The statistical distributions of the counts of organisms are generally skewed, and hence not normally distributed, nor are variances constant across treatments. We present a likelihood-ratio testing framework based on the negative binomial distribution that tests for the goodness of fit of this distribution to the observed counts, and then tests for differences in the mean and/or aggregation of the counts among treatments. Inferences about differences in means among treatments as well as the dispersion of the counts are possible. Simulations demonstrated that the statistical power of ANOV A is about the same as the likelihood-ratio testing procedure for testing equality of means, but our proposed testing procedure also provides information on dispersion. Type I error rates of Poisson regression exceeded the expected 5%, even when corrected for overdispersion. Count data on Orangecrowned Warblers (Vermivora celata) are used to demonstrate the procedure.

We examined the factors that influenced movement probabilities of snail kites (Rostrhamus sociabilis) in Florida, USA, based on birds with radio transmitters (n=282) during a three‐year period from April 1992 through April 1995. We focused on the hypotheses that increased movement probabilities were in response to low food availability or low water levels; the latter also implying low food availability for this species. An alternative hypothesis was that snail kites exhibit exploratory behavior, and corresponding increased movement probabilities, during periods of high food availability. 
Movement probabilities were not consistent with the hypotheses that low water levels or low food availability were the proximate cues to initiate movement from one wetland to another. Movement probabilities were higher during periods of relatively high food availability and were not associated with water levels; thus were consistent with the hypothesis that snail kites exhibited exploratory behavior during times of food abundance. However, we do not believe that our results are in conflict with previous reports of increased movement during extreme food scarcity; rather, that these hypotheses are not mutually exclusive and reflect different resource levels. During extreme drying events, food becomes virtually unavailable and birds must either move or die, but during times of food abundance there may be an advantage of exploratory behavior. Given that local drying events occur at frequencies of approximately every 5–10 yr in this environment, having explored wetlands throughout their range reduces the need for “blind” searching for suitable alternative habitats when such events do occur.

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Dwarf mistletoes (Arceuthobium spp.) are parasitic plants that are widely distributed in coniferous forests of the northern hemisphere. Because the effects of dwarf mistletoes on their host trees include stunted growth, reduced seed production, and death, these mistletoes may have a substantial influence on forest structure. Studies of the effects of dwarf mistletoe on forest communities have focused primarily on their influence on timber production. We studied the effects of southwestern ponderosa pine dwarf mistletoe (A. vaginatum) on the abundance and diversity of bird communities in central Colorado. Four stands, which ranged in level of mistletoe infestation from none to heavy, were selected at each of two locations. Each stand was surveyed approximately once per week during the avian breeding seasons of 1989 and 1990 by spot mapping. The number of bird registrations and bird species richness were positively correlated with the level of dwarf mistletoe, and this pattern was consistent among 24 of 28 avian species. No species had a significant negative correlation with the level of dwarf mistletoe. The relative abundance of bird species (i.e., evenness) did not differ among stands. The number of cavity‐nesting birds detected also was positively correlated with both dwarf‐mistletoe levels and number of snags. The number of snags and dwarf‐mistletoe levels also were highly correlated. Most snags had been infected as live trees by dwarf mistletoe and the mistletoe probably contributed to their death. While dwarf mistletoe has traditionally been viewed as a forest pest because of reductions in timber volume, we suggest that in areas where management goals are not strictly focused on timber production, control of dwarf mistletoe may not be justified, practical, or even desirable. Our data suggest that dwarf mistletoes may have positive influence on wildlife habitat. Consequently, we suggest that eradication efforts be reconsidered given that dwarf mistletoes have been a part of these forest ecosystems for thousands, and possibly millions, of years.

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The rate of population growth ( u ) is an important demographic parameter used to assess the viability of a population and to develop management and conservation agendas. We examined the use of resighting data to estimate u for the snail kite population in Florida from 1997-2000. The analyses consisted of (1) a robust design approach that derives an estimate of u from estimates of population size and (2) the Pradel (1996) temporal symmetry (TSM) approach that directly estimates u using an open-population capture-recapture model. Besides resighting data, both approaches required information on the number of unmarked individuals that were sighted during the sampling periods. The point estimates of u differed between the robust design and TSM approaches, but the 95% confidence intervals overlapped substantially. We believe the differences may be the result of sparse data and do not indicate the inappropriateness of either modelling technique. We focused on the results of the robust design because this approach provided estimates for all study years. Variation among these estimates was smaller than levels of variation among ad hoc estimates based on previously reported index statistics. We recommend that u of snail kites be estimated using capture-resighting methods rather than ad hoc counts.

Counts of nest starts are often used as indicators of the size of avian nesting populations, or of avian productivity. However, the accuracy of single or repeated counts of unmarked nests over time for estimating seasonal numbers of nests may be strongly affected by nest events that fall in between survey dates, or that occurred prior to or after the survey period. Accuracy may also be affected by uncertainty in the interpretation of counts due to overlap between starting and ending dates of asynchronous nests during the intervisit interval. To measure the combined magnitude of these effects on survey accuracy, we overlaid a monthly “survey” regime on known initiation and ending dates of 2055 nests of ciconiiform birds. Assuming all nests present on the date of simulated survey were counted, monthly surveys underestimated the true number of nest starts by 24–64%, depending on species and year. Using a simple model, we also demonstrate that accuracy does not increase much as survey frequency increases, and that significant estimation error can occur over a wide range of nest success values and degrees of asynchrony. We suggest that (1) these biases can be significant for surveys of many kinds of nesting birds including some territorial passerines, (2) this bias cannot be addressed by increasing survey frequency, and (3) the degree of renesting may be of critical interest for inferring breeding population size from nest count data. We suggest three possible approaches for modeling this error.