also supplied the multiple correlation (R²) to determine how close the data correlated. This was done for each explanatory variable for each passerine species.

RESULTS

Our sample determined temporal patterns for productivity index using 23,245 juveniles of the three species caught over a series 189,099.2 mist net hours from 1992-2004 and 2006-2011 (Fig. 4). The data for year 2005 was not available for analysis at this time of the study. The Dark-eyed Juncos’, Orange-crowned Warbler’s and Lincoln Sparrow’s productivity indices were related by linear regression to first arrival and weather patterns. Linear regression compares one dependent variable, productivity, to one explanatory variable. Due to the single explanatory variable, no result was statistically significant, however some trends were observed.

Our survey included 11,671 juvenile Dark-eyed Junco. The first arrival of Dark-eyed Juncos at CFMS had a slight positive relationship of 0.062 Julian days per productivity with a multiple correlation (R²) value of 0.0025. The annual mean temperature had a negative effect on productivity with -0.673 degrees Celsius per productivity with a R² of 0.3328 (Fig. 5). The annual brooding season temperature had a slightly negative impact in productivity with -0.0136 degrees Celsius per productivity with an R² of 0.026. Total annual precipitation had a slight negative relationship with productivity with a relationship of -0.0468 centimeters of precipitation per productivity with a R² of 0.0132. Total annual snowfall had a negative effect on productivity with -1.1688 centimeters of snowfall per productivity with a R² of 0.0332.

Our survey included 7,309 juvenile Orange-crowned Warblers. The first arrival of Orange-crowned Warblers at CFMS had a negative relationship of -0.4545 Julian days per productivity with a R² value of 0.0779 (Fig. 6). The annual mean temperature had a negative effect on productivity with -0.5226 degrees Celsius per productivity with a R² of 0.1406. The annual brooding season temperature had a slightly positive impact in productivity with .0449 degrees Celsius per productivity with an R² of 0.016. Total annual precipitation had a slight negative relationship with productivity with a relationship of -0.0017 inches of precipitation per productivity with a R² of 0.000001. Total annual snowfall had a negative effect on productivity with -1.9743 centimeters of snowfall per productivity with a R² of 0.2122.

Our survey included 4,265 juvenile Lincoln Sparrows. The first arrival of Lincoln Sparrows at CFMS had a positive relationship of 2.1291 Julian days per productivity with a R² value of 0.0084 (Fig. 7). The annual mean temperature had a negative effect on productivity with -1.6123 degrees Celsius per productivity with a R² of 0.1246. The annual brooding season temperature had a slightly negative impact in productivity with -0.0042 degrees Celsius per productivity with an R² of 0.00001. Total annual precipitation had a slight negative relationship with productivity with a relationship of -0.1057 centimeters of precipitation per productivity with a R² of 0.0036. Total annual snowfall had a slightly negative effect on productivity with -3.7836 centimeters of snowfall per productivity with a R² of 0.0185.

DISCUSSION

In determining the temporal patterns of the productivity index, there is a visual relationship between the three species productivity each year. While the productivity of each species is different than the others and reacts differently to environmental factors, all the species have joint peaks of productivity at 1994-1995, 1997-1998 and 2009. They all have productivity crashes at 1996, 2002-2003 and 2008 (Fig. 4).

While the peaks and crashes in productivity appear to be related, each species has a different relationship to first arrival and weather patterns. These different relationships vary in magnitude and direction. All species in this study have lower numbers of juveniles during fall migration when the annual mean temperatures are high. As temperatures are expected to increase in interior Alaska due to climate change (Dybala 2013), these species may experience reduced productivity. This negative relationship between passerine productivity with temperature could be attributed to a variety of factors including food abundance, heat affects or the increased abundance of parasites. Interestingly, the annual brooding season temperature had little effect on the productivity index compared to annual mean temperature. This possibly could be because the forage available each summer is dependent on the over-wintering conditions or other in-direct affects.

Interestingly the Lincoln Sparrow productivity remained more consistent than the other species’ productivity indices. This shows that Lincoln Sparrows are less affected by positive or negative changes. This may be because of the foraging behavior or nesting behavior of Lincoln Sparrows being different than the other species. This may also be because the Lincoln Sparrows are able to use more alternatives to use available forage. More research exploring Lincoln Sparrows resilience should be explored.

The Dark-eyed Junco and Orange-crowned Warbler have higher productivity indexes in years with less snowfall. This may be a direct cause of decreasing available forage or nest sites, but it maybe indirect with snowfall decreasing early spring forage by other methods or it could be a correlation. More research exploring snowfall depth’s impacts on passerines should be explored. Doing a productivity study for a wider range of passerines with stratification by foraging behaviors would allow more insight on particular weather’s effects of various foraging behaviors.

Orange-crowned Warblers’ and Lincoln Sparrows’ productivity indices have opposite relations to arrival dates. Orange-crowned Warblers have lower productivity when they arrive late (Fig. 6). Conversely, Lincoln Sparrows benefit from earlier arrivals (Fig. 7). Arrival timing is expected to continue to alter with climate changes (Butler 2013). Arrival dates have not been correlated to breeding dates, therefore arrival does not necessarily mean the birds are beginning the breeding season. A more definitive way to find the beginning of the breeding season is checking cloacal protrusions on males and brooding patches on females to note active breeding behaviors.

Annual precipitation had little relation to productivity for any of the species possibly because the annual precipitation is not as important as precipitation during the brooding season itself (DeSante 1987). A further study looking at consecutive rain days during the brood season may have a larger effect on nest survival.

Juveniles of most passerine species depart their breeding grounds in Alaska significantly earlier than adults (Benson 2012). The juvenile to mist net hours is not accounting for departure of juveniles, this biases the data because an early autumn migration will reduce the total juveniles caught. This will lead to underestimating the productivity index in those years. To correct for this, the last date of juvenile capture should mark the end of the season.

This study likely consistently underestimates productivity. However, because this index is used with explanatory variables to find influential factors of productivity, the question is if the productivity index a good representation of actual productivity and not biased by detection probability variation. In this study, detection bias is not considered an issue because the same methods for detection with the same nets were used each year. The variation of detection probability between years would be minimal. A future passerine study of productivity could be coupled with juvenile mist net effort to determine if indeed there is a detection bias.

MANAGEMENT IMPLICATIONS

Modeling passerine productivity index’s response to various weather, allows predictions of passerine recruitment due to weather. The correlation between annual mean temperature and productivity index shows a decrease in productivity during warmer years. This can be used to predict passerine abundance changes due to potential annual temperatures. As recruitment to a population declines, the population abundance will decline. To stabilize recruitment into the population, passerines may nest in more northern latitudes or high altitudes to avoid this ecological trap.

Acknowledgments

I thank T. J. Brinkman, J. A. Curl, A. Harding Scurr for review comments and contributions to this manuscript.

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Figure 1. Location of Creamer’s Field Migratory Waterfowl Refuge in Interior Alaska.

Figure 2. Creamer’s Field Migratory Waterfowl Refuge in Fairbanks, Alaska.

Figure 3. Mist net locations of the Creamer’s Field Migration Station.

Figure 4. Relationships between annual productivity indices of Dark-eyed Juncos, Orange-crowned Warblers, and Lincoln Sparrows and time in years.

Figure 5. Relationship between annual mean temperature in Celsius and productivity index of Dark-eyed Juncos.

Figure 6. Relationship between date of first arrival and productivity index of Orange-crowned Warblers.

Figure 7. Relationship between date of first arrival and productivity index of Lincoln Sparrows.