An inexpensive method for remotely monitoring nest activity

Abstract

ABSTRACT In studies of avian nest success, investigators often face the difficult task of periodically checking nest status while at the same time limiting observer influence on nest survival. Remotely monitoring nests using temperature data loggers is one method that allows for continuous data capture regarding nest status (i.e., active vs. inactive) without the negative effects associated with repeated nest checks. We used small temperature data loggers (Thermochron iButtons) to remotely monitor nests of Long-billed Curlews (Numenius americanus) in northeastern Nevada. Data loggers programmed to record temperature at 10-min and 20-min intervals were placed in curlew nests. Data loggers were set to collect data throughout the nesting cycle to determine onset of incubation and timing of nest failure. On average, Long-billed Curlews began incubating approximately 3 d after the first egg was laid and onset of incubation coincided with the laying of the third egg. iButtons allowed us to determine when incubation was terminated in 17 of 23 unsuccessful Long-billed Curlew nests, including 13 of 17 depredated nests. The presence of iButtons in Long-billed Curlew nests did not affect daily survival rate, egg hatchability or rate of nest abandonment. iButtons are an efficient and practical means for remotely monitoring nests of large egg-laying birds, such as the Long-billed Curlew. En estudios de éxito de anidamiento, los investigadores muchas veces se encuentran con la dificultad de examinar el nido, tratando de limitar la infuencia del observador en la sobrevivencia de las aves. Uno de los métodos de monitoreo remoto es el uso de bitácoras electrónicas de temperatura que permiten tomar datos de forma contínua en un nido (e.g., activo vs inactivo) sin los efectos negativos asociados con el examen periodico del mismo. Utilizamos bitácoras electrónicas de temperatura (Thermochron iButtons) para monitorear nidos de chorlo (Numenius americanus) en el norte de Nevada. El instrumento se programó para tomar la temperatura dentro del nido a intérvalos de 10 a 20 minutos. El instrumento se utilizó para tomar datos a través del ciclo de anidamiento del ave para determinar el comienzo de la incubación y el momento en que el nido fuera abandonado. En promedio los chorlos comienzan a incubar aproximadamente tres dias después de puesto el primer huevo, lo que coincide con la puesta del tercer huevo. iButtons permitió determinar el final del periodo de incubacion de 13 de 17 nidos depredados. La presencia del instrumento en los nidos de chorlo no afecto la tasa diaria de sobrevivencia, tampoco el eclosionamiento o la tasa de abandono. Las bitácoras utilizadas son eficientes y además una forma práctica de monitorear a remoto los nidos de aves grandes tales como el chorlo estudiado. Studies of avian nesting biology and nest success often require researchers to make repeated nest visits. Frequent nest visitation for monitoring purposes, however, may be detrimental because it can reduce nest attendance by parents (Verboven et al. 2001), increase likelihood of nest abandonment (Ellison and Cleary 1978, Götmark 1992), increase egg mortality (Robert and Ralph 1975), and increase the probability of nest predation (Lenington 1979, Westmoreland and Best 1985, Major 1990). To counteract observer influences on nest survival, researchers commonly visit nests infrequently (e.g., every 3 d or weekly) to minimize nest disturbance. Yet, when time-specific effects on nest survival are present, longer between-visit intervals mean less precise information (Shaffer 2004). Daily records of nest status (i.e., active or not active), on the other hand, allow investigators to determine the exact day of nest failure and obtain precise information regarding time-specific effects. Recently, a small (diameter 11.5 mm, width: 6 mm, weight: 3.3 g), rugged, self-contained temperature data logger (Thermochron iButton, Maxim Integrated Products, Inc., Sunnyvale, CA) has been shown to be effective at documenting incubation behavior by recording temperature fluctuations in nests (Badyaev et al. 2003, Cooper and Mills 2005, Cooper et al. 2005). iButtons are preprogrammed to record time-stamped temperature readings (accuracy ± 1°C) at set intervals ranging from once every minute to once every 255 min. Perhaps the greatest advantage of iButtons compared to other temperature data loggers is cost. Thermochron iButtons can be purchased in bulk for as little as U.S. $14 per unit whereas traditional data loggers typically cost more than U.S. $100 per unit. The hardware needed to program iButtons through a personal computer is also inexpensive (approximately U.S. $32) and software is provided at no charge by the manufacturer. Previous studies using iButtons focused on incubation patterns of uniparental, intermittent incubators during focal intervals (e.g., over a 24 h cycle) and not the duration of the nesting cycle. Furthermore, studies using iButtons have focused on species that lay eggs smaller than the iButtons. We examined the effectiveness of using Thermochron iButtons to monitor nests over the entire nesting period in a species that lays large eggs. Introducing foreign objects into or around bird nests for remote-monitoring purposes may increase the conspicuousness of nests to potential nest predators or cause nest abandonment (Pietz and Granfors 2000, Renfrew and Ribic 2003). Artificial objects, such as a camera near the nest, could also have the opposite effect of increasing nest survival (Thompson et al. 1999, Herranz et al. 2002), perhaps by keeping specific predators from approaching the nest. Such confounding factors can lead to erroneous conclusions regarding nest success. Evaluating the effects of remote monitoring equipment on nest survival is therefore critical for extrapolating results to a broader context. Here we report the use of iButtons as a means for monitoring nests of Long-billed Curlews (Numenius americanus). The goals of our study were to (1) examine the effectiveness of iButtons for monitoring nests, (2) determine the onset of incubation, (3) determine the time of nest predation, and (4) examine the possible effects of iButtons on nest survival. Study site and nest searching Our study was conducted in 2004 and 2005 on two large cattle ranches (∼1600 ha) in northern Ruby Valley, Nevada (40°41′52″N, 115°14′00″W; elevation 1800 m). These ranches consist of irrigated hayfields and adjacent herbaceous rangeland habitat that support large numbers of nesting Long-billed Curlews. We initiated nest searches in April, shortly after curlews arrived at the site, and continued searching for nests through mid-June. Nest searches involved observing curlews in the act of nest building and observing individuals walking to nests and assuming an incubation posture. After discovery, nests were monitored from a distance every 3–4 d until failure or hatching. In addition, nests were visited periodically when we attempted to trap adults. For each nest we calculated nest initiation date (NID). When nests were found with one egg, we assumed that day to be the date of initiation. For nests found with 2–3 eggs, we estimated NID by counting back and assuming egg-laying intervals of 1.5 d (see Discussion). When nests were found after clutch completion, eggs were floated to determine stage of development (Westerskov 1950, Hays and Lecroy 1971) and estimate clutch completion date. We then counted back, assuming 5 d for egg laying, to obtain a NID estimate. iButton installation and data capture We used Thermochron iButtons (Model DS1921G) to monitor 44 Long-billed Curlew nests during 2004 (N= 21) and 2005 (N= 23). iButtons were placed in nests opportunistically when nests were discovered. As such, the timing of iButton installation varied from shortly after the first egg was laid to 2 weeks into incubation (mean nest age when iButton installed = 5.47 ± 0.83 [SE] d). However, in 31 of 44 nests, iButtons were installed prior to clutch completion (i.e., before the fourth egg was laid). We used two methods to install iButtons. In 2004 unaltered iButtons were placed in the nest bowl and lightly covered with existing nest materials (grasses and forbs). However, several iButtons installed in this manner were removed either by the incubating parents or a nest predator. Once removed from nests, iButtons could not be recovered and data were lost. In 2005 preprogrammed iButtons were glued to the top of 25-cm long aluminum stakes and covered with 1-mm thick black polyester. A small length of black nylon thread was used to secure the polyester to the iButton and stake. Stakes were then pushed by hand into the center of the nest until the covered iButton was flush with the bottom of the nest. No nest materials were used to cover iButtons in 2005. This method was effective in retaining iButtons after predation events and all iButtons installed in 2005 were recovered. To minimize the number of nest visits and disturbance, we programmed iButtons to collect data less frequently than in previous studies (Badyeav et al. 2003, Cooper and Mills 2005). In 2004 iButtons were programmed to collect data every 10 min. At this frequency iButtons collected temperature data for 14.2 d before memory was exhausted. Because it takes 33 d from the laying of the first egg until hatching (5 d for egg laying and 28 d for incubation), an iButton placed in a nest during egg laying that survived until hatching needed to be replaced twice to cover the entire life of the nest. In 2005 we programmed iButtons to collect data every 20 min, allowing iButton memory to last 28.4 d. At this frequency iButtons had to be replaced at most only once to cover the entire life of a nest. iButtons were exchanged opportunistically and usually when we attempted to trap adults at nests. One limitation of the data collection frequencies we used is that it did not allow for the detection of periods when an incubating bird left the nest for only a few minutes. This was a necessary trade-off to limit disturbance at nests while collecting data throughout incubation. Analysis After retrieval from nests, iButton data text files were converted into Microsoft Excel format, plotted as a line graph, and assessed visually. Temperature readings at each nest were compared to measures of ambient temperature derived from an iButton placed on the ground at a central location on the study site (2004) or from a RAWS weather station located approximately 60 km from the study site (2005). This station is in the same valley and at the same elevation as our study site and experiences similar weather patterns. Because the centrally located ambient-temperature iButton on the site was destroyed by ranching equipment in 2005, we used weather station data for comparison purposes. All ambient temperature readings were collected at hourly intervals. We conducted observations from a distance for all nests with iButtons throughout the incubation period. For each observation we recorded time of day, duration of observation, and whether a curlew was at the nest. These data were then matched with time-stamped data obtained from the iButtons to give us temperature readings at the nest during known on and off bouts. Using this information, combined with measures of ambient temperature, we assigned nests as being attended or not attended during each time-stamped temperature reading. Timing of onset of incubation was determined by following iButton nest temperature plots and ambient temperature plots forward through time until the two signals diverged. Prior to incubation, nest attentiveness consists of sporadic, short duration on bouts. Temperature signals prior to incubation, therefore, closely follow ambient temperature. When incubation began, nest temperature became relatively constant, whereas ambient temperature continued to fluctuate with the daily cycle. Timing of termination of incubation, or nest failure, was assessed by following iButton nest temperature plots and ambient temperature plots backwards through time until the two signals diverged. This yielded the last moment the nest was being incubated. Temperature signals from nest iButtons and ambient temperature readings after this event were similar and both followed the cyclical daily fluctuations in ambient temperature. To test for potential effects of iButtons on nest survival, we compared daily survival rates of nests with iButtons to control nests without iButtons. We included only nests that were depredated, abandoned, trampled by cattle, or where eggs hatched. Nests with iButtons that were flooded (N= 3), destroyed by farm equipment (N= 1), or that failed due to trapping activities (N= 1) were not included in survival analysis. Similarly, control nests in these categories were not considered. All nests that received iButtons and met the above criteria were included in survival analysis, regardless of whether the iButton was recovered or not. We used nest survival analysis in Program MARK (White and Burnham 1999, Dinsmore et al. 2002) to model daily survival rates of Long-billed Curlew nests. We considered eight a priori models: (1) a model with constant daily survival rate (DSR), S(·); (2) a model where DSR varied by the treatment effect of iButton presence/absence, S(iButton); (3) a model where DSR varied by year, S(year); (4) a model where DSR varied by NID, S(NID); (5) a model where DSR varied by year and iButton presence/absence, S(year + iButton); (6) a model where DSR varied by a year and iButton presence/absence interaction, S(year + iButton + year×iButton); (7) a model where DSR varied by iButton presence/absence and NID, S(iButton + NID); and (8) a model where DSR varied by an iButton presence/absence and NID interaction, S(iButton + NID + iButton × NID). We selected the best fit models using Akaike's Information Criterion corrected for small sample size (AICc; Akaike 1973). Although most iButtons were installed in early season curlew nests and in nests found within a few days of initiation, we continued to discover curlew nests throughout the breeding season. As a result curlew nests without iButtons had greater mean NID and age at discovery than curlew nests with iButtons. To control for potential difference in nest survival associated with these discrepancies, we included as controls only nests for which NID and nest age at discovery values were within the range of values of nests with iButtons. Another priority was to maintain nest visitation rates iButton nests and control nests to confounding the effect of iButtons on nest survival with number of visits to the nest. To maintain nest visitation iButton nests and control nests, iButtons were installed and replaced during nest visitation events that at all nests regardless of treatment (e.g., of the nest and trapping We used to test whether of nests to the treatment and control for specific by the as iButton nest or control nest and of NID, nest age at discovery, and number of nest visits per number of in the model a in the of iButton and control nests which could influence in nest survival analysis was using with set at are as mean ± We recovered iButton temperature data from of 44 Long-billed Curlew nests of nests in 2004 and 23 of 23 nests in 2005). iButtons were removed from nests by curlews or nest predators in 2004 and no data were At two nests that survived until hatching in curlews removed iButtons installed shortly before hatching. As a we did not obtain data for the last d of incubation at these nests. At nest in the iButton was the first iButton was removed by the incubating birds, and a iButton was recovered. This in a in the data at this nest. In 2005, all iButtons were recovered. of incubation recovered were installed prior to clutch completion, with 13 installed with one egg in the nest, eight with two eggs in the nest, and with eggs in the nest. of incubation was determined for 23 nests by documenting the day when nest temperature became (i.e., nest temperature to follow the cyclical by ambient The other nests were either or flooded prior to clutch completion and onset of incubation. For nests where iButtons were installed when one egg was in the nest, incubation began 3 d ± For nests where iButtons were installed when two eggs were in the nest, incubation began approximately one and a days ± At two of these nests, we onset of incubation to coincide with the laying of the third egg. for nests where iButtons were installed when eggs were in the nest, was no in the onset of incubation at all one nest ± Timing of nest predation the nests where iButtons were 17 were depredated, were flooded and abandoned, two were trampled by cattle, one was after cattle disturbance, and one failed because of egg during a trapping 17 nests, we determined the time of nest predation for 13 by documenting the when incubation was For the nests, we were to determine when predation because eggs were not being incubated. At two nests, had not been when nest predation At the other two nests, the parents were intermittent because of in and around the nest. iButton temperature data for one depredated nest is shown in Nest was depredated and on At the temperature in the nest and ambient temperature until the iButton was recovered a few days Nest iButton temperature data and ambient temperature for Nest text for 13 nests where we could determine when incubation was were and and and No nests with iButtons were and In addition, iButtons the time of nest failure for two nests trampled by cattle, the nest due to cattle disturbance, and one of the flooded nests. had not in the other two flooded nests we were to determine the time of failure. Nest survival and egg hatchability We daily survival rate for Long-billed Curlew nests in 2004 and 2005 with iButtons and Although iButton data often yielded the exact day of nest failure, the same data were not for nests without iButtons. As a result nest survival analysis was only on data from nest or trapping that were the same for iButton nests and control nests. analysis that nests with iButtons and control nests without iButtons did not with to the examined likelihood all our of nests to the iButton treatment and control treatment was and for the effect of iButtons on nest survival. Nest survival was best by a model with a constant daily survival rate = ± that did not an effect of iButton presence or an effect of iButton on nest survival received little support The model of a constant daily survival rate with an iButton effect was greater than two values from the top model that did not an iButton effect A likelihood test these two models that iButton presence did not affect Long-billed Curlew nest survival of daily survival rate of iButton nests and control nests the model with an iButton effect were ± and ± The model a year × iButton × also received little that the method of iButton installation did not affect nest survival. We no effect of iButtons on egg hatchability with of eggs from nests with iButtons hatching compared to from nests without iButtons. a nest where only one of eggs because of disturbance by ranching activities is hatchability for nests with iButtons was iButtons provided an effective and to monitor Long-billed Curlew nests and to onset of incubation, daily nest and timing of nest failure. of incubation was determined for all nests where iButtons were installed prior to clutch completion that survived until clutch In our study most Long-billed Curlews began incubating after the third egg was laid and before the fourth egg was behavior has been in species of and 1979, et al. et al. and may also in et al. iButton data also the egg-laying intervals of Long-billed Curlews. began with the third egg, and it approximately 3 d the first egg is and 1.5 d the egg is for incubation to that curlews on average, one egg every 1.5 d. This to an egg-laying period of d from the first egg to the fourth egg. Our data also that onset of incubation varied from to d after the first egg was that in egg-laying frequency and that may incubating prior to the laying of the third egg. As with nest iButton data abandonment that nest predation and predation Curlews may have nests with predation following at a Nest however, is in Long-billed Curlews and after clutch nests with iButtons that were in our were flooded and the other by For all nests in 2004 and 2005, abandonment for other than cattle disturbance or was of that predation termination of incubation in most to nest iButton data may increase the of nest For in Ruby Valley, present, curlew nests are a nest in 2005, a curlew nest was found to have been However, data from the recovered iButton that the nest had failed a day before due to using only nest this nest have been as destroyed by of iButtons iButtons placed in Long-billed Curlew nests were often removed from the nest by incubating or We iButtons to stakes in 2005 and were lost. iButtons in black also iButtons less and removed the to cover iButtons with nest of installation iButtons had no on nesting Long-billed Curlews. nest survival results obtained from nests with iButtons were of all Long-billed Curlew nests in our In addition, no Long-billed Curlews nests because of iButtons and no eggs were by iButtons. Our methods did not an of nest temperature. Long-billed Curlew eggs are large as by mm, As a 6 iButtons at the bottom of a curlew nest were a few from the of incubating recorded by iButtons were less than the temperature of the eggs during incubation. measures of nest temperature are data loggers in eggs be more (e.g., and We our methods were effective at presence or of an incubating when used in with of ambient temperature. However, the ambient temperature at our study site from to such ambient the temperature increase associated with an incubating bird on the nest was and our of incubation At where ambient temperature closely iButton temperature readings in the nest, of incubation status be We the in Ruby Valley, Nevada for allowing us to this study on We also our for in curlew nests. was provided through the Nevada of the of the Nevada of and the Nevada We Cooper and one for on a previous of this