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Movement Ecology and Stopover Duration of Northern Waterthrush and Yellow-Rumped Warbler During Spring Migration Along the Upper Mississippi River

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Movement Ecology and Stopover Duration of Northern Waterthrush and Yellow-Rumped Warbler During Spring Migration Along the Upper Mississippi River Movement Ecology and Stopover Duration of Northern Waterthrush and Yellow-rumped Warbler during Spring Migration along the Upper Mississippi River Thesis Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the Graduate School of The Ohio State University By David L. Slager, B.S. Environment and Natural Resources Graduate Program The Ohio State University 2011 Thesis Committee: Dr. Paul G. Rodewald, Advisor Dr. Robert J. Gates Dr. Amanda D. Rodewald Copyright by David L. Slager 2011 Abstract Because distances traveled by Nearctic-Neotropical migratory songbirds are too great to cover in a single flight, birds must stop-over to rest and refuel along the way. Migrant birds spend the majority of their time and energy at stopover sites, highlighting the importance of stopover to a successful migration. Since migrant songbirds stop-over at locations not previously encountered, their efficiency in exploring and exploiting unfamiliar locations should influence their rate of fat deposition. The decision of when to depart from a stopover site is central to maintaining a successful migratory schedule because early arrival on the breeding grounds is known to confer reproductive advantages to migratory songbirds. However, little is known about which intrinsic and extrinsic factors influence movement patterns and departure decisions of migratory birds during stopover. My research objectives were to (1) describe the movement patterns of migrants during stopover, (2) determine the minimum stopover duration of migrants, (3) assess the influences of habitat, sex, energetic condition, and time of year on movements and stopover duration, and (4) examine relationships between weather conditions and the probability of migratory departure. I examined fine-scale movement patterns and stopover duration of 43 Northern Waterthrushes (Parkesia noveboracensis) and 30 "Myrtle" Yellow-rumped Warblers (Setophaga coronata coronata) during migratory stopover at Trempealeau National ii Wildlife Refuge in western Wisconsin during April-May of 2009 and 2010. I experimentally translocated and radio-tracked birds to investigate how migrants adjust movement patterns and stopover duration upon encountering unfamiliar environments. To determine movement patterns of migrants during stopover, I radio-located birds every 30 min on the day of release and the day after release. I calculated daily movement rate (m/hr) by dividing the total distance moved by the total time radio-tracked, daily displacement by taking the straight-line distance between the first point of the day (or release point) and the bird's location at 20:00, and daily linearity by dividing linear displacement by the total distance moved. Yellow-rumped Warblers occupied a variety of habitats and moved at a rate of 157 ± 54 m/hr (mean ± sd), showing no differences in movement metrics between the first and second days after release. An information theoretic approach using Akaike's Information Criterion (AICc) revealed that ordinal date, energetic condition, sex, release point, and year explained no more variation in daily movement metrics for day 1 and day 2 than a null model. Energetic condition at capture and daily measures of movement rate, displacement, and linearity showed no change over the migratory period. Yellow-rumped Warblers did not show a pronounced period of exploration followed by settling, a result contrasting with most studies of songbird movements during stopover. One possible explanation is that these generalist warblers find a wide variety of stopover habitats to be suitable, reducing the benefits of locating a particular habitat via exploration. By contrast, Northern Waterthrushes moved at a median rate of 89 m/hr on day 1 and 64 m/hr on day 2, showing strong decreases in movement rate (43%), displacement iii (71%), and linearity (55%) over the 2 days following release, a pattern consistent with exploration followed by settling. Waterthrushes released in upland forest habitat almost invariably moved to wetter habitats within a few hours of release. Most individual waterthrushes, regardless of release habitat, were fairly sedentary after the day of release. Models best supported by Akaike's Information Criterion (AICc) revealed prominent differences in determinants of movement on day 1 and day 2. Waterthrushes released in upland forest habitat, at a later ordinal date, and in higher energetic condition had higher movement rates on day 1. In contrast, females and birds tracked in 2009 had higher movement rates on day 2. Northern Waterthrushes released in upland forest habitat and at later ordinal dates exhibited greater displacement on day 1; birds tracked in 2009 showed greater displacement on day 2. Birds released in upland forest habitat had increased linearity on day 1, whereas males and birds tracked in 2009 showed increased linearity on day 2. The fact that waterthrushes released in less suitable upland forest habitat showed movement characteristics on day 1 consistent with greater exploration than birds released in more suitable bottomland forest habitat suggests exploration as a method by which migrants locate suitable stopover microsites when encountering unfamiliar areas. Although waterthrushes with higher fat stores had higher movement rates on day 1, movements on day 2 were independent of energetic condition, suggesting that lean and fat birds explore habitat differently but converge on similar movement patterns after settling. A possible explanation for increased daily movement rates and displacement later in the spring is that urgency to arrive on the breeding grounds forces more rapid exploration. The fact that female waterthrushes had higher movement rates iv and lower linearity than males on day 2 despite no inter-sexual differences in movements on day 1 is difficult to explain, but suggests males and females have a similar approach to initial exploration of habitat yet differ in movement patterns following settlement. To investigate minimum stopover duration, I radio-located each bird at least once per day until departure. If a radio frequency was not detected for 7 days, I assumed the bird had departed on migration on the night of the last detection. Models best supported by Akaike's Information Criterion (AICc) indicated that Yellow-rumped Warblers with lower fat stores and those captured later in the season had longer stopover durations. The best-supported models for Northern Waterthrush showed that females released earlier in the season, in 2010, in bottomland forest habitat, and with lower fat stores had a longer minimum stopover duration. The negative relationship between energetic condition and stopover duration for both species studied matches the pattern emerging from other studies. Declining stopover duration over the season in Northern Waterthrushes is consistent with the idea that the primary constraints facing birds shift from energy to time as arrival timing on the breeding grounds becomes more urgent later in the spring. The tendency for Yellow-rumped Warblers to show shorter stopover durations earlier in the spring is more difficult to explain and underscores that spring migration is not a homogeneous period with regard to stopover decisions. Male Northern Waterthrushes had shorter stopover durations than females even after controlling for the earlier arrival dates of males, which may be explained in part by the need of males to arrive on the breeding grounds early to establish territories prior to the arrival of females. Surprisingly, waterthrushes released in less suitable upland forest habitat had shorter v stopover durations than those released in more suitable bottomland forest, despite the fact that all birds eventually settled in wet habitats. Most studies of songbirds indicate limited daily displacement of migrants at stopover sites, so it is possible that where a migrant makes landfall ultimately determines the extent of stopover resources available, due to an upper bound to exploratory movements. To examine how weather conditions predicted the probability of migratory departure, I used Random Forests (an extension of classification trees) to determine how a suite of nightly weather variables predicted migrants' departure decisions after controlling for time of season. The Random Forests model predicted the highest probability of Yellow-rumped Warbler departure on nights with decreasing barometric pressure and a more easterly wind component. The model predicted Northern Waterthrush departure on nights with a higher temperatures and more easterly winds. After controlling for time of year, I found that departure in both species is predicted by conditions associated with passing warm fronts. My work suggests that energetic condition and date determine the duration of stopover whereas favorable weather conditions fine-tune the precise date of departure. Overall, my results show that a diverse suite of factors including habitat, condition, sex, year, date, and weather mediates the movement patterns and stopover duration of migrants. I found that birds, especially in the case of Northern Waterthrushes, explore limited areas during stopover and that habitat and energetic condition may strongly influence stopover duration. Understanding inter-specific differences in stopover biology is an essential part of any holistic conservation strategy for migrants. vi The fact that events during stopover can influence arrival timing and reproductive success on the breeding grounds highlights the importance of conserving
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